Release version 3.3 GA (2021.1.0).

See #3865

.mvn/wrapper/maven-wrapper.properties

@@ -1 +1,2 @@
-distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.5.4/apache-maven-3.5.4-bin.zip
+#Mon Oct 11 14:30:24 CEST 2021
+distributionUrl=https\://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.8.3/apache-maven-3.8.3-bin.zip

CI.adoc

@@ -1,6 +1,6 @@
 = Continuous Integration
 
-image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-mongodb%2Fmaster&subject=Moore%20(master)[link=https://jenkins.spring.io/view/SpringData/job/spring-data-mongodb/]
+image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-mongodb%2Fmain&subject=Moore%20(main)[link=https://jenkins.spring.io/view/SpringData/job/spring-data-mongodb/]
 image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-mongodb%2F2.1.x&subject=Lovelace%20(2.1.x)[link=https://jenkins.spring.io/view/SpringData/job/spring-data-mongodb/]
 image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-mongodb%2F1.10.x&subject=Ingalls%20(1.10.x)[link=https://jenkins.spring.io/view/SpringData/job/spring-data-mongodb/]
 

CONTRIBUTING.adoc

@@ -1,3 +1,3 @@
 = Spring Data contribution guidelines
 
-You find the contribution guidelines for Spring Data projects https://github.com/spring-projects/spring-data-build/blob/master/CONTRIBUTING.adoc[here].
+You find the contribution guidelines for Spring Data projects https://github.com/spring-projects/spring-data-build/blob/main/CONTRIBUTING.adoc[here].

Jenkinsfile

@@ -3,7 +3,7 @@ pipeline {
 
 	triggers {
 		pollSCM 'H/10 * * * *'
-		upstream(upstreamProjects: "spring-data-commons/master", threshold: hudson.model.Result.SUCCESS)
+		upstream(upstreamProjects: "spring-data-commons/main", threshold: hudson.model.Result.SUCCESS)
 	}
 
 	options {
@@ -14,6 +14,22 @@ pipeline {
 	stages {
 		stage("Docker images") {
 			parallel {
+			    stage('Publish JDK 8 + MongoDB 5.0') {
+					when {
+						changeset "ci/openjdk8-mongodb-5.0/**"
+					}
+					agent { label 'data' }
+					options { timeout(time: 30, unit: 'MINUTES') }
+
+					steps {
+						script {
+							def image = docker.build("springci/spring-data-openjdk8-with-mongodb-5.0.0", "ci/openjdk8-mongodb-5.0/")
+								docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
+									image.push()
+								}
+							}
+						}
+					}
 				stage('Publish JDK 8 + MongoDB 4.0') {
 					when {
 						changeset "ci/openjdk8-mongodb-4.0/**"
@@ -46,16 +62,16 @@ pipeline {
 						}
 					}
 				}
-				stage('Publish JDK 15 + MongoDB 4.4') {
+				stage('Publish JDK 16 + MongoDB 4.4') {
 					when {
-						changeset "ci/openjdk15-mongodb-4.4/**"
+						changeset "ci/openjdk16-mongodb-4.4/**"
 					}
 					agent { label 'data' }
 					options { timeout(time: 30, unit: 'MINUTES') }
 
 					steps {
 						script {
-							def image = docker.build("springci/spring-data-openjdk15-with-mongodb-4.4.4", "ci/openjdk15-mongodb-4.4/")
+							def image = docker.build("springci/spring-data-openjdk16-with-mongodb-4.4.4", "ci/openjdk16-mongodb-4.4/")
 							docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
 								image.push()
 							}
@@ -67,8 +83,9 @@ pipeline {
 
 		stage("test: baseline (jdk8)") {
 			when {
+				beforeAgent(true)
 				anyOf {
-					branch 'master'
+					branch(pattern: "main|(\\d\\.\\d\\.x)", comparator: "REGEXP")
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -76,6 +93,9 @@ pipeline {
 				label 'data'
 			}
 			options { timeout(time: 30, unit: 'MINUTES') }
+			environment {
+				ARTIFACTORY = credentials('02bd1690-b54f-4c9f-819d-a77cb7a9822c')
+			}
 			steps {
 				script {
 					docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
@@ -85,7 +105,7 @@ pipeline {
 							sh 'sleep 10'
 							sh 'mongo --eval "rs.initiate({_id: \'rs0\', members:[{_id: 0, host: \'127.0.0.1:27017\'}]});"'
 							sh 'sleep 15'
-							sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
+							sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -s settings.xml clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
 						}
 					}
 				}
@@ -94,8 +114,9 @@ pipeline {
 
 		stage("Test other configurations") {
 			when {
+				beforeAgent(true)
 				allOf {
-					branch 'master'
+					branch(pattern: "main|(\\d\\.\\d\\.x)", comparator: "REGEXP")
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -105,6 +126,9 @@ pipeline {
 						label 'data'
 					}
 					options { timeout(time: 30, unit: 'MINUTES') }
+					environment {
+						ARTIFACTORY = credentials('02bd1690-b54f-4c9f-819d-a77cb7a9822c')
+					}
 					steps {
 						script {
 							docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
@@ -114,7 +138,7 @@ pipeline {
 									sh 'sleep 10'
 									sh 'mongo --eval "rs.initiate({_id: \'rs0\', members:[{_id: 0, host: \'127.0.0.1:27017\'}]});"'
 									sh 'sleep 15'
-									sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
+									sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -s settings.xml clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
 								}
 							}
 						}
@@ -126,6 +150,9 @@ pipeline {
 						label 'data'
 					}
 					options { timeout(time: 30, unit: 'MINUTES') }
+					environment {
+						ARTIFACTORY = credentials('02bd1690-b54f-4c9f-819d-a77cb7a9822c')
+					}
 					steps {
 						script {
 							docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
@@ -135,28 +162,55 @@ pipeline {
 									sh 'sleep 10'
 									sh 'mongo --eval "rs.initiate({_id: \'rs0\', members:[{_id: 0, host: \'127.0.0.1:27017\'}]});"'
 									sh 'sleep 15'
-									sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
+									sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -s settings.xml clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
 								}
 							}
 						}
 					}
 				}
 
-				stage("test: baseline (jdk15)") {
+				stage("test: mongodb 5.0 (jdk8)") {
+                	agent {
+                		label 'data'
+                	}
+                	options { timeout(time: 30, unit: 'MINUTES') }
+                	environment {
+                		ARTIFACTORY = credentials('02bd1690-b54f-4c9f-819d-a77cb7a9822c')
+                	}
+                	steps {
+                		script {
+                			docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
+                				docker.image('springci/spring-data-openjdk8-with-mongodb-5.0.0:latest').inside('-v $HOME:/tmp/jenkins-home') {
+                					sh 'mkdir -p /tmp/mongodb/db /tmp/mongodb/log'
+                					sh 'mongod --setParameter transactionLifetimeLimitSeconds=90 --setParameter maxTransactionLockRequestTimeoutMillis=10000 --dbpath /tmp/mongodb/db --replSet rs0 --fork --logpath /tmp/mongodb/log/mongod.log &'
+                					sh 'sleep 10'
+                					sh 'mongo --eval "rs.initiate({_id: \'rs0\', members:[{_id: 0, host: \'127.0.0.1:27017\'}]});"'
+                					sh 'sleep 15'
+                					sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -s settings.xml clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
+                				}
+                			}
+                		}
+                	}
+                }
+
+				stage("test: baseline (jdk16)") {
 					agent {
 						label 'data'
 					}
 					options { timeout(time: 30, unit: 'MINUTES') }
+					environment {
+						ARTIFACTORY = credentials('02bd1690-b54f-4c9f-819d-a77cb7a9822c')
+					}
 					steps {
 						script {
 							docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
-								docker.image('springci/spring-data-openjdk15-with-mongodb-4.4.4:latest').inside('-v $HOME:/tmp/jenkins-home') {
+								docker.image('springci/spring-data-openjdk16-with-mongodb-4.4.4:latest').inside('-v $HOME:/tmp/jenkins-home') {
 									sh 'mkdir -p /tmp/mongodb/db /tmp/mongodb/log'
 									sh 'mongod --setParameter transactionLifetimeLimitSeconds=90 --setParameter maxTransactionLockRequestTimeoutMillis=10000 --dbpath /tmp/mongodb/db --replSet rs0 --fork --logpath /tmp/mongodb/log/mongod.log &'
 									sh 'sleep 10'
 									sh 'mongo --eval "rs.initiate({_id: \'rs0\', members:[{_id: 0, host: \'127.0.0.1:27017\'}]});"'
 									sh 'sleep 15'
-									sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -Pjava11 clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
+									sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -s settings.xml -Pjava11 clean dependency:list test -Duser.name=jenkins -Dsort -U -B'
 								}
 							}
 						}
@@ -167,8 +221,9 @@ pipeline {
 
 		stage('Release to artifactory') {
 			when {
+				beforeAgent(true)
 				anyOf {
-					branch 'master'
+					branch(pattern: "main|(\\d\\.\\d\\.x)", comparator: "REGEXP")
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -185,7 +240,7 @@ pipeline {
 				script {
 					docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
 						docker.image('adoptopenjdk/openjdk8:latest').inside('-v $HOME:/tmp/jenkins-home') {
-							sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -Pci,artifactory ' +
+							sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -s settings.xml -Pci,artifactory ' +
 									'-Dartifactory.server=https://repo.spring.io ' +
 									"-Dartifactory.username=${ARTIFACTORY_USR} " +
 									"-Dartifactory.password=${ARTIFACTORY_PSW} " +
@@ -198,35 +253,6 @@ pipeline {
 				}
 			}
 		}
-
-		stage('Publish documentation') {
-			when {
-				branch 'master'
-			}
-			agent {
-				label 'data'
-			}
-			options { timeout(time: 20, unit: 'MINUTES') }
-
-			environment {
-				ARTIFACTORY = credentials('02bd1690-b54f-4c9f-819d-a77cb7a9822c')
-			}
-
-			steps {
-				script {
-					docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
-						docker.image('adoptopenjdk/openjdk8:latest').inside('-v $HOME:/tmp/jenkins-home') {
-							sh 'MAVEN_OPTS="-Duser.name=jenkins -Duser.home=/tmp/jenkins-home" ./mvnw -Pci,distribute ' +
-									'-Dartifactory.server=https://repo.spring.io ' +
-									"-Dartifactory.username=${ARTIFACTORY_USR} " +
-									"-Dartifactory.password=${ARTIFACTORY_PSW} " +
-									"-Dartifactory.distribution-repository=temp-private-local " +
-									'-Dmaven.test.skip=true clean deploy -U -B'
-						}
-					}
-				}
-			}
-		}
 	}
 
 	post {

LICENSE.txt

@@ -0,0 +1,202 @@
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README.adoc

@@ -1,6 +1,6 @@
 image:https://spring.io/badges/spring-data-mongodb/ga.svg[Spring Data MongoDB,link=https://projects.spring.io/spring-data-mongodb#quick-start] image:https://spring.io/badges/spring-data-mongodb/snapshot.svg[Spring Data MongoDB,link=https://projects.spring.io/spring-data-mongodb#quick-start]
 
-= Spring Data MongoDB image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-mongodb%2Fmaster&subject=Build[link=https://jenkins.spring.io/view/SpringData/job/spring-data-mongodb/] https://gitter.im/spring-projects/spring-data[image:https://badges.gitter.im/spring-projects/spring-data.svg[Gitter]]
+= Spring Data MongoDB image:https://jenkins.spring.io/buildStatus/icon?job=spring-data-mongodb%2Fmain&subject=Build[link=https://jenkins.spring.io/view/SpringData/job/spring-data-mongodb/] https://gitter.im/spring-projects/spring-data[image:https://badges.gitter.im/spring-projects/spring-data.svg[Gitter]]
 
 The primary goal of the https://projects.spring.io/spring-data[Spring Data] project is to make it easier to build Spring-powered applications that use new data access technologies such as non-relational databases, map-reduce frameworks, and cloud based data services.
 
@@ -8,10 +8,12 @@ The Spring Data MongoDB project aims to provide a familiar and consistent Spring
 The Spring Data MongoDB project provides integration with the MongoDB document database.
 Key functional areas of Spring Data MongoDB are a POJO centric model for interacting with a MongoDB `+Document+` and easily writing a repository style data access layer.
 
+[[code-of-conduct]]
 == Code of Conduct
 
 This project is governed by the https://github.com/spring-projects/.github/blob/e3cc2ff230d8f1dca06535aa6b5a4a23815861d4/CODE_OF_CONDUCT.md[Spring Code of Conduct]. By participating, you are expected to uphold this code of conduct. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.
 
+[[getting-started]]
 == Getting Started
 
 Here is a quick teaser of an application using Spring Data Repositories in Java:
@@ -59,6 +61,7 @@ class ApplicationConfig extends AbstractMongoClientConfiguration {
 }
 ----
 
+[[maven-configuration]]
 === Maven configuration
 
 Add the Maven dependency:
@@ -68,24 +71,25 @@ Add the Maven dependency:
 <dependency>
   <groupId>org.springframework.data</groupId>
   <artifactId>spring-data-mongodb</artifactId>
-  <version>${version}.RELEASE</version>
+  <version>${version}</version>
 </dependency>
 ----
 
-If you'd rather like the latest snapshots of the upcoming major version, use our Maven snapshot repository and declare the appropriate dependency version.
+If you'd rather like the latest snapshots of the upcoming major version, use our Maven snapshot repository
+and declare the appropriate dependency version.
 
 [source,xml]
 ----
 <dependency>
   <groupId>org.springframework.data</groupId>
   <artifactId>spring-data-mongodb</artifactId>
-  <version>${version}.BUILD-SNAPSHOT</version>
+  <version>${version}-SNAPSHOT</version>
 </dependency>
 
 <repository>
-  <id>spring-libs-snapshot</id>
+  <id>spring-snapshot</id>
   <name>Spring Snapshot Repository</name>
-  <url>https://repo.spring.io/libs-snapshot</url>
+  <url>https://repo.spring.io/snapshot</url>
 </repository>
 ----
 
@@ -98,7 +102,7 @@ Some of the changes affect the initial setup configuration as well as compile/ru
 
 .Changed XML Namespace Elements and Attributes:
 |===
-Element / Attribute | 2.x | 3.x
+| Element / Attribute | 2.x | 3.x
 
 | `<mongo:mongo-client />`
 | Used to create a `com.mongodb.MongoClient`
@@ -116,7 +120,7 @@ Use `<mongo:client-settings cluster-hosts="..." />` instead
 
 .Removed XML Namespace Elements and Attributes:
 |===
-Element / Attribute | Replacement in 3.x | Comment
+| Element / Attribute | Replacement in 3.x | Comment
 
 | `<mongo:db-factory mongo-ref="..." />`
 | `<mongo:db-factory mongo-client-ref="..." />`
@@ -133,7 +137,7 @@ Element / Attribute | Replacement in 3.x | Comment
 
 .New XML Namespace Elements and Attributes:
 |===
-Element | Comment
+| Element | Comment
 
 | `<mongo:db-factory mongo-client-ref="..." />`
 | Replacement for `<mongo:db-factory mongo-ref="..." />`
@@ -153,7 +157,7 @@ Element | Comment
 
 .Java API changes
 |===
-Type | Comment
+| Type | Comment
 
 | `MongoClientFactoryBean`
 | Creates `com.mongodb.client.MongoClient` instead of `com.mongodb.MongoClient` +
@@ -174,7 +178,7 @@ Uses `MongoClientSettings` instead of `MongoClientOptions`.
 
 .Removed Java API:
 |===
-2.x | Replacement in 3.x | Comment
+| 2.x | Replacement in 3.x | Comment
 
 | `MongoClientOptionsFactoryBean`
 | `MongoClientSettingsFactoryBean`
@@ -226,6 +230,7 @@ static class Config extends AbstractMongoClientConfiguration {
 ----
 ====
 
+[[getting-help]]
 == Getting Help
 
 Having trouble with Spring Data? We’d love to help!
@@ -239,6 +244,7 @@ If you are just starting out with Spring, try one of the https://spring.io/guide
 You can also chat with the community on https://gitter.im/spring-projects/spring-data[Gitter].
 * Report bugs with Spring Data MongoDB at https://github.com/spring-projects/spring-data-mongodb/issues[github.com/spring-projects/spring-data-mongodb/issues].
 
+[[reporting-issues]]
 == Reporting Issues
 
 Spring Data uses Github as issue tracking system to record bugs and feature requests.
@@ -249,10 +255,85 @@ If you want to raise an issue, please follow the recommendations below:
 * Please provide as much information as possible with the issue report, we like to know the version of Spring Data that you are using, the JVM version, Stacktrace, etc.
 * If you need to paste code, or include a stack trace use https://guides.github.com/features/mastering-markdown/[Markdown] code fences +++```+++.
 
+[[guides]]
+== Guides
+
+The https://spring.io/[spring.io] site contains several guides that show how to use Spring Data step-by-step:
+
+* https://spring.io/guides/gs/accessing-data-mongodb/[Accessing Data with MongoDB] is a very basic guide that shows you how to create a simple application and how to access data using repositories.
+* https://spring.io/guides/gs/accessing-mongodb-data-rest/[Accessing MongoDB Data with REST] is a guide to creating a REST web service exposing data stored in MongoDB through repositories.
+
+[[examples]]
+== Examples
+
+* https://github.com/spring-projects/spring-data-examples/[Spring Data Examples] contains example projects that explain specific features in more detail.
+
+[[building-from-source]]
 == Building from Source
 
-You don’t need to build from source to use Spring Data (binaries in https://repo.spring.io[repo.spring.io]), but if you want to try out the latest and greatest, Spring Data can be easily built with the https://github.com/takari/maven-wrapper[maven wrapper].
-You also need JDK 1.8.
+You do not need to build from source to use Spring Data. Binaries are available in https://repo.spring.io[repo.spring.io].
+and accessible from Maven using the Maven configuration noted <<maven-configuration,above>>.
+
+NOTE: Configuration for Gradle is similar to Maven.
+
+The best way to get started is by creating a Spring Boot project using MongoDB on https://start.spring.io[start.spring.io].
+Follow this https://start.spring.io/#type=maven-project&language=java&platformVersion=2.5.4&packaging=jar&jvmVersion=1.8&groupId=com.example&artifactId=demo&name=demo&description=Demo%20project%20for%20Spring%20Boot&packageName=com.example.demo&dependencies=data-mongodb[link]
+to build an imperative application and this https://start.spring.io/#type=maven-project&language=java&platformVersion=2.5.4&packaging=jar&jvmVersion=1.8&groupId=com.example&artifactId=demo&name=demo&description=Demo%20project%20for%20Spring%20Boot&packageName=com.example.demo&dependencies=data-mongodb-reactive[link]
+to build a reactive one.
+
+However, if you want to try out the latest and greatest, Spring Data can be easily built with the https://github.com/takari/maven-wrapper[maven wrapper]
+and minimally JDK 8 (https://www.oracle.com/java/technologies/downloads/[JDK downloads]).
+
+In order to build Spring Data MongoDB, first you will need to https://www.mongodb.com/try/download/community[download]
+and https://docs.mongodb.com/manual/installation/[install a MongoDB distribution].
+
+Once you have installed MongoDB, you need to start a MongoDB server. It is convenient to set an environment variable to
+your MongoDB installation (e.g. `MONGODB_HOME`).
+
+To run the full test suite a https://docs.mongodb.com/manual/tutorial/deploy-replica-set/[MongoDB Replica Set] is required.
+
+To run the MongoDB server enter the following command from a command-line:
+
+[source,bash]
+----
+$ $MONGODB_HOME/bin/mongod --dbpath $MONGODB_HOME/runtime/data --ipv6 --port 27017 --replSet rs0
+...
+"msg":"Successfully connected to host"
+----
+
+Once the MongoDB server starts up, you should see the message (`msg`), "_Successfully connected to host_".
+
+Notice the `--dbpath` option to the `mongod` command. You can set this to anything you like, but in this case, we set
+the absolute path to a sub-directory (`runtime/data/`) under the MongoDB installation directory (in `$MONGODB_HOME`).
+
+You need to initialize the MongoDB replica set only once on the first time the MongoDB server is started.
+To initialize the replica set, start a mongo client:
+
+[source,bash]
+----
+$ $MONGODB_HOME/bin/mongo
+MongoDB server version: 5.0.0
+...
+----
+
+Then enter the following command:
+
+[source,bash]
+----
+mongo> rs.initiate({ _id: 'rs0', members: [ { _id: 0, host: '127.0.0.1:27017' } ] })
+----
+
+Finally, on UNIX-based system (for example, Linux or Mac OS X) you may need to adjust the `ulimit`.
+In case you need to, you can adjust the `ulimit` with the following command (32768 is just a recommendation):
+
+[source,bash]
+----
+$ ulimit -n 32768
+----
+
+You can use `ulimit -a` again to verify the `ulimit` on "_open files_" was set appropriately.
+
+Now you are ready to build Spring Data MongoDB. Simply enter the following `mvnw` (Maven Wrapper) command:
 
 [source,bash]
 ----
@@ -261,7 +342,8 @@ You also need JDK 1.8.
 
 If you want to build with the regular `mvn` command, you will need https://maven.apache.org/run-maven/index.html[Maven v3.5.0 or above].
 
-_Also see link:CONTRIBUTING.adoc[CONTRIBUTING.adoc] if you wish to submit pull requests, and in particular please sign the https://cla.pivotal.io/sign/spring[Contributor’s Agreement] before your first non-trivial change._
+_Also see link:CONTRIBUTING.adoc[CONTRIBUTING.adoc] if you wish to submit pull requests, and in particular, please sign
+the https://cla.pivotal.io/sign/spring[Contributor’s Agreement] before your first non-trivial change._
 
 === Building reference documentation
 
@@ -274,17 +356,7 @@ Building the documentation builds also the project without running tests.
 
 The generated documentation is available from `target/site/reference/html/index.html`.
 
-== Guides
-
-The https://spring.io/[spring.io] site contains several guides that show how to use Spring Data step-by-step:
-
-* https://spring.io/guides/gs/accessing-data-mongodb/[Accessing Data with MongoDB] is a very basic guide that shows you how to create a simple application and how to access data using repositories.
-* https://spring.io/guides/gs/accessing-mongodb-data-rest/[Accessing MongoDB Data with REST] is a guide to creating a REST web service exposing data stored in MongoDB through repositories.
-
-== Examples
-
-* https://github.com/spring-projects/spring-data-examples/[Spring Data Examples] contains example projects that explain specific features in more detail.
-
+[[license]]
 == License
 
 Spring Data MongoDB is Open Source software released under the https://www.apache.org/licenses/LICENSE-2.0.html[Apache 2.0 license].

ci/openjdk11-mongodb-4.4/Dockerfile

@@ -4,6 +4,9 @@ ENV TZ=Etc/UTC
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN set -eux; \
+	sed -i -e 's/archive.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/security.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/http/https/g' /etc/apt/sources.list ; \
 	apt-get update && apt-get install -y apt-transport-https apt-utils gnupg2 ; \
 	apt-key adv --keyserver hkps://keyserver.ubuntu.com:443 --recv 656408E390CFB1F5 ; \
 	echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.4 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-4.4.list; \

ci/openjdk16-mongodb-4.4/Dockerfile

@@ -1,9 +1,12 @@
-FROM adoptopenjdk/openjdk15:latest
+FROM adoptopenjdk/openjdk16:latest
 
 ENV TZ=Etc/UTC
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN set -eux; \
+	sed -i -e 's/archive.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/security.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/http/https/g' /etc/apt/sources.list ; \
 	apt-get update && apt-get install -y apt-transport-https apt-utils gnupg2 ; \
 	apt-key adv --keyserver hkps://keyserver.ubuntu.com:443 --recv 656408E390CFB1F5 ; \
 	echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.4 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-4.4.list; \

ci/openjdk8-mongodb-4.0/Dockerfile

@@ -4,6 +4,9 @@ ENV TZ=Etc/UTC
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN RUN set -eux; \
+	sed -i -e 's/archive.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/security.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/http/https/g' /etc/apt/sources.list ; \
 	apt-get update && apt-get install -y apt-transport-https apt-utils gnupg2 ; \
 	apt-key adv --keyserver hkps://keyserver.ubuntu.com:443 --recv 9DA31620334BD75D9DCB49F368818C72E52529D4 ; \
 	echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.0 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-4.0.list; \

ci/openjdk8-mongodb-4.4/Dockerfile

@@ -4,6 +4,9 @@ ENV TZ=Etc/UTC
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN set -eux; \
+	sed -i -e 's/archive.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/security.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/http/https/g' /etc/apt/sources.list ; \
 	apt-get update && apt-get install -y apt-transport-https apt-utils gnupg2 ; \
 	apt-key adv --keyserver hkps://keyserver.ubuntu.com:443 --recv 656408E390CFB1F5 ; \
 	echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.4 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-4.4.list; \

ci/openjdk8-mongodb-5.0/Dockerfile

@@ -0,0 +1,20 @@
+FROM adoptopenjdk/openjdk8:latest
+
+ENV TZ=Etc/UTC
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN set -eux; \
+	sed -i -e 's/archive.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/security.ubuntu.com/mirror.one.com/g' /etc/apt/sources.list; \
+	sed -i -e 's/http/https/g' /etc/apt/sources.list ; \
+	apt-get update && apt-get install -y apt-transport-https apt-utils gnupg2 wget ; \
+	# MongoDB 5.0 release signing key
+	apt-key adv --keyserver hkps://keyserver.ubuntu.com:443 --recv B00A0BD1E2C63C11 ; \
+	# Needed when MongoDB creates a 5.0 folder.
+	echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/5.0 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-5.0.list; \
+	echo ${TZ} > /etc/timezone;
+
+RUN apt-get update; \
+	apt-get install -y mongodb-org=5.0.3 mongodb-org-server=5.0.3 mongodb-org-shell=5.0.3 mongodb-org-mongos=5.0.3 mongodb-org-tools=5.0.3; \
+	apt-get clean; \
+	rm -rf /var/lib/apt/lists/*;

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.2.0</version>
+	<version>3.3.0</version>
 	<packaging>pom</packaging>
 
 	<name>Spring Data MongoDB</name>
@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.springframework.data.build</groupId>
 		<artifactId>spring-data-parent</artifactId>
-		<version>2.5.0</version>
+		<version>2.6.0</version>
 	</parent>
 
 	<modules>
@@ -26,8 +26,8 @@
 	<properties>
 		<project.type>multi</project.type>
 		<dist.id>spring-data-mongodb</dist.id>
-		<springdata.commons>2.5.0</springdata.commons>
-		<mongo>4.2.3</mongo>
+		<springdata.commons>2.6.0</springdata.commons>
+		<mongo>4.4.0</mongo>
 		<mongo.reactivestreams>${mongo}</mongo.reactivestreams>
 		<jmh.version>1.19</jmh.version>
 	</properties>
@@ -141,11 +141,11 @@
 			<id>sonatype-libs-snapshot</id>
 			<url>https://oss.sonatype.org/content/repositories/snapshots</url>
 			<releases>
-        <enabled>false</enabled>
-      </releases>
+				<enabled>false</enabled>
+			</releases>
 			<snapshots>
-        <enabled>true</enabled>
-      </snapshots>
+				<enabled>true</enabled>
+			</snapshots>
 		</repository>
 	</repositories>
 
@@ -158,11 +158,6 @@
 			<id>spring-libs-milestone</id>
 			<url>https://repo.spring.io/libs-milestone</url>
 		</pluginRepository>
-		<pluginRepository>
-			<id>bintray-plugins</id>
-			<name>bintray-plugins</name>
-			<url>https://jcenter.bintray.com</url>
-		</pluginRepository>
 	</pluginRepositories>
 
 </project>

settings.xml

@@ -0,0 +1,29 @@
+<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
+          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+          xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0
+                      https://maven.apache.org/xsd/settings-1.0.0.xsd">
+
+	<servers>
+		<server>
+			<id>spring-plugins-release</id>
+			<username>${env.ARTIFACTORY_USR}</username>
+			<password>${env.ARTIFACTORY_PSW}</password>
+		</server>
+		<server>
+			<id>spring-libs-snapshot</id>
+			<username>${env.ARTIFACTORY_USR}</username>
+			<password>${env.ARTIFACTORY_PSW}</password>
+		</server>
+		<server>
+			<id>spring-libs-milestone</id>
+			<username>${env.ARTIFACTORY_USR}</username>
+			<password>${env.ARTIFACTORY_PSW}</password>
+		</server>
+		<server>
+			<id>spring-libs-release</id>
+			<username>${env.ARTIFACTORY_USR}</username>
+			<password>${env.ARTIFACTORY_PSW}</password>
+		</server>
+	</servers>
+
+</settings>

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.2.0</version>
+		<version>3.3.0</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -14,7 +14,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.2.0</version>
+		<version>3.3.0</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -11,7 +11,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.2.0</version>
+		<version>3.3.0</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 
@@ -87,6 +87,13 @@
 			<optional>true</optional>
 		</dependency>
 
+		<dependency>
+			<groupId>com.google.code.findbugs</groupId>
+			<artifactId>jsr305</artifactId>
+			<version>3.0.2</version>
+			<optional>true</optional>
+		</dependency>
+
 		<!-- reactive -->
 
 		<dependency>
@@ -310,6 +317,15 @@
 			<scope>test</scope>
 		</dependency>
 
+		<!-- jMolecules -->
+
+		<dependency>
+			<groupId>org.jmolecules</groupId>
+			<artifactId>jmolecules-ddd</artifactId>
+			<version>${jmolecules}</version>
+			<scope>test</scope>
+		</dependency>
+
 	</dependencies>
 
 	<build>

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/BindableMongoExpression.java

@@ -31,7 +31,7 @@ import org.springframework.util.StringUtils;
  * expression. The expression will be wrapped within <code>{ ... }</code> if necessary. The actual parsing and parameter
  * binding of placeholders like {@code ?0} is delayed upon first call on the the target {@link Document} via
  * {@link #toDocument()}.
- * <p />
+ * <br />
  *
  * <pre class="code">
  * $toUpper : $name                -> { '$toUpper' : '$name' }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoCollectionUtils.java

@@ -20,8 +20,8 @@ import org.springframework.util.StringUtils;
 
 /**
  * Helper class featuring helper methods for working with MongoDb collections.
- * <p/>
- * <p/>
+ * <br />
+ * <br />
  * Mainly intended for internal use within the framework.
  *
  * @author Thomas Risberg

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoDatabaseUtils.java

@@ -30,7 +30,7 @@ import com.mongodb.client.MongoDatabase;
  * Helper class for managing a {@link MongoDatabase} instances via {@link MongoDatabaseFactory}. Used for obtaining
  * {@link ClientSession session bound} resources, such as {@link MongoDatabase} and
  * {@link com.mongodb.client.MongoCollection} suitable for transactional usage.
- * <p />
+ * <br />
  * <strong>Note:</strong> Intended for internal usage only.
  *
  * @author Christoph Strobl
@@ -43,7 +43,7 @@ public class MongoDatabaseUtils {
 	/**
 	 * Obtain the default {@link MongoDatabase database} form the given {@link MongoDatabaseFactory factory} using
 	 * {@link SessionSynchronization#ON_ACTUAL_TRANSACTION native session synchronization}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the current
 	 * {@link Thread} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -56,7 +56,7 @@ public class MongoDatabaseUtils {
 
 	/**
 	 * Obtain the default {@link MongoDatabase database} form the given {@link MongoDatabaseFactory factory}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the current
 	 * {@link Thread} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -71,7 +71,7 @@ public class MongoDatabaseUtils {
 	/**
 	 * Obtain the {@link MongoDatabase database} with given name form the given {@link MongoDatabaseFactory factory} using
 	 * {@link SessionSynchronization#ON_ACTUAL_TRANSACTION native session synchronization}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the current
 	 * {@link Thread} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -85,7 +85,7 @@ public class MongoDatabaseUtils {
 
 	/**
 	 * Obtain the {@link MongoDatabase database} with given name form the given {@link MongoDatabaseFactory factory}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the current
 	 * {@link Thread} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -104,7 +104,8 @@ public class MongoDatabaseUtils {
 
 		Assert.notNull(factory, "Factory must not be null!");
 
-		if (!TransactionSynchronizationManager.isSynchronizationActive()) {
+		if (sessionSynchronization == SessionSynchronization.NEVER
+				|| !TransactionSynchronizationManager.isSynchronizationActive()) {
 			return StringUtils.hasText(dbName) ? factory.getMongoDatabase(dbName) : factory.getMongoDatabase();
 		}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoExpression.java

@@ -18,7 +18,7 @@ package org.springframework.data.mongodb;
 /**
  * Wrapper object for MongoDB expressions like {@code $toUpper : $name} that manifest as {@link org.bson.Document} when
  * passed on to the driver.
- * <p />
+ * <br />
  * A set of predefined {@link MongoExpression expressions}, including a
  * {@link org.springframework.data.mongodb.core.aggregation.AggregationSpELExpression SpEL based variant} for method
  * like expressions (eg. {@code toUpper(name)}) are available via the

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoResourceHolder.java

@@ -24,7 +24,7 @@ import com.mongodb.client.ClientSession;
 /**
  * MongoDB specific {@link ResourceHolderSupport resource holder}, wrapping a {@link ClientSession}.
  * {@link MongoTransactionManager} binds instances of this class to the thread.
- * <p />
+ * <br />
  * <strong>Note:</strong> Intended for internal usage only.
  *
  * @author Christoph Strobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoTransactionManager.java

@@ -37,18 +37,18 @@ import com.mongodb.client.ClientSession;
 /**
  * A {@link org.springframework.transaction.PlatformTransactionManager} implementation that manages
  * {@link ClientSession} based transactions for a single {@link MongoDatabaseFactory}.
- * <p />
+ * <br />
  * Binds a {@link ClientSession} from the specified {@link MongoDatabaseFactory} to the thread.
- * <p />
+ * <br />
  * {@link TransactionDefinition#isReadOnly() Readonly} transactions operate on a {@link ClientSession} and enable causal
  * consistency, and also {@link ClientSession#startTransaction() start}, {@link ClientSession#commitTransaction()
  * commit} or {@link ClientSession#abortTransaction() abort} a transaction.
- * <p />
+ * <br />
  * Application code is required to retrieve the {@link com.mongodb.client.MongoDatabase} via
  * {@link MongoDatabaseUtils#getDatabase(MongoDatabaseFactory)} instead of a standard
  * {@link MongoDatabaseFactory#getMongoDatabase()} call. Spring classes such as
  * {@link org.springframework.data.mongodb.core.MongoTemplate} use this strategy implicitly.
- * <p />
+ * <br />
  * By default failure of a {@literal commit} operation raises a {@link TransactionSystemException}. One may override
  * {@link #doCommit(MongoTransactionObject)} to implement the
  * <a href="https://docs.mongodb.com/manual/core/transactions/#retry-commit-operation">Retry Commit Operation</a>
@@ -69,11 +69,11 @@ public class MongoTransactionManager extends AbstractPlatformTransactionManager
 
 	/**
 	 * Create a new {@link MongoTransactionManager} for bean-style usage.
-	 * <p />
+	 * <br />
 	 * <strong>Note:</strong>The {@link MongoDatabaseFactory db factory} has to be
 	 * {@link #setDbFactory(MongoDatabaseFactory) set} before using the instance. Use this constructor to prepare a
 	 * {@link MongoTransactionManager} via a {@link org.springframework.beans.factory.BeanFactory}.
-	 * <p />
+	 * <br />
 	 * Optionally it is possible to set default {@link TransactionOptions transaction options} defining
 	 * {@link com.mongodb.ReadConcern} and {@link com.mongodb.WriteConcern}.
 	 *
@@ -212,8 +212,8 @@ public class MongoTransactionManager extends AbstractPlatformTransactionManager
 	 * By default those labels are ignored, nevertheless one might check for
 	 * {@link MongoException#UNKNOWN_TRANSACTION_COMMIT_RESULT_LABEL transient commit errors labels} and retry the the
 	 * commit. <br />
+	 * <pre>
 	 * <code>
-	 *     <pre>
 	 * int retries = 3;
 	 * do {
 	 *     try {
@@ -226,8 +226,8 @@ public class MongoTransactionManager extends AbstractPlatformTransactionManager
 	 *     }
 	 *     Thread.sleep(500);
 	 * } while (--retries > 0);
-	 *     </pre>
 	 * </code>
+	 * </pre>
 	 *
 	 * @param transactionObject never {@literal null}.
 	 * @throws Exception in case of transaction errors.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoDatabaseUtils.java

@@ -36,7 +36,7 @@ import com.mongodb.reactivestreams.client.MongoDatabase;
  * Helper class for managing reactive {@link MongoDatabase} instances via {@link ReactiveMongoDatabaseFactory}. Used for
  * obtaining {@link ClientSession session bound} resources, such as {@link MongoDatabase} and {@link MongoCollection}
  * suitable for transactional usage.
- * <p />
+ * <br />
  * <strong>Note:</strong> Intended for internal usage only.
  *
  * @author Mark Paluch
@@ -75,7 +75,7 @@ public class ReactiveMongoDatabaseUtils {
 	/**
 	 * Obtain the default {@link MongoDatabase database} form the given {@link ReactiveMongoDatabaseFactory factory} using
 	 * {@link SessionSynchronization#ON_ACTUAL_TRANSACTION native session synchronization}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the subscriber
 	 * {@link Context} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -88,7 +88,7 @@ public class ReactiveMongoDatabaseUtils {
 
 	/**
 	 * Obtain the default {@link MongoDatabase database} form the given {@link ReactiveMongoDatabaseFactory factory}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the subscriber
 	 * {@link Context} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -104,7 +104,7 @@ public class ReactiveMongoDatabaseUtils {
 	/**
 	 * Obtain the {@link MongoDatabase database} with given name form the given {@link ReactiveMongoDatabaseFactory
 	 * factory} using {@link SessionSynchronization#ON_ACTUAL_TRANSACTION native session synchronization}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the subscriber
 	 * {@link Context} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -119,7 +119,7 @@ public class ReactiveMongoDatabaseUtils {
 	/**
 	 * Obtain the {@link MongoDatabase database} with given name form the given {@link ReactiveMongoDatabaseFactory
 	 * factory}.
-	 * <p />
+	 * <br />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the subscriber
 	 * {@link Context} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
 	 *
@@ -138,6 +138,10 @@ public class ReactiveMongoDatabaseUtils {
 
 		Assert.notNull(factory, "DatabaseFactory must not be null!");
 
+		if (sessionSynchronization == SessionSynchronization.NEVER) {
+			return getMongoDatabaseOrDefault(dbName, factory);
+		}
+
 		return TransactionSynchronizationManager.forCurrentTransaction()
 				.filter(TransactionSynchronizationManager::isSynchronizationActive) //
 				.flatMap(synchronizationManager -> {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoResourceHolder.java

@@ -24,7 +24,7 @@ import com.mongodb.reactivestreams.client.ClientSession;
 /**
  * MongoDB specific resource holder, wrapping a {@link ClientSession}. {@link ReactiveMongoTransactionManager} binds
  * instances of this class to the subscriber context.
- * <p />
+ * <br />
  * <strong>Note:</strong> Intended for internal usage only.
  *
  * @author Mark Paluch

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoTransactionManager.java

@@ -38,21 +38,21 @@ import com.mongodb.reactivestreams.client.ClientSession;
  * A {@link org.springframework.transaction.ReactiveTransactionManager} implementation that manages
  * {@link com.mongodb.reactivestreams.client.ClientSession} based transactions for a single
  * {@link org.springframework.data.mongodb.ReactiveMongoDatabaseFactory}.
- * <p />
+ * <br />
  * Binds a {@link ClientSession} from the specified
  * {@link org.springframework.data.mongodb.ReactiveMongoDatabaseFactory} to the subscriber
  * {@link reactor.util.context.Context}.
- * <p />
+ * <br />
  * {@link org.springframework.transaction.TransactionDefinition#isReadOnly() Readonly} transactions operate on a
  * {@link ClientSession} and enable causal consistency, and also {@link ClientSession#startTransaction() start},
  * {@link com.mongodb.reactivestreams.client.ClientSession#commitTransaction() commit} or
  * {@link ClientSession#abortTransaction() abort} a transaction.
- * <p />
+ * <br />
  * Application code is required to retrieve the {@link com.mongodb.reactivestreams.client.MongoDatabase} via
  * {@link org.springframework.data.mongodb.ReactiveMongoDatabaseUtils#getDatabase(ReactiveMongoDatabaseFactory)} instead
  * of a standard {@link org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getMongoDatabase()} call. Spring
  * classes such as {@link org.springframework.data.mongodb.core.ReactiveMongoTemplate} use this strategy implicitly.
- * <p />
+ * <br />
  * By default failure of a {@literal commit} operation raises a {@link TransactionSystemException}. You can override
  * {@link #doCommit(TransactionSynchronizationManager, ReactiveMongoTransactionObject)} to implement the
  * <a href="https://docs.mongodb.com/manual/core/transactions/#retry-commit-operation">Retry Commit Operation</a>
@@ -71,11 +71,11 @@ public class ReactiveMongoTransactionManager extends AbstractReactiveTransaction
 
 	/**
 	 * Create a new {@link ReactiveMongoTransactionManager} for bean-style usage.
-	 * <p />
+	 * <br />
 	 * <strong>Note:</strong>The {@link org.springframework.data.mongodb.ReactiveMongoDatabaseFactory db factory} has to
 	 * be {@link #setDatabaseFactory(ReactiveMongoDatabaseFactory)} set} before using the instance. Use this constructor
 	 * to prepare a {@link ReactiveMongoTransactionManager} via a {@link org.springframework.beans.factory.BeanFactory}.
-	 * <p />
+	 * <br />
 	 * Optionally it is possible to set default {@link TransactionOptions transaction options} defining
 	 * {@link com.mongodb.ReadConcern} and {@link com.mongodb.WriteConcern}.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/SessionAwareMethodInterceptor.java

@@ -35,7 +35,7 @@ import com.mongodb.session.ClientSession;
 /**
  * {@link MethodInterceptor} implementation looking up and invoking an alternative target method having
  * {@link ClientSession} as its first argument. This allows seamless integration with the existing code base.
- * <p />
+ * <br />
  * The {@link MethodInterceptor} is aware of methods on {@code MongoCollection} that my return new instances of itself
  * like (eg. {@link com.mongodb.reactivestreams.client.MongoCollection#withWriteConcern(WriteConcern)} and decorate them
  * if not already proxied.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/SessionSynchronization.java

@@ -15,13 +15,20 @@
  */
 package org.springframework.data.mongodb;
 
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.ReactiveMongoTemplate;
+
 /**
- * {@link SessionSynchronization} is used along with {@link org.springframework.data.mongodb.core.MongoTemplate} to
- * define in which type of transactions to participate if any.
+ * {@link SessionSynchronization} is used along with {@code MongoTemplate} to define in which type of transactions to
+ * participate if any.
  *
  * @author Christoph Strobl
  * @author Mark Paluch
  * @since 2.1
+ * @see MongoTemplate#setSessionSynchronization(SessionSynchronization)
+ * @see MongoDatabaseUtils#getDatabase(MongoDatabaseFactory, SessionSynchronization)
+ * @see ReactiveMongoTemplate#setSessionSynchronization(SessionSynchronization)
+ * @see ReactiveMongoDatabaseUtils#getDatabase(ReactiveMongoDatabaseFactory, SessionSynchronization)
  */
 public enum SessionSynchronization {
 
@@ -34,5 +41,12 @@ public enum SessionSynchronization {
 	/**
 	 * Synchronize with native MongoDB transactions initiated via {@link MongoTransactionManager}.
 	 */
-	ON_ACTUAL_TRANSACTION;
+	ON_ACTUAL_TRANSACTION,
+
+	/**
+	 * Do not participate in ongoing transactions.
+	 *
+	 * @since 3.2.5
+	 */
+	NEVER;
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/SpringDataMongoDB.java

@@ -48,7 +48,7 @@ public class SpringDataMongoDB {
 
 	/**
 	 * Fetches the "Implementation-Version" manifest attribute from the jar file.
-	 * <p />
+	 * <br />
 	 * Note that some ClassLoaders do not expose the package metadata, hence this class might not be able to determine the
 	 * version in all environments. In this case the current Major version is returned as a fallback.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/config/MongoConfigurationSupport.java

@@ -172,8 +172,7 @@ public abstract class MongoConfigurationSupport {
 
 	/**
 	 * Configures whether to abbreviate field names for domain objects by configuring a
-	 * {@link CamelCaseAbbreviatingFieldNamingStrategy} on the {@link MongoMappingContext} instance created. For advanced
-	 * customization needs, consider overriding {@link #mappingMongoConverter()}.
+	 * {@link CamelCaseAbbreviatingFieldNamingStrategy} on the {@link MongoMappingContext} instance created.
 	 *
 	 * @return
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/config/MongoParsingUtils.java

@@ -22,9 +22,12 @@ import java.util.Map;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.CustomEditorConfigurer;
 import org.springframework.beans.factory.support.BeanDefinitionBuilder;
+import org.springframework.beans.factory.support.BeanDefinitionValidationException;
 import org.springframework.beans.factory.support.ManagedMap;
 import org.springframework.beans.factory.xml.BeanDefinitionParser;
 import org.springframework.data.mongodb.core.MongoClientSettingsFactoryBean;
+import org.springframework.data.mongodb.core.MongoServerApiFactoryBean;
+import org.springframework.util.StringUtils;
 import org.springframework.util.xml.DomUtils;
 import org.w3c.dom.Element;
 
@@ -112,6 +115,20 @@ abstract class MongoParsingUtils {
 		// Field level encryption
 		setPropertyReference(clientOptionsDefBuilder, settingsElement, "encryption-settings-ref", "autoEncryptionSettings");
 
+		// ServerAPI
+		if (StringUtils.hasText(settingsElement.getAttribute("server-api-version"))) {
+
+			MongoServerApiFactoryBean serverApiFactoryBean = new MongoServerApiFactoryBean();
+			serverApiFactoryBean.setVersion(settingsElement.getAttribute("server-api-version"));
+			try {
+				clientOptionsDefBuilder.addPropertyValue("serverApi", serverApiFactoryBean.getObject());
+			} catch (Exception exception) {
+				throw new BeanDefinitionValidationException("Non parsable server-api.", exception);
+			}
+		} else {
+			setPropertyReference(clientOptionsDefBuilder, settingsElement, "server-api-ref", "serverApi");
+		}
+
 		// and the rest
 
 		mongoClientBuilder.addPropertyValue("mongoClientSettings", clientOptionsDefBuilder.getBeanDefinition());

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/config/PersistentEntitiesFactoryBean.java

@@ -28,7 +28,7 @@ import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
  * @author Christoph Strobl
  * @since 3.1
  */
-class PersistentEntitiesFactoryBean implements FactoryBean<PersistentEntities> {
+public class PersistentEntitiesFactoryBean implements FactoryBean<PersistentEntities> {
 
 	private final MappingMongoConverter converter;
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ChangeStreamOptions.java

@@ -242,13 +242,13 @@ public class ChangeStreamOptions {
 
 		/**
 		 * Set the filter to apply.
-		 * <p/>
+		 * <br />
 		 * Fields on aggregation expression root level are prefixed to map to fields contained in
 		 * {@link ChangeStreamDocument#getFullDocument() fullDocument}. However {@literal operationType}, {@literal ns},
 		 * {@literal documentKey} and {@literal fullDocument} are reserved words that will be omitted, and therefore taken
 		 * as given, during the mapping procedure. You may want to have a look at the
 		 * <a href="https://docs.mongodb.com/manual/reference/change-events/">structure of Change Events</a>.
-		 * <p/>
+		 * <br />
 		 * Use {@link org.springframework.data.mongodb.core.aggregation.TypedAggregation} to ensure filter expressions are
 		 * mapped to domain type fields.
 		 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/CollectionOptions.java

@@ -17,8 +17,11 @@ package org.springframework.data.mongodb.core;
 
 import java.util.Optional;
 
+import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.data.mongodb.core.schema.MongoJsonSchema;
+import org.springframework.data.mongodb.core.timeseries.Granularity;
+import org.springframework.data.mongodb.core.timeseries.GranularityDefinition;
 import org.springframework.data.mongodb.core.validation.Validator;
 import org.springframework.data.util.Optionals;
 import org.springframework.lang.Nullable;
@@ -42,6 +45,7 @@ public class CollectionOptions {
 	private @Nullable Boolean capped;
 	private @Nullable Collation collation;
 	private ValidationOptions validationOptions;
+	private @Nullable TimeSeriesOptions timeSeriesOptions;
 
 	/**
 	 * Constructs a new <code>CollectionOptions</code> instance.
@@ -54,17 +58,19 @@ public class CollectionOptions {
 	 */
 	@Deprecated
 	public CollectionOptions(@Nullable Long size, @Nullable Long maxDocuments, @Nullable Boolean capped) {
-		this(size, maxDocuments, capped, null, ValidationOptions.none());
+		this(size, maxDocuments, capped, null, ValidationOptions.none(), null);
 	}
 
 	private CollectionOptions(@Nullable Long size, @Nullable Long maxDocuments, @Nullable Boolean capped,
-			@Nullable Collation collation, ValidationOptions validationOptions) {
+			@Nullable Collation collation, ValidationOptions validationOptions,
+			@Nullable TimeSeriesOptions timeSeriesOptions) {
 
 		this.maxDocuments = maxDocuments;
 		this.size = size;
 		this.capped = capped;
 		this.collation = collation;
 		this.validationOptions = validationOptions;
+		this.timeSeriesOptions = timeSeriesOptions;
 	}
 
 	/**
@@ -78,7 +84,7 @@ public class CollectionOptions {
 
 		Assert.notNull(collation, "Collation must not be null!");
 
-		return new CollectionOptions(null, null, null, collation, ValidationOptions.none());
+		return new CollectionOptions(null, null, null, collation, ValidationOptions.none(), null);
 	}
 
 	/**
@@ -88,7 +94,21 @@ public class CollectionOptions {
 	 * @since 2.0
 	 */
 	public static CollectionOptions empty() {
-		return new CollectionOptions(null, null, null, null, ValidationOptions.none());
+		return new CollectionOptions(null, null, null, null, ValidationOptions.none(), null);
+	}
+
+	/**
+	 * Quick way to set up {@link CollectionOptions} for a Time Series collection. For more advanced settings use
+	 * {@link #timeSeries(TimeSeriesOptions)}.
+	 *
+	 * @param timeField The name of the property which contains the date in each time series document. Must not be
+	 *          {@literal null}.
+	 * @return new instance of {@link CollectionOptions}.
+	 * @see #timeSeries(TimeSeriesOptions)
+	 * @since 3.3
+	 */
+	public static CollectionOptions timeSeries(String timeField) {
+		return empty().timeSeries(TimeSeriesOptions.timeSeries(timeField));
 	}
 
 	/**
@@ -99,7 +119,7 @@ public class CollectionOptions {
 	 * @since 2.0
 	 */
 	public CollectionOptions capped() {
-		return new CollectionOptions(size, maxDocuments, true, collation, validationOptions);
+		return new CollectionOptions(size, maxDocuments, true, collation, validationOptions, null);
 	}
 
 	/**
@@ -110,7 +130,7 @@ public class CollectionOptions {
 	 * @since 2.0
 	 */
 	public CollectionOptions maxDocuments(long maxDocuments) {
-		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions);
+		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions, timeSeriesOptions);
 	}
 
 	/**
@@ -121,7 +141,7 @@ public class CollectionOptions {
 	 * @since 2.0
 	 */
 	public CollectionOptions size(long size) {
-		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions);
+		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions, timeSeriesOptions);
 	}
 
 	/**
@@ -132,7 +152,7 @@ public class CollectionOptions {
 	 * @since 2.0
 	 */
 	public CollectionOptions collation(@Nullable Collation collation) {
-		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions);
+		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions, timeSeriesOptions);
 	}
 
 	/**
@@ -252,7 +272,20 @@ public class CollectionOptions {
 	public CollectionOptions validation(ValidationOptions validationOptions) {
 
 		Assert.notNull(validationOptions, "ValidationOptions must not be null!");
-		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions);
+		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions, timeSeriesOptions);
+	}
+
+	/**
+	 * Create new {@link CollectionOptions} with the given {@link TimeSeriesOptions}.
+	 *
+	 * @param timeSeriesOptions must not be {@literal null}.
+	 * @return new instance of {@link CollectionOptions}.
+	 * @since 3.3
+	 */
+	public CollectionOptions timeSeries(TimeSeriesOptions timeSeriesOptions) {
+
+		Assert.notNull(timeSeriesOptions, "TimeSeriesOptions must not be null!");
+		return new CollectionOptions(size, maxDocuments, capped, collation, validationOptions, timeSeriesOptions);
 	}
 
 	/**
@@ -303,6 +336,16 @@ public class CollectionOptions {
 		return validationOptions.isEmpty() ? Optional.empty() : Optional.of(validationOptions);
 	}
 
+	/**
+	 * Get the {@link TimeSeriesOptions} if available.
+	 *
+	 * @return {@link Optional#empty()} if not specified.
+	 * @since 3.3
+	 */
+	public Optional<TimeSeriesOptions> getTimeSeriesOptions() {
+		return Optional.ofNullable(timeSeriesOptions);
+	}
+
 	/**
 	 * Encapsulation of ValidationOptions options.
 	 *
@@ -385,7 +428,7 @@ public class CollectionOptions {
 		/**
 		 * Get the {@code validationAction} to perform.
 		 *
-		 * @return @return {@link Optional#empty()} if not set.
+		 * @return {@link Optional#empty()} if not set.
 		 */
 		public Optional<ValidationAction> getValidationAction() {
 			return Optional.ofNullable(validationAction);
@@ -398,4 +441,89 @@ public class CollectionOptions {
 			return !Optionals.isAnyPresent(getValidator(), getValidationAction(), getValidationLevel());
 		}
 	}
+
+	/**
+	 * Options applicable to Time Series collections.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 * @see <a href=
+	 *      "https://docs.mongodb.com/manual/core/timeseries-collections">https://docs.mongodb.com/manual/core/timeseries-collections</a>
+	 */
+	public static class TimeSeriesOptions {
+
+		private final String timeField;
+
+		private @Nullable final String metaField;
+
+		private final GranularityDefinition granularity;
+
+		private TimeSeriesOptions(String timeField, @Nullable String metaField, GranularityDefinition granularity) {
+
+			Assert.hasText(timeField, "Time field must not be empty or null!");
+
+			this.timeField = timeField;
+			this.metaField = metaField;
+			this.granularity = granularity;
+		}
+
+		/**
+		 * Create a new instance of {@link TimeSeriesOptions} using the given field as its {@literal timeField}. The one,
+		 * that contains the date in each time series document. <br />
+		 * {@link Field#name() Annotated fieldnames} will be considered during the mapping process.
+		 *
+		 * @param timeField must not be {@literal null}.
+		 * @return new instance of {@link TimeSeriesOptions}.
+		 */
+		public static TimeSeriesOptions timeSeries(String timeField) {
+			return new TimeSeriesOptions(timeField, null, Granularity.DEFAULT);
+		}
+
+		/**
+		 * Set the name of the field which contains metadata in each time series document. Should not be the {@literal id}
+		 * nor {@link TimeSeriesOptions#timeSeries(String)} timeField} nor point to an {@literal array} or
+		 * {@link java.util.Collection}. <br />
+		 * {@link Field#name() Annotated fieldnames} will be considered during the mapping process.
+		 *
+		 * @param metaField must not be {@literal null}.
+		 * @return new instance of {@link TimeSeriesOptions}.
+		 */
+		public TimeSeriesOptions metaField(String metaField) {
+			return new TimeSeriesOptions(timeField, metaField, granularity);
+		}
+
+		/**
+		 * Select the {@link GranularityDefinition} parameter to define how data in the time series collection is organized.
+		 * Select one that is closest to the time span between incoming measurements.
+		 *
+		 * @return new instance of {@link TimeSeriesOptions}.
+		 * @see Granularity
+		 */
+		public TimeSeriesOptions granularity(GranularityDefinition granularity) {
+			return new TimeSeriesOptions(timeField, metaField, granularity);
+		}
+
+		/**
+		 * @return never {@literal null}.
+		 */
+		public String getTimeField() {
+			return timeField;
+		}
+
+		/**
+		 * @return can be {@literal null}. Might be an {@literal empty} {@link String} as well, so maybe check via
+		 *         {@link org.springframework.util.StringUtils#hasText(String)}.
+		 */
+		@Nullable
+		public String getMetaField() {
+			return metaField;
+		}
+
+		/**
+		 * @return never {@literal null}.
+		 */
+		public GranularityDefinition getGranularity() {
+			return granularity;
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/EncryptionAlgorithms.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+/**
+ * Encryption algorithms supported by MongoDB Client Side Field Level Encryption.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public final class EncryptionAlgorithms {
+
+	public static final String AEAD_AES_256_CBC_HMAC_SHA_512_Deterministic = "AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic";
+	public static final String AEAD_AES_256_CBC_HMAC_SHA_512_Random = "AEAD_AES_256_CBC_HMAC_SHA_512-Random";
+
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/EntityOperations.java

@@ -29,19 +29,23 @@ import org.springframework.data.mapping.PersistentEntity;
 import org.springframework.data.mapping.PersistentPropertyAccessor;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mapping.model.ConvertingPropertyAccessor;
+import org.springframework.data.mongodb.core.CollectionOptions.TimeSeriesOptions;
 import org.springframework.data.mongodb.core.convert.MongoWriter;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.data.mongodb.core.mapping.MongoSimpleTypes;
+import org.springframework.data.mongodb.core.mapping.TimeSeries;
 import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.timeseries.Granularity;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 import org.springframework.util.ObjectUtils;
+import org.springframework.util.StringUtils;
 
 /**
  * Common operations performed on an entity in the context of it's mapping metadata.
@@ -778,6 +782,24 @@ class EntityOperations {
 		 * @return
 		 */
 		Optional<Collation> getCollation(Query query);
+
+		/**
+		 * Derive the applicable {@link CollectionOptions} for the given type.
+		 *
+		 * @return never {@literal null}.
+		 * @since 3.3
+		 */
+		CollectionOptions getCollectionOptions();
+
+		/**
+		 * Map the fields of a given {@link TimeSeriesOptions} against the target domain type to consider potentially
+		 * annotated field names.
+		 *
+		 * @param options must not be {@literal null}.
+		 * @return never {@literal null}.
+		 * @since 3.3
+		 */
+		TimeSeriesOptions mapTimeSeriesOptions(TimeSeriesOptions options);
 	}
 
 	/**
@@ -817,6 +839,16 @@ class EntityOperations {
 
 			return query.getCollation();
 		}
+
+		@Override
+		public CollectionOptions getCollectionOptions() {
+			return CollectionOptions.empty();
+		}
+
+		@Override
+		public TimeSeriesOptions mapTimeSeriesOptions(TimeSeriesOptions options) {
+			return options;
+		}
 	}
 
 	/**
@@ -854,6 +886,58 @@ class EntityOperations {
 
 			return Optional.ofNullable(entity.getCollation());
 		}
+
+		@Override
+		public CollectionOptions getCollectionOptions() {
+
+			CollectionOptions collectionOptions = CollectionOptions.empty();
+			if (entity.hasCollation()) {
+				collectionOptions = collectionOptions.collation(entity.getCollation());
+			}
+
+			if (entity.isAnnotationPresent(TimeSeries.class)) {
+
+				TimeSeries timeSeries = entity.getRequiredAnnotation(TimeSeries.class);
+
+				if (entity.getPersistentProperty(timeSeries.timeField()) == null) {
+					throw new MappingException(String.format("Time series field '%s' does not exist in type %s",
+							timeSeries.timeField(), entity.getName()));
+				}
+
+				TimeSeriesOptions options = TimeSeriesOptions.timeSeries(timeSeries.timeField());
+				if (StringUtils.hasText(timeSeries.metaField())) {
+
+					if (entity.getPersistentProperty(timeSeries.metaField()) == null) {
+						throw new MappingException(
+								String.format("Meta field '%s' does not exist in type %s", timeSeries.metaField(), entity.getName()));
+					}
+
+					options = options.metaField(timeSeries.metaField());
+				}
+				if (!Granularity.DEFAULT.equals(timeSeries.granularity())) {
+					options = options.granularity(timeSeries.granularity());
+				}
+				collectionOptions = collectionOptions.timeSeries(options);
+			}
+
+			return collectionOptions;
+		}
+
+		@Override
+		public TimeSeriesOptions mapTimeSeriesOptions(TimeSeriesOptions source) {
+
+			TimeSeriesOptions target = TimeSeriesOptions.timeSeries(mappedNameOrDefault(source.getTimeField()));
+
+			if (StringUtils.hasText(source.getMetaField())) {
+				target = target.metaField(mappedNameOrDefault(source.getMetaField()));
+			}
+			return target.granularity(source.getGranularity());
+		}
+
+		private String mappedNameOrDefault(String name) {
+			MongoPersistentProperty persistentProperty = entity.getPersistentProperty(name);
+			return persistentProperty != null ? persistentProperty.getFieldName() : name;
+		}
 	}
 
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableFindOperation.java

@@ -125,7 +125,7 @@ public interface ExecutableFindOperation {
 
 		/**
 		 * Get the number of matching elements.
-		 * <p />
+		 * <br />
 		 * This method uses an {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions) aggregation
 		 * execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees shard,
 		 * session and transaction compliance. In case an inaccurate count satisfies the applications needs use

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableUpdateOperation.java

@@ -89,7 +89,7 @@ public interface ExecutableUpdateOperation {
 
 	/**
 	 * Trigger
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * execution by calling one of the terminating methods.
 	 *
 	 * @author Mark Paluch

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/FindAndReplaceOptions.java

@@ -17,7 +17,7 @@ package org.springframework.data.mongodb.core;
 
 /**
  * Options for
- * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>.
+ * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>.
  * <br />
  * Defaults to
  * <dl>

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/IndexConverters.java

@@ -115,6 +115,10 @@ abstract class IndexConverters {
 				ops = ops.collation(fromDocument(indexOptions.get("collation", Document.class)));
 			}
 
+			if (indexOptions.containsKey("wildcardProjection")) {
+				ops.wildcardProjection(indexOptions.get("wildcardProjection", Document.class));
+			}
+
 			return ops;
 		};
 	}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MappedDocument.java

@@ -156,5 +156,14 @@ public class MappedDocument {
 		public List<ArrayFilter> getArrayFilters() {
 			return delegate.getArrayFilters();
 		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.query.UpdateDefinition#hasArrayFilters()
+		 */
+		@Override
+		public boolean hasArrayFilters() {
+			return delegate.hasArrayFilters();
+		}
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MappingMongoJsonSchemaCreator.java

@@ -20,13 +20,20 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.List;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+import org.bson.Document;
 
 import org.springframework.data.mapping.PersistentProperty;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
+import org.springframework.data.mongodb.core.mapping.Encrypted;
 import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+import org.springframework.data.mongodb.core.schema.IdentifiableJsonSchemaProperty.ArrayJsonSchemaProperty;
+import org.springframework.data.mongodb.core.schema.IdentifiableJsonSchemaProperty.EncryptedJsonSchemaProperty;
 import org.springframework.data.mongodb.core.schema.IdentifiableJsonSchemaProperty.ObjectJsonSchemaProperty;
 import org.springframework.data.mongodb.core.schema.JsonSchemaObject;
 import org.springframework.data.mongodb.core.schema.JsonSchemaObject.Type;
@@ -34,10 +41,12 @@ import org.springframework.data.mongodb.core.schema.JsonSchemaProperty;
 import org.springframework.data.mongodb.core.schema.MongoJsonSchema;
 import org.springframework.data.mongodb.core.schema.MongoJsonSchema.MongoJsonSchemaBuilder;
 import org.springframework.data.mongodb.core.schema.TypedJsonSchemaObject;
+import org.springframework.data.util.ClassTypeInformation;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.ObjectUtils;
+import org.springframework.util.StringUtils;
 
 /**
  * {@link MongoJsonSchemaCreator} implementation using both {@link MongoConverter} and {@link MappingContext} to obtain
@@ -52,6 +61,7 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 
 	private final MongoConverter converter;
 	private final MappingContext<MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
+	private final Predicate<JsonSchemaPropertyContext> filter;
 
 	/**
 	 * Create a new instance of {@link MappingMongoJsonSchemaCreator}.
@@ -61,10 +71,24 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 	@SuppressWarnings("unchecked")
 	MappingMongoJsonSchemaCreator(MongoConverter converter) {
 
+		this(converter, (MappingContext<MongoPersistentEntity<?>, MongoPersistentProperty>) converter.getMappingContext(),
+				(property) -> true);
+	}
+
+	@SuppressWarnings("unchecked")
+	MappingMongoJsonSchemaCreator(MongoConverter converter,
+			MappingContext<MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext,
+			Predicate<JsonSchemaPropertyContext> filter) {
+
 		Assert.notNull(converter, "Converter must not be null!");
 		this.converter = converter;
-		this.mappingContext = (MappingContext<MongoPersistentEntity<?>, MongoPersistentProperty>) converter
-				.getMappingContext();
+		this.mappingContext = mappingContext;
+		this.filter = filter;
+	}
+
+	@Override
+	public MongoJsonSchemaCreator filter(Predicate<JsonSchemaPropertyContext> filter) {
+		return new MappingMongoJsonSchemaCreator(converter, mappingContext, filter);
 	}
 
 	/*
@@ -77,11 +101,29 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 		MongoPersistentEntity<?> entity = mappingContext.getRequiredPersistentEntity(type);
 		MongoJsonSchemaBuilder schemaBuilder = MongoJsonSchema.builder();
 
+		{
+			Encrypted encrypted = entity.findAnnotation(Encrypted.class);
+			if (encrypted != null) {
+
+				Document encryptionMetadata = new Document();
+
+				Collection<Object> encryptionKeyIds = entity.getEncryptionKeyIds();
+				if (!CollectionUtils.isEmpty(encryptionKeyIds)) {
+					encryptionMetadata.append("keyId", encryptionKeyIds);
+				}
+
+				if (StringUtils.hasText(encrypted.algorithm())) {
+					encryptionMetadata.append("algorithm", encrypted.algorithm());
+				}
+
+				schemaBuilder.encryptionMetadata(encryptionMetadata);
+			}
+		}
+
 		List<JsonSchemaProperty> schemaProperties = computePropertiesForEntity(Collections.emptyList(), entity);
 		schemaBuilder.properties(schemaProperties.toArray(new JsonSchemaProperty[0]));
 
 		return schemaBuilder.build();
-
 	}
 
 	private List<JsonSchemaProperty> computePropertiesForEntity(List<MongoPersistentProperty> path,
@@ -93,6 +135,11 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 
 			List<MongoPersistentProperty> currentPath = new ArrayList<>(path);
 
+			if (!filter.test(new PropertyContext(
+					currentPath.stream().map(PersistentProperty::getName).collect(Collectors.joining(".")), nested))) {
+				continue;
+			}
+
 			if (path.contains(nested)) { // cycle guard
 				schemaProperties.add(createSchemaProperty(computePropertyFieldName(CollectionUtils.lastElement(currentPath)),
 						Object.class, false));
@@ -114,21 +161,88 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 		Class<?> rawTargetType = computeTargetType(property); // target type before conversion
 		Class<?> targetType = converter.getTypeMapper().getWriteTargetTypeFor(rawTargetType); // conversion target type
 
-		if (property.isEntity() && ObjectUtils.nullSafeEquals(rawTargetType, targetType)) {
+		if (!isCollection(property) && property.isEntity() && ObjectUtils.nullSafeEquals(rawTargetType, targetType)) {
 			return createObjectSchemaPropertyForEntity(path, property, required);
 		}
 
 		String fieldName = computePropertyFieldName(property);
 
-		if (property.isCollectionLike()) {
-			return createSchemaProperty(fieldName, targetType, required);
+		JsonSchemaProperty schemaProperty;
+		if (isCollection(property)) {
+			schemaProperty = createArraySchemaProperty(fieldName, property, required);
 		} else if (property.isMap()) {
-			return createSchemaProperty(fieldName, Type.objectType(), required);
+			schemaProperty = createSchemaProperty(fieldName, Type.objectType(), required);
 		} else if (ClassUtils.isAssignable(Enum.class, targetType)) {
-			return createEnumSchemaProperty(fieldName, targetType, required);
+			schemaProperty = createEnumSchemaProperty(fieldName, targetType, required);
+		} else {
+			schemaProperty = createSchemaProperty(fieldName, targetType, required);
 		}
 
-		return createSchemaProperty(fieldName, targetType, required);
+		return applyEncryptionDataIfNecessary(property, schemaProperty);
+	}
+
+	private JsonSchemaProperty createArraySchemaProperty(String fieldName, MongoPersistentProperty property,
+			boolean required) {
+
+		ArrayJsonSchemaProperty schemaProperty = JsonSchemaProperty.array(fieldName);
+
+		if (isSpecificType(property)) {
+			schemaProperty = potentiallyEnhanceArraySchemaProperty(property, schemaProperty);
+		}
+
+		return createPotentiallyRequiredSchemaProperty(schemaProperty, required);
+	}
+
+	@SuppressWarnings({ "unchecked", "rawtypes" })
+	private ArrayJsonSchemaProperty potentiallyEnhanceArraySchemaProperty(MongoPersistentProperty property,
+			ArrayJsonSchemaProperty schemaProperty) {
+
+		MongoPersistentEntity<?> persistentEntity = mappingContext
+				.getPersistentEntity(property.getTypeInformation().getRequiredComponentType());
+
+		if (persistentEntity != null) {
+
+			List<JsonSchemaProperty> nestedProperties = computePropertiesForEntity(Collections.emptyList(), persistentEntity);
+
+			if (nestedProperties.isEmpty()) {
+				return schemaProperty;
+			}
+
+			return schemaProperty
+					.items(JsonSchemaObject.object().properties(nestedProperties.toArray(new JsonSchemaProperty[0])));
+		}
+
+		if (ClassUtils.isAssignable(Enum.class, property.getActualType())) {
+
+			List<Object> possibleValues = getPossibleEnumValues((Class<Enum>) property.getActualType());
+
+			return schemaProperty
+					.items(createSchemaObject(computeTargetType(property.getActualType(), possibleValues), possibleValues));
+		}
+
+		return schemaProperty.items(JsonSchemaObject.of(property.getActualType()));
+	}
+
+	private boolean isSpecificType(MongoPersistentProperty property) {
+		return !ClassTypeInformation.OBJECT.equals(property.getTypeInformation().getActualType());
+	}
+
+	private JsonSchemaProperty applyEncryptionDataIfNecessary(MongoPersistentProperty property,
+			JsonSchemaProperty schemaProperty) {
+
+		Encrypted encrypted = property.findAnnotation(Encrypted.class);
+		if (encrypted == null) {
+			return schemaProperty;
+		}
+
+		EncryptedJsonSchemaProperty enc = new EncryptedJsonSchemaProperty(schemaProperty);
+		if (StringUtils.hasText(encrypted.algorithm())) {
+			enc = enc.algorithm(encrypted.algorithm());
+		}
+		if (!ObjectUtils.isEmpty(encrypted.keyId())) {
+			enc = enc.keys(property.getEncryptionKeyIds());
+		}
+		return enc;
 	}
 
 	private JsonSchemaProperty createObjectSchemaPropertyForEntity(List<MongoPersistentProperty> path,
@@ -142,15 +256,12 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 				target.properties(nestedProperties.toArray(new JsonSchemaProperty[0])), required);
 	}
 
+	@SuppressWarnings({ "unchecked", "rawtypes" })
 	private JsonSchemaProperty createEnumSchemaProperty(String fieldName, Class<?> targetType, boolean required) {
 
-		List<Object> possibleValues = new ArrayList<>();
+		List<Object> possibleValues = getPossibleEnumValues((Class<Enum>) targetType);
 
-		for (Object enumValue : EnumSet.allOf((Class) targetType)) {
-			possibleValues.add(converter.convertToMongoType(enumValue));
-		}
-
-		targetType = possibleValues.isEmpty() ? targetType : possibleValues.iterator().next().getClass();
+		targetType = computeTargetType(targetType, possibleValues);
 		return createSchemaProperty(fieldName, targetType, required, possibleValues);
 	}
 
@@ -161,14 +272,20 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 	JsonSchemaProperty createSchemaProperty(String fieldName, Object type, boolean required,
 			Collection<?> possibleValues) {
 
+		TypedJsonSchemaObject schemaObject = createSchemaObject(type, possibleValues);
+
+		return createPotentiallyRequiredSchemaProperty(JsonSchemaProperty.named(fieldName).with(schemaObject), required);
+	}
+
+	private TypedJsonSchemaObject createSchemaObject(Object type, Collection<?> possibleValues) {
+
 		TypedJsonSchemaObject schemaObject = type instanceof Type ? JsonSchemaObject.of(Type.class.cast(type))
 				: JsonSchemaObject.of(Class.class.cast(type));
 
 		if (!CollectionUtils.isEmpty(possibleValues)) {
 			schemaObject = schemaObject.possibleValues(possibleValues);
 		}
-
-		return createPotentiallyRequiredSchemaProperty(JsonSchemaProperty.named(fieldName).with(schemaObject), required);
+		return schemaObject;
 	}
 
 	private String computePropertyFieldName(PersistentProperty property) {
@@ -199,12 +316,53 @@ class MappingMongoJsonSchemaCreator implements MongoJsonSchemaCreator {
 		return mongoProperty.getFieldType() != mongoProperty.getActualType() ? Object.class : mongoProperty.getFieldType();
 	}
 
-	static JsonSchemaProperty createPotentiallyRequiredSchemaProperty(JsonSchemaProperty property, boolean required) {
+	private static Class<?> computeTargetType(Class<?> fallback, List<Object> possibleValues) {
+		return possibleValues.isEmpty() ? fallback : possibleValues.iterator().next().getClass();
+	}
 
-		if (!required) {
+	private <E extends Enum<E>> List<Object> getPossibleEnumValues(Class<E> targetType) {
+
+		EnumSet<E> enumSet = EnumSet.allOf(targetType);
+		List<Object> possibleValues = new ArrayList<>(enumSet.size());
+
+		for (Object enumValue : enumSet) {
+			possibleValues.add(converter.convertToMongoType(enumValue));
+		}
+
+		return possibleValues;
+	}
+
+	private static boolean isCollection(MongoPersistentProperty property) {
+		return property.isCollectionLike() && !property.getType().equals(byte[].class);
+	}
+
+	static JsonSchemaProperty createPotentiallyRequiredSchemaProperty(JsonSchemaProperty property, boolean required) {
+		return required ? JsonSchemaProperty.required(property) : property;
+	}
+
+	class PropertyContext implements JsonSchemaPropertyContext {
+
+		private final String path;
+		private final MongoPersistentProperty property;
+
+		public PropertyContext(String path, MongoPersistentProperty property) {
+			this.path = path;
+			this.property = property;
+		}
+
+		@Override
+		public String getPath() {
+			return path;
+		}
+
+		@Override
+		public MongoPersistentProperty getProperty() {
 			return property;
 		}
 
-		return JsonSchemaProperty.required(property);
+		@Override
+		public <T> MongoPersistentEntity<T> resolveEntity(MongoPersistentProperty property) {
+			return (MongoPersistentEntity<T>) mappingContext.getPersistentEntity(property);
+		}
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoClientSettingsFactoryBean.java

@@ -36,6 +36,7 @@ import com.mongodb.MongoClientSettings.Builder;
 import com.mongodb.ReadConcern;
 import com.mongodb.ReadPreference;
 import com.mongodb.ServerAddress;
+import com.mongodb.ServerApi;
 import com.mongodb.WriteConcern;
 import com.mongodb.connection.ClusterConnectionMode;
 import com.mongodb.connection.ClusterType;
@@ -113,6 +114,7 @@ public class MongoClientSettingsFactoryBean extends AbstractFactoryBean<MongoCli
 	// encryption and retry
 
 	private @Nullable AutoEncryptionSettings autoEncryptionSettings;
+	private @Nullable ServerApi serverApi;
 
 	/**
 	 * @param socketConnectTimeoutMS in msec
@@ -395,6 +397,15 @@ public class MongoClientSettingsFactoryBean extends AbstractFactoryBean<MongoCli
 		this.autoEncryptionSettings = autoEncryptionSettings;
 	}
 
+	/**
+	 * @param serverApi can be {@literal null}.
+	 * @see MongoClientSettings.Builder#serverApi(ServerApi)
+	 * @since 3.3
+	 */
+	public void setServerApi(@Nullable ServerApi serverApi) {
+		this.serverApi = serverApi;
+	}
+
 	@Override
 	public Class<?> getObjectType() {
 		return MongoClientSettings.class;
@@ -476,9 +487,11 @@ public class MongoClientSettingsFactoryBean extends AbstractFactoryBean<MongoCli
 		if (retryWrites != null) {
 			builder = builder.retryWrites(retryWrites);
 		}
-
 		if (uUidRepresentation != null) {
-			builder.uuidRepresentation(uUidRepresentation);
+			builder = builder.uuidRepresentation(uUidRepresentation);
+		}
+		if (serverApi != null) {
+			builder = builder.serverApi(serverApi);
 		}
 
 		return builder.build();

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoDatabaseFactorySupport.java

@@ -33,7 +33,7 @@ import com.mongodb.client.MongoDatabase;
 /**
  * Common base class for usage with both {@link com.mongodb.client.MongoClients} defining common properties such as
  * database name and exception translator.
- * <p/>
+ * <br />
  * Not intended to be used directly.
  *
  * @author Christoph Strobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoDbFactorySupport.java

@@ -20,7 +20,7 @@ import org.springframework.dao.support.PersistenceExceptionTranslator;
 /**
  * Common base class for usage with both {@link com.mongodb.client.MongoClients} defining common properties such as
  * database name and exception translator.
- * <p/>
+ * <br />
  * Not intended to be used directly.
  *
  * @author Christoph Strobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoJsonSchemaCreator.java

@@ -15,7 +15,23 @@
  */
 package org.springframework.data.mongodb.core;
 
+import java.util.HashSet;
+import java.util.Set;
+import java.util.function.Predicate;
+
+import org.springframework.data.mapping.PersistentProperty;
+import org.springframework.data.mapping.context.MappingContext;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
+import org.springframework.data.mongodb.core.convert.MongoCustomConversions;
+import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
+import org.springframework.data.mongodb.core.mapping.Encrypted;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
+import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
+import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+import org.springframework.data.mongodb.core.mapping.MongoSimpleTypes;
+import org.springframework.data.mongodb.core.mapping.Unwrapped.Nullable;
+import org.springframework.data.mongodb.core.schema.JsonSchemaProperty;
 import org.springframework.data.mongodb.core.schema.MongoJsonSchema;
 import org.springframework.util.Assert;
 
@@ -24,6 +40,7 @@ import org.springframework.util.Assert;
  * following mapping rules.
  * <p>
  * <strong>Required Properties</strong>
+ * </p>
  * <ul>
  * <li>Properties of primitive type</li>
  * </ul>
@@ -45,7 +62,8 @@ import org.springframework.util.Assert;
  * {@link org.springframework.data.annotation.Id _id} properties using types that can be converted into
  * {@link org.bson.types.ObjectId} like {@link String} will be mapped to {@code type : 'object'} unless there is more
  * specific information available via the {@link org.springframework.data.mongodb.core.mapping.MongoId} annotation.
- * </p>
+
+ * {@link Encrypted} properties will contain {@literal encrypt} information.
  *
  * @author Christoph Strobl
  * @since 2.2
@@ -60,6 +78,88 @@ public interface MongoJsonSchemaCreator {
 	 */
 	MongoJsonSchema createSchemaFor(Class<?> type);
 
+	/**
+	 * Filter matching {@link JsonSchemaProperty properties}.
+	 *
+	 * @param filter the {@link Predicate} to evaluate for inclusion. Must not be {@literal null}.
+	 * @return new instance of {@link MongoJsonSchemaCreator}.
+	 * @since 3.3
+	 */
+	MongoJsonSchemaCreator filter(Predicate<JsonSchemaPropertyContext> filter);
+
+	/**
+	 * The context in which a specific {@link #getProperty()} is encountered during schema creation.
+	 * 
+	 * @since 3.3
+	 */
+	interface JsonSchemaPropertyContext {
+
+		/**
+		 * The path to a given field/property in dot notation.
+		 * 
+		 * @return never {@literal null}.
+		 */
+		String getPath();
+
+		/**
+		 * The current property.
+		 * 
+		 * @return never {@literal null}.
+		 */
+		MongoPersistentProperty getProperty();
+
+		/**
+		 * Obtain the {@link MongoPersistentEntity} for a given property.
+		 * 
+		 * @param property must not be {@literal null}.
+		 * @param <T>
+		 * @return {@literal null} if the property is not an entity. It is nevertheless recommend to check
+		 *         {@link PersistentProperty#isEntity()} first.
+		 */
+		@Nullable
+		<T> MongoPersistentEntity<T> resolveEntity(MongoPersistentProperty property);
+
+	}
+
+	/**
+	 * A filter {@link Predicate} that matches {@link Encrypted encrypted properties} and those having nested ones.
+	 *
+	 * @return new instance of {@link Predicate}.
+	 * @since 3.3
+	 */
+	static Predicate<JsonSchemaPropertyContext> encryptedOnly() {
+
+		return new Predicate<JsonSchemaPropertyContext>() {
+
+			// cycle guard
+			private final Set<MongoPersistentProperty> seen = new HashSet<>();
+
+			@Override
+			public boolean test(JsonSchemaPropertyContext context) {
+				return extracted(context.getProperty(), context);
+			}
+
+			private boolean extracted(MongoPersistentProperty property, JsonSchemaPropertyContext context) {
+				if (property.isAnnotationPresent(Encrypted.class)) {
+					return true;
+				}
+
+				if (!property.isEntity() || seen.contains(property)) {
+					return false;
+				}
+
+				seen.add(property);
+
+				for (MongoPersistentProperty nested : context.resolveEntity(property)) {
+					if (extracted(nested, context)) {
+						return true;
+					}
+				}
+				return false;
+			}
+		};
+	}
+
 	/**
 	 * Creates a new {@link MongoJsonSchemaCreator} that is aware of conversions applied by the given
 	 * {@link MongoConverter}.
@@ -72,4 +172,41 @@ public interface MongoJsonSchemaCreator {
 		Assert.notNull(mongoConverter, "MongoConverter must not be null!");
 		return new MappingMongoJsonSchemaCreator(mongoConverter);
 	}
+
+	/**
+	 * Creates a new {@link MongoJsonSchemaCreator} that is aware of type mappings and potential
+	 * {@link org.springframework.data.spel.spi.EvaluationContextExtension extensions}.
+	 *
+	 * @param mappingContext must not be {@literal null}.
+	 * @return new instance of {@link MongoJsonSchemaCreator}.
+	 * @since 3.3
+	 */
+	static MongoJsonSchemaCreator create(MappingContext mappingContext) {
+
+		MappingMongoConverter converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+		converter.setCustomConversions(MongoCustomConversions.create(config -> {}));
+		converter.afterPropertiesSet();
+
+		return create(converter);
+	}
+
+	/**
+	 * Creates a new {@link MongoJsonSchemaCreator} that does not consider potential extensions - suitable for testing. We
+	 * recommend to use {@link #create(MappingContext)}.
+	 *
+	 * @return new instance of {@link MongoJsonSchemaCreator}.
+	 * @since 3.3
+	 */
+	static MongoJsonSchemaCreator create() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		mappingContext.setSimpleTypeHolder(MongoSimpleTypes.HOLDER);
+		mappingContext.afterPropertiesSet();
+
+		MappingMongoConverter converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+		converter.setCustomConversions(MongoCustomConversions.create(config -> {}));
+		converter.afterPropertiesSet();
+
+		return create(converter);
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoOperations.java

@@ -58,7 +58,7 @@ import com.mongodb.client.result.UpdateResult;
  * Interface that specifies a basic set of MongoDB operations. Implemented by {@link MongoTemplate}. Not often used but
  * a useful option for extensibility and testability (as it can be easily mocked, stubbed, or be the target of a JDK
  * proxy).
- * <p/>
+ * <br />
  * <strong>NOTE:</strong> Some operations cannot be executed within a MongoDB transaction. Please refer to the MongoDB
  * specific documentation to learn more about <a href="https://docs.mongodb.com/manual/core/transactions/">Multi
  * Document Transactions</a>.
@@ -125,7 +125,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Executes a {@link DbCallback} translating any exceptions as necessary.
-	 * <p/>
+	 * <br />
 	 * Allows for returning a result object, that is a domain object or a collection of domain objects.
 	 *
 	 * @param action callback object that specifies the MongoDB actions to perform on the passed in DB instance. Must not
@@ -138,7 +138,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Executes the given {@link CollectionCallback} on the entity collection of the specified class.
-	 * <p/>
+	 * <br />
 	 * Allows for returning a result object, that is a domain object or a collection of domain objects.
 	 *
 	 * @param entityClass class that determines the collection to use. Must not be {@literal null}.
@@ -151,7 +151,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Executes the given {@link CollectionCallback} on the collection of the given name.
-	 * <p/>
+	 * <br />
 	 * Allows for returning a result object, that is a domain object or a collection of domain objects.
 	 *
 	 * @param collectionName the name of the collection that specifies which {@link MongoCollection} instance will be
@@ -176,7 +176,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Obtain a {@link ClientSession session} bound instance of {@link SessionScoped} binding the {@link ClientSession}
 	 * provided by the given {@link Supplier} to each and every command issued against MongoDB.
-	 * <p/>
+	 * <br />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle. Use the
 	 * {@link SessionScoped#execute(SessionCallback, Consumer)} hook to potentially close the {@link ClientSession}.
 	 *
@@ -212,7 +212,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Obtain a {@link ClientSession} bound instance of {@link MongoOperations}.
-	 * <p/>
+	 * <br />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle.
 	 *
 	 * @param session must not be {@literal null}.
@@ -300,7 +300,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * is created on first interaction with the server. Collections can be explicitly created via
 	 * {@link #createCollection(Class)}. Please make sure to check if the collection {@link #collectionExists(Class)
 	 * exists} first.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param collectionName name of the collection. Must not be {@literal null}.
@@ -310,7 +310,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Check to see if a collection with a name indicated by the entity class exists.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param entityClass class that determines the name of the collection. Must not be {@literal null}.
@@ -320,7 +320,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Check to see if a collection with a given name exists.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param collectionName name of the collection. Must not be {@literal null}.
@@ -330,7 +330,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Drop the collection with the name indicated by the entity class.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param entityClass class that determines the collection to drop/delete. Must not be {@literal null}.
@@ -339,7 +339,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Drop the collection with the given name.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param collectionName name of the collection to drop/delete.
@@ -403,10 +403,10 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Query for a list of objects of type T from the collection used by the entity class.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your collection does not contain a homogeneous collection of types, this operation will not be an efficient way
 	 * to map objects since the test for class type is done in the client and not on the server.
 	 *
@@ -417,10 +417,10 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Query for a list of objects of type T from the specified collection.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your collection does not contain a homogeneous collection of types, this operation will not be an efficient way
 	 * to map objects since the test for class type is done in the client and not on the server.
 	 *
@@ -539,11 +539,11 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Execute an aggregation operation backed by a Mongo DB {@link com.mongodb.client.AggregateIterable}.
-	 * <p/>
+	 * <br />
 	 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.AggregateIterable} that
 	 * needs to be closed. The raw results will be mapped to the given entity class and are returned as stream. The name
 	 * of the inputCollection is derived from the inputType of the aggregation.
-	 * <p/>
+	 * <br />
 	 * Aggregation streaming can't be used with {@link AggregationOptions#isExplain() aggregation explain}. Enabling
 	 * explanation mode will throw an {@link IllegalArgumentException}.
 	 *
@@ -557,10 +557,10 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Execute an aggregation operation backed by a Mongo DB {@link com.mongodb.client.AggregateIterable}.
-	 * <p/>
+	 * <br />
 	 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.AggregateIterable} that
 	 * needs to be closed. The raw results will be mapped to the given entity class.
-	 * <p/>
+	 * <br />
 	 * Aggregation streaming can't be used with {@link AggregationOptions#isExplain() aggregation explain}. Enabling
 	 * explanation mode will throw an {@link IllegalArgumentException}.
 	 *
@@ -576,10 +576,10 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Execute an aggregation operation backed by a Mongo DB {@link com.mongodb.client.AggregateIterable}.
-	 * <p/>
+	 * <br />
 	 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.AggregateIterable} that
 	 * needs to be closed. The raw results will be mapped to the given entity class.
-	 * <p/>
+	 * <br />
 	 * Aggregation streaming can't be used with {@link AggregationOptions#isExplain() aggregation explain}. Enabling
 	 * explanation mode will throw an {@link IllegalArgumentException}.
 	 *
@@ -702,10 +702,10 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Map the results of an ad-hoc query on the collection for the entity class to a single instance of an object of the
 	 * specified type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -720,10 +720,10 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Map the results of an ad-hoc query on the specified collection to a single instance of an object of the specified
 	 * type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -768,10 +768,10 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Map the results of an ad-hoc query on the collection for the entity class to a List of the specified type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -784,10 +784,10 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Map the results of an ad-hoc query on the specified collection to a List of the specified type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -881,7 +881,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	}
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify </a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
@@ -897,7 +897,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	<T> T findAndModify(Query query, UpdateDefinition update, Class<T> entityClass);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify </a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
@@ -914,7 +914,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	<T> T findAndModify(Query query, UpdateDefinition update, Class<T> entityClass, String collectionName);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify </a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
 	 * {@link FindAndModifyOptions} into account.
 	 *
@@ -934,7 +934,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	<T> T findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify </a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
 	 * {@link FindAndModifyOptions} into account.
 	 *
@@ -957,7 +957,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement}
 	 * document. <br />
 	 * The collection name is derived from the {@literal replacement} type. <br />
@@ -977,7 +977,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement}
 	 * document.<br />
 	 * Options are defaulted to {@link FindAndReplaceOptions#empty()}. <br />
@@ -997,7 +997,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account.<br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -1018,7 +1018,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account.<br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -1041,7 +1041,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account.<br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -1066,7 +1066,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account.<br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -1094,7 +1094,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account.<br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -1120,9 +1120,9 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * Map the results of an ad-hoc query on the collection for the entity type to a single instance of an object of the
 	 * specified type. The first document that matches the query is returned and also removed from the collection in the
 	 * database.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -1137,10 +1137,10 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Map the results of an ad-hoc query on the specified collection to a single instance of an object of the specified
 	 * type. The first document that matches the query is returned and also removed from the collection in the database.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -1160,7 +1160,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
-	 * <p />
+	 * <br />
 	 * This method uses an
 	 * {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
@@ -1182,7 +1182,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
-	 * <p />
+	 * <br />
 	 * This method uses an
 	 * {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
@@ -1199,7 +1199,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Estimate the number of documents, in the collection {@link #getCollectionName(Class) identified by the given type},
 	 * based on collection statistics.
-	 * <p />
+	 * <br />
 	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
 	 * transactions.
 	 *
@@ -1215,7 +1215,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Estimate the number of documents in the given collection based on collection statistics.
-	 * <p />
+	 * <br />
 	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
 	 * transactions.
 	 *
@@ -1232,7 +1232,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
-	 * <p />
+	 * <br />
 	 * This method uses an
 	 * {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
@@ -1249,17 +1249,17 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Insert the object into the collection for the entity type of the object to save.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
 	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See
 	 * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation" > Spring's
 	 * Type Conversion"</a> for more details.
-	 * <p/>
+	 * <br />
 	 * Insert is used to initially store the object into the database. To update an existing object use the save method.
-	 * <p/>
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1270,12 +1270,12 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Insert the object into the specified collection.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * Insert is used to initially store the object into the database. To update an existing object use the save method.
-	 * <p/>
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1315,16 +1315,16 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Save the object to the collection for the entity type of the object to save. This will perform an insert if the
 	 * object is not already present, that is an 'upsert'.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
 	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See
 	 * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation" > Spring's
 	 * Type Conversion"</a> for more details.
-	 * <p />
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1336,16 +1336,15 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Save the object to the specified collection. This will perform an insert if the object is not already present, that
 	 * is an 'upsert'.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
-	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See <a
-	 * https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation">Spring's Type
-	 * Conversion"</a> for more details.
-	 * <p />
+	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API.
+	 * See <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation">Spring's Type Conversion</a> for more details.
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoServerApiFactoryBean.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import org.springframework.beans.factory.FactoryBean;
+import org.springframework.lang.Nullable;
+import org.springframework.util.ObjectUtils;
+
+import com.mongodb.ServerApi;
+import com.mongodb.ServerApi.Builder;
+import com.mongodb.ServerApiVersion;
+
+/**
+ * {@link FactoryBean} for creating {@link ServerApi} using the {@link ServerApi.Builder}.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public class MongoServerApiFactoryBean implements FactoryBean<ServerApi> {
+
+	private String version;
+	private @Nullable Boolean deprecationErrors;
+	private @Nullable Boolean strict;
+
+	/**
+	 * @param version the version string either as the enum name or the server version value.
+	 * @see ServerApiVersion
+	 */
+	public void setVersion(String version) {
+		this.version = version;
+	}
+
+	/**
+	 * @param deprecationErrors
+	 * @see ServerApi.Builder#deprecationErrors(boolean)
+	 */
+	public void setDeprecationErrors(@Nullable Boolean deprecationErrors) {
+		this.deprecationErrors = deprecationErrors;
+	}
+
+	/**
+	 * @param strict
+	 * @see ServerApi.Builder#strict(boolean)
+	 */
+	public void setStrict(@Nullable Boolean strict) {
+		this.strict = strict;
+	}
+
+	@Nullable
+	@Override
+	public ServerApi getObject() throws Exception {
+
+		Builder builder = ServerApi.builder().version(version());
+
+		if (deprecationErrors != null) {
+			builder = builder.deprecationErrors(deprecationErrors);
+		}
+		if (strict != null) {
+			builder = builder.strict(strict);
+		}
+		return builder.build();
+	}
+
+	@Nullable
+	@Override
+	public Class<?> getObjectType() {
+		return ServerApi.class;
+	}
+
+	private ServerApiVersion version() {
+		try {
+			// lookup by name eg. 'V1'
+			return ObjectUtils.caseInsensitiveValueOf(ServerApiVersion.values(), version);
+		} catch (IllegalArgumentException e) {
+			// or just the version number, eg. just '1'
+			return ServerApiVersion.findByValue(version);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoTemplate.java

@@ -28,6 +28,7 @@ import org.bson.Document;
 import org.bson.conversions.Bson;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+
 import org.springframework.beans.BeansException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
@@ -46,6 +47,7 @@ import org.springframework.data.geo.Distance;
 import org.springframework.data.geo.GeoResult;
 import org.springframework.data.geo.GeoResults;
 import org.springframework.data.geo.Metric;
+import org.springframework.data.mapping.MappingException;
 import org.springframework.data.mapping.callback.EntityCallbacks;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mongodb.MongoDatabaseFactory;
@@ -97,12 +99,12 @@ import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.mongodb.core.query.UpdateDefinition.ArrayFilter;
+import org.springframework.data.mongodb.core.timeseries.Granularity;
 import org.springframework.data.mongodb.core.validation.Validator;
 import org.springframework.data.mongodb.util.BsonUtils;
 import org.springframework.data.projection.SpelAwareProxyProjectionFactory;
 import org.springframework.data.util.CloseableIterator;
 import org.springframework.data.util.Optionals;
-import org.springframework.jca.cci.core.ConnectionCallback;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
@@ -336,7 +338,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	/**
 	 * Set the {@link EntityCallbacks} instance to use when invoking
 	 * {@link org.springframework.data.mapping.callback.EntityCallback callbacks} like the {@link BeforeSaveCallback}.
-	 * <p />
+	 * <br />
 	 * Overrides potentially existing {@link EntityCallbacks}.
 	 *
 	 * @param entityCallbacks must not be {@literal null}.
@@ -596,7 +598,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @see org.springframework.data.mongodb.core.MongoOperations#createCollection(java.lang.Class)
 	 */
 	public <T> MongoCollection<Document> createCollection(Class<T> entityClass) {
-		return createCollection(entityClass, CollectionOptions.empty());
+		return createCollection(entityClass, operations.forType(entityClass).getCollectionOptions());
 	}
 
 	/*
@@ -972,7 +974,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		for (Document element : results) {
 
 			GeoResult<T> geoResult = callback.doWith(element);
-			aggregate = aggregate.add(new BigDecimal(geoResult.getDistance().getValue()));
+			aggregate = aggregate.add(BigDecimal.valueOf(geoResult.getDistance().getValue()));
 			result.add(geoResult);
 		}
 
@@ -1383,7 +1385,6 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		return source.isVersionedEntity() //
 				? doSaveVersioned(source, collectionName) //
 				: (T) doSave(collectionName, objectToSave, this.mongoConverter);
-
 	}
 
 	@SuppressWarnings("unchecked")
@@ -2434,6 +2435,20 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				co.validationOptions(options);
 			}
 
+			if (collectionOptions.containsKey("timeseries")) {
+
+				Document timeSeries = collectionOptions.get("timeseries", Document.class);
+				com.mongodb.client.model.TimeSeriesOptions options = new com.mongodb.client.model.TimeSeriesOptions(
+						timeSeries.getString("timeField"));
+				if (timeSeries.containsKey("metaField")) {
+					options.metaField(timeSeries.getString("metaField"));
+				}
+				if (timeSeries.containsKey("granularity")) {
+					options.granularity(TimeSeriesGranularity.valueOf(timeSeries.getString("granularity").toUpperCase()));
+				}
+				co.timeSeriesOptions(options);
+			}
+
 			db.createCollection(collectionName, co);
 
 			MongoCollection<Document> coll = db.getCollection(collectionName, Document.class);
@@ -2588,6 +2603,19 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			collectionOptions.getValidationOptions().ifPresent(it -> it.getValidator() //
 					.ifPresent(val -> doc.put("validator", getMappedValidator(val, targetType))));
+
+			collectionOptions.getTimeSeriesOptions().map(operations.forType(targetType)::mapTimeSeriesOptions)
+					.ifPresent(it -> {
+
+						Document timeseries = new Document("timeField", it.getTimeField());
+						if (StringUtils.hasText(it.getMetaField())) {
+							timeseries.append("metaField", it.getMetaField());
+						}
+						if (!Granularity.DEFAULT.equals(it.getGranularity())) {
+							timeseries.append("granularity", it.getGranularity().name().toLowerCase());
+						}
+						doc.put("timeseries", timeseries);
+					});
 		}
 
 		return doc;
@@ -2635,7 +2663,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	/**
 	 * Map the results of an ad-hoc query on the default MongoDB collection to an object using the template's converter.
 	 * The first document that matches the query is returned and also removed from the collection in the database.
-	 * <p/>
+	 * <br />
 	 * The query document is specified as a standard Document and so is the fields specification.
 	 *
 	 * @param collectionName name of the collection to retrieve the objects from
@@ -2751,25 +2779,24 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * Internal method using callbacks to do queries against the datastore that requires reading a single object from a
 	 * collection of objects. It will take the following steps
 	 * <ol>
-	 * <li>Execute the given {@link ConnectionCallback} for a {@link Document}.</li>
+	 * <li>Execute the given {@link CollectionCallback} for a {@link Document}.</li>
 	 * <li>Apply the given {@link DocumentCallback} to each of the {@link Document}s to obtain the result.</li>
 	 * <ol>
 	 *
 	 * @param <T>
 	 * @param collectionCallback the callback to retrieve the {@link Document} with
-	 * @param objectCallback the {@link DocumentCallback} to transform {@link Document}s into the actual domain type
+	 * @param documentCallback the {@link DocumentCallback} to transform {@link Document}s into the actual domain type
 	 * @param collectionName the collection to be queried
 	 * @return
 	 */
 	@Nullable
 	private <T> T executeFindOneInternal(CollectionCallback<Document> collectionCallback,
-			DocumentCallback<T> objectCallback, String collectionName) {
+			DocumentCallback<T> documentCallback, String collectionName) {
 
 		try {
 
-			T result = objectCallback
-					.doWith(collectionCallback.doInCollection(getAndPrepareCollection(doGetDatabase(), collectionName)));
-			return result;
+			Document document = collectionCallback.doInCollection(getAndPrepareCollection(doGetDatabase(), collectionName));
+			return document != null ? documentCallback.doWith(document) : null;
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
 		}
@@ -2779,7 +2806,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * Internal method using callback to do queries against the datastore that requires reading a collection of objects.
 	 * It will take the following steps
 	 * <ol>
-	 * <li>Execute the given {@link ConnectionCallback} for a {@link FindIterable}.</li>
+	 * <li>Execute the given {@link CollectionCallback} for a {@link FindIterable}.</li>
 	 * <li>Prepare that {@link FindIterable} with the given {@link CursorPreparer} (will be skipped if
 	 * {@link CursorPreparer} is {@literal null}</li>
 	 * <li>Iterate over the {@link FindIterable} and applies the given {@link DocumentCallback} to each of the
@@ -2789,36 +2816,27 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @param <T>
 	 * @param collectionCallback the callback to retrieve the {@link FindIterable} with
 	 * @param preparer the {@link CursorPreparer} to potentially modify the {@link FindIterable} before iterating over it
-	 * @param objectCallback the {@link DocumentCallback} to transform {@link Document}s into the actual domain type
+	 * @param documentCallback the {@link DocumentCallback} to transform {@link Document}s into the actual domain type
 	 * @param collectionName the collection to be queried
 	 * @return
 	 */
 	private <T> List<T> executeFindMultiInternal(CollectionCallback<FindIterable<Document>> collectionCallback,
-			CursorPreparer preparer, DocumentCallback<T> objectCallback, String collectionName) {
+			CursorPreparer preparer, DocumentCallback<T> documentCallback, String collectionName) {
 
 		try {
 
-			MongoCursor<Document> cursor = null;
-
-			try {
-
-				cursor = preparer
-						.initiateFind(getAndPrepareCollection(doGetDatabase(), collectionName), collectionCallback::doInCollection)
-						.iterator();
+			try (MongoCursor<Document> cursor = preparer
+					.initiateFind(getAndPrepareCollection(doGetDatabase(), collectionName), collectionCallback::doInCollection)
+					.iterator()) {
 
 				List<T> result = new ArrayList<>();
 
 				while (cursor.hasNext()) {
 					Document object = cursor.next();
-					result.add(objectCallback.doWith(object));
+					result.add(documentCallback.doWith(object));
 				}
 
 				return result;
-			} finally {
-
-				if (cursor != null) {
-					cursor.close();
-				}
 			}
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
@@ -2828,23 +2846,12 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	private void executeQueryInternal(CollectionCallback<FindIterable<Document>> collectionCallback,
 			CursorPreparer preparer, DocumentCallbackHandler callbackHandler, String collectionName) {
 
-		try {
+		try (MongoCursor<Document> cursor = preparer
+				.initiateFind(getAndPrepareCollection(doGetDatabase(), collectionName), collectionCallback::doInCollection)
+				.iterator()) {
 
-			MongoCursor<Document> cursor = null;
-
-			try {
-
-				cursor = preparer
-						.initiateFind(getAndPrepareCollection(doGetDatabase(), collectionName), collectionCallback::doInCollection)
-						.iterator();
-
-				while (cursor.hasNext()) {
-					callbackHandler.processDocument(cursor.next());
-				}
-			} finally {
-				if (cursor != null) {
-					cursor.close();
-				}
+			while (cursor.hasNext()) {
+				callbackHandler.processDocument(cursor.next());
 			}
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
@@ -3143,8 +3150,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 	interface DocumentCallback<T> {
 
-		@Nullable
-		T doWith(@Nullable Document object);
+		T doWith(Document object);
 	}
 
 	/**
@@ -3168,22 +3174,19 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			this.collectionName = collectionName;
 		}
 
-		@Nullable
-		public T doWith(@Nullable Document document) {
+		public T doWith(Document document) {
 
-			T source = null;
+			maybeEmitEvent(new AfterLoadEvent<>(document, type, collectionName));
+			T entity = reader.read(type, document);
 
-			if (document != null) {
-				maybeEmitEvent(new AfterLoadEvent<>(document, type, collectionName));
-				source = reader.read(type, document);
+			if (entity == null) {
+				throw new MappingException(String.format("EntityReader %s returned null", reader));
 			}
 
-			if (source != null) {
-				maybeEmitEvent(new AfterConvertEvent<>(document, source, collectionName));
-				source = maybeCallAfterConvert(source, document, collectionName);
-			}
+			maybeEmitEvent(new AfterConvertEvent<>(document, entity, collectionName));
+			entity = maybeCallAfterConvert(entity, document, collectionName);
 
-			return source;
+			return entity;
 		}
 	}
 
@@ -3216,8 +3219,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		 * @see org.springframework.data.mongodb.core.MongoTemplate.DocumentCallback#doWith(org.bson.Document)
 		 */
 		@SuppressWarnings("unchecked")
-		@Nullable
-		public T doWith(@Nullable Document document) {
+		public T doWith(Document document) {
 
 			if (document == null) {
 				return null;
@@ -3228,15 +3230,16 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			maybeEmitEvent(new AfterLoadEvent<>(document, targetType, collectionName));
 
-			Object source = reader.read(typeToRead, document);
-			Object result = targetType.isInterface() ? projectionFactory.createProjection(targetType, source) : source;
+			Object entity = reader.read(typeToRead, document);
 
-			if (result != null) {
-				maybeEmitEvent(new AfterConvertEvent<>(document, result, collectionName));
-				result = maybeCallAfterConvert(result, document, collectionName);
+			if (entity == null) {
+				throw new MappingException(String.format("EntityReader %s returned null", reader));
 			}
 
-			return (T) result;
+			Object result = targetType.isInterface() ? projectionFactory.createProjection(targetType, entity) : entity;
+
+			maybeEmitEvent(new AfterConvertEvent<>(document, result, collectionName));
+			return (T) maybeCallAfterConvert(result, document, collectionName);
 		}
 	}
 
@@ -3373,8 +3376,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			this.metric = metric;
 		}
 
-		@Nullable
-		public GeoResult<T> doWith(@Nullable Document object) {
+		public GeoResult<T> doWith(Document object) {
 
 			double distance = Double.NaN;
 			if (object.containsKey(distanceField)) {
@@ -3401,10 +3403,6 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		/**
 		 * Creates a new {@link CloseableIterableCursorAdapter} backed by the given {@link MongoCollection}.
-		 *
-		 * @param cursor
-		 * @param exceptionTranslator
-		 * @param objectReadCallback
 		 */
 		CloseableIterableCursorAdapter(MongoIterable<Document> cursor, PersistenceExceptionTranslator exceptionTranslator,
 				DocumentCallback<T> objectReadCallback) {
@@ -3448,8 +3446,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			try {
 				Document item = cursor.next();
-				T converted = objectReadCallback.doWith(item);
-				return converted;
+				return objectReadCallback.doWith(item);
 			} catch (RuntimeException ex) {
 				throw potentiallyConvertRuntimeException(ex, exceptionTranslator);
 			}
@@ -3495,7 +3492,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	/**
 	 * {@link MongoTemplate} extension bound to a specific {@link ClientSession} that is applied when interacting with the
 	 * server through the driver API.
-	 * <p />
+	 * <br />
 	 * The prepare steps for {@link MongoDatabase} and {@link MongoCollection} proxy the target and invoke the desired
 	 * target method matching the actual arguments plus a {@link ClientSession}.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/QueryOperations.java

@@ -613,7 +613,7 @@ class QueryOperations {
 
 		UpdateContext(MappedDocument update, boolean upsert) {
 
-			super(new BasicQuery(new Document(BsonUtils.asMap(update.getIdFilter()))));
+			super(new BasicQuery(BsonUtils.asDocument(update.getIdFilter())));
 			this.multi = false;
 			this.upsert = upsert;
 			this.mappedDocument = update;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveChangeStreamOperation.java

@@ -71,7 +71,7 @@ public interface ReactiveChangeStreamOperation {
 		/**
 		 * Start listening to changes. The stream will not be completed unless the {@link org.reactivestreams.Subscription}
 		 * is {@link org.reactivestreams.Subscription#cancel() canceled}.
-		 * <p />
+		 * <br />
 		 * However, the stream may become dead, or invalid, if all watched collections, databases are dropped.
 		 */
 		Flux<ChangeStreamEvent<T>> listen();

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveFindOperation.java

@@ -91,10 +91,10 @@ public interface ReactiveFindOperation {
 		 * Get all matching elements using a {@link com.mongodb.CursorType#TailableAwait tailable cursor}. The stream will
 		 * not be completed unless the {@link org.reactivestreams.Subscription} is
 		 * {@link org.reactivestreams.Subscription#cancel() canceled}.
-		 * <p />
+		 * <br />
 		 * However, the stream may become dead, or invalid, if either the query returns no match or the cursor returns the
 		 * document at the "end" of the collection and then the application deletes that document.
-		 * <p />
+		 * <br />
 		 * A stream that is no longer in use must be {@link reactor.core.Disposable#dispose()} disposed} otherwise the
 		 * streams will linger and exhaust resources. <br/>
 		 * <strong>NOTE:</strong> Requires a capped collection.
@@ -106,7 +106,7 @@ public interface ReactiveFindOperation {
 
 		/**
 		 * Get the number of matching elements.
-		 * <p />
+		 * <br />
 		 * This method uses an
 		 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 		 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMongoOperations.java

@@ -59,7 +59,7 @@ import com.mongodb.reactivestreams.client.MongoCollection;
  * Implemented by {@link ReactiveMongoTemplate}. Not often used but a useful option for extensibility and testability
  * (as it can be easily mocked, stubbed, or be the target of a JDK proxy). Command execution using
  * {@link ReactiveMongoOperations} is deferred until subscriber subscribes to the {@link Publisher}.
- * <p />
+ * <br />
  * <strong>NOTE:</strong> Some operations cannot be executed within a MongoDB transaction. Please refer to the MongoDB
  * specific documentation to learn more about <a href="https://docs.mongodb.com/manual/core/transactions/">Multi
  * Document Transactions</a>.
@@ -121,7 +121,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Executes a {@link ReactiveDatabaseCallback} translating any exceptions as necessary.
-	 * <p/>
+	 * <br />
 	 * Allows for returning a result object, that is a domain object or a collection of domain objects.
 	 *
 	 * @param action callback object that specifies the MongoDB actions to perform on the passed in DB instance. Must not
@@ -133,7 +133,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Executes the given {@link ReactiveCollectionCallback} on the entity collection of the specified class.
-	 * <p/>
+	 * <br />
 	 * Allows for returning a result object, that is a domain object or a collection of domain objects.
 	 *
 	 * @param entityClass class that determines the collection to use. Must not be {@literal null}.
@@ -145,7 +145,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Executes the given {@link ReactiveCollectionCallback} on the collection of the given name.
-	 * <p/>
+	 * <br />
 	 * Allows for returning a result object, that is a domain object or a collection of domain objects.
 	 *
 	 * @param collectionName the name of the collection that specifies which {@link MongoCollection} instance will be
@@ -159,7 +159,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Obtain a {@link ClientSession session} bound instance of {@link SessionScoped} binding the {@link ClientSession}
 	 * provided by the given {@link Supplier} to each and every command issued against MongoDB.
-	 * <p />
+	 * <br />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle. Use
 	 * {@link ReactiveSessionScoped#execute(ReactiveSessionCallback, Consumer)} to provide a hook for processing the
 	 * {@link ClientSession} when done.
@@ -178,7 +178,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Obtain a {@link ClientSession session} bound instance of {@link SessionScoped} binding a new {@link ClientSession}
 	 * with given {@literal sessionOptions} to each and every command issued against MongoDB.
-	 * <p />
+	 * <br />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle. Use
 	 * {@link ReactiveSessionScoped#execute(ReactiveSessionCallback, Consumer)} to provide a hook for processing the
 	 * {@link ClientSession} when done.
@@ -192,7 +192,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Obtain a {@link ClientSession session} bound instance of {@link ReactiveSessionScoped} binding the
 	 * {@link ClientSession} provided by the given {@link Publisher} to each and every command issued against MongoDB.
-	 * <p />
+	 * <br />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle. Use
 	 * {@link ReactiveSessionScoped#execute(ReactiveSessionCallback, Consumer)} to provide a hook for processing the
 	 * {@link ClientSession} when done.
@@ -205,7 +205,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Obtain a {@link ClientSession} bound instance of {@link ReactiveMongoOperations}.
-	 * <p />
+	 * <br />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle.
 	 *
 	 * @param session must not be {@literal null}.
@@ -218,7 +218,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * Initiate a new {@link ClientSession} and obtain a {@link ClientSession session} bound instance of
 	 * {@link ReactiveSessionScoped}. Starts the transaction and adds the {@link ClientSession} to each and every command
 	 * issued against MongoDB.
-	 * <p/>
+	 * <br />
 	 * Each {@link ReactiveSessionScoped#execute(ReactiveSessionCallback) execution} initiates a new managed transaction
 	 * that is {@link ClientSession#commitTransaction() committed} on success. Transactions are
 	 * {@link ClientSession#abortTransaction() rolled back} upon errors.
@@ -233,7 +233,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * Obtain a {@link ClientSession session} bound instance of {@link ReactiveSessionScoped}, start the transaction and
 	 * bind the {@link ClientSession} provided by the given {@link Publisher} to each and every command issued against
 	 * MongoDB.
-	 * <p/>
+	 * <br />
 	 * Each {@link ReactiveSessionScoped#execute(ReactiveSessionCallback) execution} initiates a new managed transaction
 	 * that is {@link ClientSession#commitTransaction() committed} on success. Transactions are
 	 * {@link ClientSession#abortTransaction() rolled back} upon errors.
@@ -293,7 +293,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * created on first interaction with the server. Collections can be explicitly created via
 	 * {@link #createCollection(Class)}. Please make sure to check if the collection {@link #collectionExists(Class)
 	 * exists} first.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param collectionName name of the collection.
@@ -303,7 +303,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Check to see if a collection with a name indicated by the entity class exists.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param entityClass class that determines the name of the collection. Must not be {@literal null}.
@@ -313,7 +313,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Check to see if a collection with a given name exists.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param collectionName name of the collection. Must not be {@literal null}.
@@ -323,7 +323,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Drop the collection with the name indicated by the entity class.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param entityClass class that determines the collection to drop/delete. Must not be {@literal null}.
@@ -332,7 +332,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Drop the collection with the given name.
-	 * <p/>
+	 * <br />
 	 * Translate any exceptions as necessary.
 	 *
 	 * @param collectionName name of the collection to drop/delete.
@@ -341,10 +341,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Query for a {@link Flux} of objects of type T from the collection used by the entity class.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your collection does not contain a homogeneous collection of types, this operation will not be an efficient way
 	 * to map objects since the test for class type is done in the client and not on the server.
 	 *
@@ -355,10 +355,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Query for a {@link Flux} of objects of type T from the specified collection.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your collection does not contain a homogeneous collection of types, this operation will not be an efficient way
 	 * to map objects since the test for class type is done in the client and not on the server.
 	 *
@@ -371,10 +371,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Map the results of an ad-hoc query on the collection for the entity class to a single instance of an object of the
 	 * specified type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -388,10 +388,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Map the results of an ad-hoc query on the specified collection to a single instance of an object of the specified
 	 * type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -435,10 +435,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Map the results of an ad-hoc query on the collection for the entity class to a {@link Flux} of the specified type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -451,10 +451,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Map the results of an ad-hoc query on the specified collection to a {@link Flux} of the specified type.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -566,10 +566,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Execute an aggregation operation.
-	 * <p/>
+	 * <br />
 	 * The raw results will be mapped to the given entity class and are returned as stream. The name of the
 	 * inputCollection is derived from the {@link TypedAggregation#getInputType() aggregation input type}.
-	 * <p/>
+	 * <br />
 	 * Aggregation streaming cannot be used with {@link AggregationOptions#isExplain() aggregation explain} nor with
 	 * {@link AggregationOptions#getCursorBatchSize()}. Enabling explanation mode or setting batch size cause
 	 * {@link IllegalArgumentException}.
@@ -584,10 +584,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Execute an aggregation operation.
-	 * <p/>
+	 * <br />
 	 * The raw results will be mapped to the given {@code ouputType}. The name of the inputCollection is derived from the
 	 * {@code inputType}.
-	 * <p/>
+	 * <br />
 	 * Aggregation streaming cannot be used with {@link AggregationOptions#isExplain() aggregation explain} nor with
 	 * {@link AggregationOptions#getCursorBatchSize()}. Enabling explanation mode or setting batch size cause
 	 * {@link IllegalArgumentException}.
@@ -604,9 +604,9 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Execute an aggregation operation.
-	 * <p/>
+	 * <br />
 	 * The raw results will be mapped to the given entity class.
-	 * <p/>
+	 * <br />
 	 * Aggregation streaming cannot be used with {@link AggregationOptions#isExplain() aggregation explain} nor with
 	 * {@link AggregationOptions#getCursorBatchSize()}. Enabling explanation mode or setting batch size cause
 	 * {@link IllegalArgumentException}.
@@ -676,7 +676,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	<T> Flux<GeoResult<T>> geoNear(NearQuery near, Class<T> entityClass, String collectionName);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify</a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
@@ -691,7 +691,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, Class<T> entityClass);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify</a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
@@ -707,7 +707,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, Class<T> entityClass, String collectionName);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify</a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
 	 * {@link FindAndModifyOptions} into account.
 	 *
@@ -725,7 +725,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify</a>
 	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
 	 * {@link FindAndModifyOptions} into account.
 	 *
@@ -746,7 +746,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement}
 	 * document. <br />
 	 * Options are defaulted to {@link FindAndReplaceOptions#empty()}. <br />
@@ -764,7 +764,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement}
 	 * document. <br />
 	 * Options are defaulted to {@link FindAndReplaceOptions#empty()}. <br />
@@ -783,7 +783,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account. <br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -803,7 +803,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account. <br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -825,7 +825,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account. <br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -849,7 +849,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account. <br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -876,7 +876,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Triggers
-	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace<a/>
+	 * <a href="https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/">findOneAndReplace</a>
 	 * to replace a single document matching {@link Criteria} of given {@link Query} with the {@code replacement} document
 	 * taking {@link FindAndReplaceOptions} into account. <br />
 	 * <strong>NOTE:</strong> The replacement entity must not hold an {@literal id}.
@@ -902,9 +902,9 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * Map the results of an ad-hoc query on the collection for the entity type to a single instance of an object of the
 	 * specified type. The first document that matches the query is returned and also removed from the collection in the
 	 * database.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -918,10 +918,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Map the results of an ad-hoc query on the specified collection to a single instance of an object of the specified
 	 * type. The first document that matches the query is returned and also removed from the collection in the database.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -940,7 +940,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
-	 * <p />
+	 * <br />
 	 * This method uses an
 	 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
@@ -962,7 +962,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
-	 * <p />
+	 * <br />
 	 * This method uses an
 	 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
@@ -983,7 +983,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
-	 * <p />
+	 * <br />
 	 * This method uses an
 	 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
 	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
@@ -1001,7 +1001,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Estimate the number of documents, in the collection {@link #getCollectionName(Class) identified by the given type},
 	 * based on collection statistics.
-	 * <p />
+	 * <br />
 	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
 	 * transactions.
 	 *
@@ -1017,7 +1017,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Estimate the number of documents in the given collection based on collection statistics.
-	 * <p />
+	 * <br />
 	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
 	 * transactions.
 	 *
@@ -1029,17 +1029,17 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Insert the object into the collection for the entity type of the object to save.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
 	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See
 	 * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation" > Spring's
 	 * Type Conversion"</a> for more details.
-	 * <p/>
+	 * <br />
 	 * Insert is used to initially store the object into the database. To update an existing object use the save method.
-	 * <p />
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1050,12 +1050,12 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Insert the object into the specified collection.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * Insert is used to initially store the object into the database. To update an existing object use the save method.
-	 * <p />
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1094,15 +1094,15 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Insert the object into the collection for the entity type of the object to save.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
 	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See
 	 * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation" > Spring's
 	 * Type Conversion"</a> for more details.
-	 * <p/>
+	 * <br />
 	 * Insert is used to initially store the object into the database. To update an existing object use the save method.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1140,16 +1140,16 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Save the object to the collection for the entity type of the object to save. This will perform an insert if the
 	 * object is not already present, that is an 'upsert'.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
 	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See
 	 * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation" > Spring's
 	 * Type Conversion"</a> for more details.
-	 * <p />
+	 * <br />
 	 * The {@code objectToSave} must not be collection-like.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
@@ -1161,15 +1161,14 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Save the object to the specified collection. This will perform an insert if the object is not already present, that
 	 * is an 'upsert'.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
-	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See <a
-	 * https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation">Spring's Type
-	 * Conversion"</a> for more details.
+	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API.
+	 * See <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation">Spring's Type Conversion</a> for more details.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
 	 * @param collectionName name of the collection to store the object in. Must not be {@literal null}.
@@ -1181,15 +1180,14 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Save the object to the collection for the entity type of the object to save. This will perform an insert if the
 	 * object is not already present, that is an 'upsert'.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
-	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See
-	 * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation" > Spring's
-	 * Type Conversion"</a> for more details.
+	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API.
+	 * See <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation"> Spring's Type Conversion</a> for more details.
 	 *
 	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
 	 * @return the saved object.
@@ -1199,17 +1197,16 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Save the object to the specified collection. This will perform an insert if the object is not already present, that
 	 * is an 'upsert'.
-	 * <p/>
+	 * <br />
 	 * The object is converted to the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * If your object has an "Id' property, it will be set with the generated Id from MongoDB. If your Id property is a
 	 * String then MongoDB ObjectId will be used to populate that string. Otherwise, the conversion from ObjectId to your
-	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API. See <a
-	 * https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation">Spring's Type
-	 * Conversion"</a> for more details.
+	 * property type will be handled by Spring's BeanWrapper class that leverages Type Conversion API.
+	 * See <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#validation">Spring's Type Conversion</a> for more details.
 	 *
-	 * @param objectToSave the object to store in the collection. Must not be {@literal null}.
+	 * @param objectToSave the object to store in the collReactiveMongoOperationsection. Must not be {@literal null}.
 	 * @param collectionName name of the collection to store the object in. Must not be {@literal null}.
 	 * @return the saved object.
 	 */
@@ -1481,10 +1478,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * type. The stream uses a {@link com.mongodb.CursorType#TailableAwait tailable} cursor that may be an infinite
 	 * stream. The stream will not be completed unless the {@link org.reactivestreams.Subscription} is
 	 * {@link Subscription#cancel() canceled}.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -1500,10 +1497,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * type. The stream uses a {@link com.mongodb.CursorType#TailableAwait tailable} cursor that may be an infinite
 	 * stream. The stream will not be completed unless the {@link org.reactivestreams.Subscription} is
 	 * {@link Subscription#cancel() canceled}.
-	 * <p/>
+	 * <br />
 	 * The object is converted from the MongoDB native representation using an instance of {@see MongoConverter}. Unless
 	 * configured otherwise, an instance of {@link MappingMongoConverter} will be used.
-	 * <p/>
+	 * <br />
 	 * The query is specified as a {@link Query} which can be created either using the {@link BasicQuery} or the more
 	 * feature rich {@link Query}.
 	 *
@@ -1520,10 +1517,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * the configured default database via the reactive infrastructure. Use the optional provided {@link Aggregation} to
 	 * filter events. The stream will not be completed unless the {@link org.reactivestreams.Subscription} is
 	 * {@link Subscription#cancel() canceled}.
-	 * <p />
+	 * <br />
 	 * The {@link ChangeStreamEvent#getBody()} is mapped to the {@literal resultType} while the
 	 * {@link ChangeStreamEvent#getRaw()} contains the unmodified payload.
-	 * <p />
+	 * <br />
 	 * Use {@link ChangeStreamOptions} to set arguments like {@link ChangeStreamOptions#getResumeToken() the resumseToken}
 	 * for resuming change streams.
 	 *
@@ -1544,10 +1541,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * the given collection via the reactive infrastructure. Use the optional provided {@link Aggregation} to filter
 	 * events. The stream will not be completed unless the {@link org.reactivestreams.Subscription} is
 	 * {@link Subscription#cancel() canceled}.
-	 * <p />
+	 * <br />
 	 * The {@link ChangeStreamEvent#getBody()} is mapped to the {@literal resultType} while the
 	 * {@link ChangeStreamEvent#getRaw()} contains the unmodified payload.
-	 * <p />
+	 * <br />
 	 * Use {@link ChangeStreamOptions} to set arguments like {@link ChangeStreamOptions#getResumeToken() the resumseToken}
 	 * for resuming change streams.
 	 *
@@ -1569,10 +1566,10 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * Subscribe to a MongoDB <a href="https://docs.mongodb.com/manual/changeStreams/">Change Stream</a> via the reactive
 	 * infrastructure. Use the optional provided {@link Aggregation} to filter events. The stream will not be completed
 	 * unless the {@link org.reactivestreams.Subscription} is {@link Subscription#cancel() canceled}.
-	 * <p />
+	 * <br />
 	 * The {@link ChangeStreamEvent#getBody()} is mapped to the {@literal resultType} while the
 	 * {@link ChangeStreamEvent#getRaw()} contains the unmodified payload.
-	 * <p />
+	 * <br />
 	 * Use {@link ChangeStreamOptions} to set arguments like {@link ChangeStreamOptions#getResumeToken() the resumseToken}
 	 * for resuming change streams.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMongoTemplate.java

@@ -22,7 +22,15 @@ import reactor.core.publisher.Mono;
 import reactor.util.function.Tuple2;
 import reactor.util.function.Tuples;
 
-import java.util.*;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Consumer;
 import java.util.function.Function;
@@ -36,6 +44,7 @@ import org.reactivestreams.Publisher;
 import org.reactivestreams.Subscriber;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+
 import org.springframework.beans.BeansException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
@@ -51,6 +60,7 @@ import org.springframework.data.convert.EntityReader;
 import org.springframework.data.geo.Distance;
 import org.springframework.data.geo.GeoResult;
 import org.springframework.data.geo.Metric;
+import org.springframework.data.mapping.MappingException;
 import org.springframework.data.mapping.PersistentEntity;
 import org.springframework.data.mapping.callback.ReactiveEntityCallbacks;
 import org.springframework.data.mapping.context.MappingContext;
@@ -100,6 +110,7 @@ import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.mongodb.core.query.UpdateDefinition.ArrayFilter;
+import org.springframework.data.mongodb.core.timeseries.Granularity;
 import org.springframework.data.mongodb.core.validation.Validator;
 import org.springframework.data.mongodb.util.BsonUtils;
 import org.springframework.data.projection.SpelAwareProxyProjectionFactory;
@@ -351,7 +362,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * Set the {@link ReactiveEntityCallbacks} instance to use when invoking
 	 * {@link org.springframework.data.mapping.callback.EntityCallback callbacks} like the
 	 * {@link ReactiveBeforeSaveCallback}.
-	 * <p />
+	 * <br />
 	 * Overrides potentially existing {@link ReactiveEntityCallbacks}.
 	 *
 	 * @param entityCallbacks must not be {@literal null}.
@@ -655,7 +666,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#createCollection(java.lang.Class)
 	 */
 	public <T> Mono<MongoCollection<Document>> createCollection(Class<T> entityClass) {
-		return createCollection(entityClass, CollectionOptions.empty());
+		return createCollection(entityClass, operations.forType(entityClass).getCollectionOptions());
 	}
 
 	/*
@@ -964,7 +975,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		return doAggregate(aggregation, collectionName, null, outputType);
 	}
 
-	protected <O> Flux<O> doAggregate(Aggregation aggregation, String collectionName,  @Nullable Class<?> inputType, Class<O> outputType) {
+	protected <O> Flux<O> doAggregate(Aggregation aggregation, String collectionName, @Nullable Class<?> inputType,
+			Class<O> outputType) {
 
 		Assert.notNull(aggregation, "Aggregation pipeline must not be null!");
 		Assert.hasText(collectionName, "Collection name must not be null or empty!");
@@ -976,19 +988,18 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		AggregationDefinition ctx = queryOperations.createAggregation(aggregation, inputType);
 
 		if (LOGGER.isDebugEnabled()) {
-			LOGGER.debug("Streaming aggregation: {} in collection {}", serializeToJsonSafely(ctx.getAggregationPipeline()), collectionName);
+			LOGGER.debug("Streaming aggregation: {} in collection {}", serializeToJsonSafely(ctx.getAggregationPipeline()),
+					collectionName);
 		}
 
 		ReadDocumentCallback<O> readCallback = new ReadDocumentCallback<>(mongoConverter, outputType, collectionName);
-		return execute(collectionName,
-				collection -> aggregateAndMap(collection, ctx.getAggregationPipeline(), ctx.isOutOrMerge(), options,
-						readCallback,
-						ctx.getInputType()));
+		return execute(collectionName, collection -> aggregateAndMap(collection, ctx.getAggregationPipeline(),
+				ctx.isOutOrMerge(), options, readCallback, ctx.getInputType()));
 	}
 
 	private <O> Flux<O> aggregateAndMap(MongoCollection<Document> collection, List<Document> pipeline,
-			boolean isOutOrMerge,
-			AggregationOptions options, ReadDocumentCallback<O> readCallback, @Nullable Class<?> inputType) {
+			boolean isOutOrMerge, AggregationOptions options, ReadDocumentCallback<O> readCallback,
+			@Nullable Class<?> inputType) {
 
 		AggregatePublisher<Document> cursor = collection.aggregate(pipeline, Document.class)
 				.allowDiskUse(options.isAllowDiskUse());
@@ -2495,6 +2506,20 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			result.validationOptions(validationOptions);
 		});
 
+		collectionOptions.getTimeSeriesOptions().map(operations.forType(entityType)::mapTimeSeriesOptions).ifPresent(it -> {
+
+			TimeSeriesOptions options = new TimeSeriesOptions(it.getTimeField());
+
+			if (StringUtils.hasText(it.getMetaField())) {
+				options.metaField(it.getMetaField());
+			}
+			if (!Granularity.DEFAULT.equals(it.getGranularity())) {
+				options.granularity(TimeSeriesGranularity.valueOf(it.getGranularity().name().toUpperCase()));
+			}
+
+			result.timeSeriesOptions(options);
+		});
+
 		return result;
 	}
 
@@ -2512,7 +2537,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	/**
 	 * Map the results of an ad-hoc query on the default MongoDB collection to an object using the template's converter.
 	 * The first document that matches the query is returned and also removed from the collection in the database.
-	 * <p/>
+	 * <br />
 	 * The query document is specified as a standard Document and so is the fields specification.
 	 *
 	 * @param collectionName name of the collection to retrieve the objects from
@@ -3152,13 +3177,14 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 			maybeEmitEvent(new AfterLoadEvent<>(document, type, collectionName));
 
-			T source = reader.read(type, document);
-			if (source != null) {
-				maybeEmitEvent(new AfterConvertEvent<>(document, source, collectionName));
-				return maybeCallAfterConvert(source, document, collectionName);
+			T entity = reader.read(type, document);
+
+			if (entity == null) {
+				throw new MappingException(String.format("EntityReader %s returned null", reader));
 			}
 
-			return Mono.empty();
+			maybeEmitEvent(new AfterConvertEvent<>(document, entity, collectionName));
+			return maybeCallAfterConvert(entity, document, collectionName);
 		}
 	}
 
@@ -3196,16 +3222,17 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 			maybeEmitEvent(new AfterLoadEvent<>(document, typeToRead, collectionName));
 
-			Object source = reader.read(typeToRead, document);
-			Object result = targetType.isInterface() ? projectionFactory.createProjection(targetType, source) : source;
+			Object entity = reader.read(typeToRead, document);
 
-			T castEntity = (T) result;
-			if (castEntity != null) {
-				maybeEmitEvent(new AfterConvertEvent<>(document, castEntity, collectionName));
-				return maybeCallAfterConvert(castEntity, document, collectionName);
+			if (entity == null) {
+				throw new MappingException(String.format("EntityReader %s returned null", reader));
 			}
 
-			return Mono.empty();
+			Object result = targetType.isInterface() ? projectionFactory.createProjection(targetType, entity) : entity;
+
+			T castEntity = (T) result;
+			maybeEmitEvent(new AfterConvertEvent<>(document, castEntity, collectionName));
+			return maybeCallAfterConvert(castEntity, document, collectionName);
 		}
 	}
 
@@ -3363,7 +3390,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	/**
 	 * {@link MongoTemplate} extension bound to a specific {@link ClientSession} that is applied when interacting with the
 	 * server through the driver API.
-	 * <p />
+	 * <br />
 	 * The prepare steps for {@link MongoDatabase} and {@link MongoCollection} proxy the target and invoke the desired
 	 * target method matching the actual arguments plus a {@link ClientSession}.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveSessionCallback.java

@@ -32,7 +32,7 @@ public interface ReactiveSessionCallback<T> {
 	/**
 	 * Execute operations against a MongoDB instance via session bound {@link ReactiveMongoOperations}. The session is
 	 * inferred directly into the operation so that no further interaction is necessary.
-	 * <p />
+	 * <br />
 	 * Please note that only Spring Data-specific abstractions like {@link ReactiveMongoOperations#find(Query, Class)} and
 	 * others are enhanced with the {@link com.mongodb.session.ClientSession}. When obtaining plain MongoDB gateway
 	 * objects like {@link com.mongodb.reactivestreams.client.MongoCollection} or

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveSessionScoped.java

@@ -33,7 +33,7 @@ public interface ReactiveSessionScoped {
 
 	/**
 	 * Executes the given {@link ReactiveSessionCallback} within the {@link com.mongodb.session.ClientSession}.
-	 * <p/>
+	 * <br />
 	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
 	 * closed} when done.
 	 *
@@ -47,7 +47,7 @@ public interface ReactiveSessionScoped {
 
 	/**
 	 * Executes the given {@link ReactiveSessionCallback} within the {@link com.mongodb.session.ClientSession}.
-	 * <p/>
+	 * <br />
 	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
 	 * closed} when done.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ScriptOperations.java

@@ -23,7 +23,7 @@ import org.springframework.lang.Nullable;
 
 
 /**
- * Script operations on {@link com.mongodb.DB} level. Allows interaction with server side JavaScript functions.
+ * Script operations on {@link com.mongodb.client.MongoDatabase} level. Allows interaction with server side JavaScript functions.
  *
  * @author Christoph Strobl
  * @author Oliver Gierke
@@ -72,10 +72,10 @@ public interface ScriptOperations {
 	Object call(String scriptName, Object... args);
 
 	/**
-	 * Checks {@link DB} for existence of {@link ServerSideJavaScript} with given name.
+	 * Checks {@link com.mongodb.client.MongoDatabase} for existence of {@literal ServerSideJavaScript} with given name.
 	 *
 	 * @param scriptName must not be {@literal null} or empty.
-	 * @return false if no {@link ServerSideJavaScript} with given name exists.
+	 * @return false if no {@literal ServerSideJavaScript} with given name exists.
 	 */
 	boolean exists(String scriptName);
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SessionCallback.java

@@ -31,7 +31,7 @@ public interface SessionCallback<T> {
 	/**
 	 * Execute operations against a MongoDB instance via session bound {@link MongoOperations}. The session is inferred
 	 * directly into the operation so that no further interaction is necessary.
-	 * <p />
+	 * <br />
 	 * Please note that only Spring Data-specific abstractions like {@link MongoOperations#find(Query, Class)} and others
 	 * are enhanced with the {@link com.mongodb.session.ClientSession}. When obtaining plain MongoDB gateway objects like
 	 * {@link com.mongodb.client.MongoCollection} or {@link com.mongodb.client.MongoDatabase} via eg.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SessionScoped.java

@@ -23,7 +23,7 @@ import com.mongodb.client.ClientSession;
 
 /**
  * Gateway interface to execute {@link ClientSession} bound operations against MongoDB via a {@link SessionCallback}.
- * <p />
+ * <br />
  * The very same bound {@link ClientSession} is used for all invocations of {@code execute} on the instance.
  *
  * @author Christoph Strobl
@@ -34,7 +34,7 @@ public interface SessionScoped {
 
 	/**
 	 * Executes the given {@link SessionCallback} within the {@link com.mongodb.session.ClientSession}.
-	 * <p/>
+	 * <br />
 	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
 	 * closed} when done.
 	 *
@@ -49,7 +49,7 @@ public interface SessionScoped {
 
 	/**
 	 * Executes the given {@link SessionCallback} within the {@link com.mongodb.session.ClientSession}.
-	 * <p/>
+	 * <br />
 	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
 	 * closed} when done.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AccumulatorOperators.java

@@ -142,11 +142,118 @@ public class AccumulatorOperators {
 			return usesFieldRef() ? StdDevSamp.stdDevSampOf(fieldReference) : StdDevSamp.stdDevSampOf(expression);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that uses the previous input (field/expression) and the value of the
+		 * given field to calculate the population covariance of the two.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 * @since 3.3
+		 */
+		public CovariancePop covariancePop(String fieldReference) {
+			return covariancePop().and(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that uses the previous input (field/expression) and the result of the
+		 * given {@link AggregationExpression expression} to calculate the population covariance of the two.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 * @since 3.3
+		 */
+		public CovariancePop covariancePop(AggregationExpression expression) {
+			return covariancePop().and(expression);
+		}
+
+		private CovariancePop covariancePop() {
+			return usesFieldRef() ? CovariancePop.covariancePopOf(fieldReference) : CovariancePop.covariancePopOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that uses the previous input (field/expression) and the value of the
+		 * given field to calculate the sample covariance of the two.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 * @since 3.3
+		 */
+		public CovarianceSamp covarianceSamp(String fieldReference) {
+			return covarianceSamp().and(fieldReference);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that uses the previous input (field/expression) and the result of the
+		 * given {@link AggregationExpression expression} to calculate the sample covariance of the two.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 * @since 3.3
+		 */
+		public CovarianceSamp covarianceSamp(AggregationExpression expression) {
+			return covarianceSamp().and(expression);
+		}
+
+		private CovarianceSamp covarianceSamp() {
+			return usesFieldRef() ? CovarianceSamp.covarianceSampOf(fieldReference)
+					: CovarianceSamp.covarianceSampOf(expression);
+		}
+
+		/**
+		 * Creates new {@link ExpMovingAvgBuilder} that to build {@link AggregationExpression expMovingAvg} that calculates
+		 * the exponential moving average of numeric values
+		 *
+		 * @return new instance of {@link ExpMovingAvg}.
+		 * @since 3.3
+		 */
+		public ExpMovingAvgBuilder expMovingAvg() {
+
+			ExpMovingAvg expMovingAvg = usesFieldRef() ? ExpMovingAvg.expMovingAvgOf(fieldReference)
+					: ExpMovingAvg.expMovingAvgOf(expression);
+			return new ExpMovingAvgBuilder() {
+
+				@Override
+				public ExpMovingAvg historicalDocuments(int numberOfHistoricalDocuments) {
+					return expMovingAvg.n(numberOfHistoricalDocuments);
+				}
+
+				@Override
+				public ExpMovingAvg alpha(double exponentialDecayValue) {
+					return expMovingAvg.alpha(exponentialDecayValue);
+				}
+			};
+		}
+
 		private boolean usesFieldRef() {
 			return fieldReference != null;
 		}
 	}
 
+	/**
+	 * Builder for {@link ExpMovingAvg}.
+	 *
+	 * @since 3.3
+	 */
+	public interface ExpMovingAvgBuilder {
+
+		/**
+		 * Define the number of historical documents with significant mathematical weight.
+		 *
+		 * @param numberOfHistoricalDocuments
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		ExpMovingAvg historicalDocuments(int numberOfHistoricalDocuments);
+
+		/**
+		 * Define the exponential decay value.
+		 *
+		 * @param exponentialDecayValue
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		ExpMovingAvg alpha(double exponentialDecayValue);
+
+	}
+
 	/**
 	 * {@link AggregationExpression} for {@code $sum}.
 	 *
@@ -658,4 +765,185 @@ public class AccumulatorOperators {
 			return super.toDocument(value, context);
 		}
 	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $covariancePop}.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class CovariancePop extends AbstractAggregationExpression {
+
+		private CovariancePop(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Creates new {@link CovariancePop}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 */
+		public static CovariancePop covariancePopOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new CovariancePop(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link CovariancePop}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 */
+		public static CovariancePop covariancePopOf(AggregationExpression expression) {
+			return new CovariancePop(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link CovariancePop} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 */
+		public CovariancePop and(String fieldReference) {
+			return new CovariancePop(append(asFields(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link CovariancePop} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link CovariancePop}.
+		 */
+		public CovariancePop and(AggregationExpression expression) {
+			return new CovariancePop(append(expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$covariancePop";
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $covarianceSamp}.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class CovarianceSamp extends AbstractAggregationExpression {
+
+		private CovarianceSamp(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Creates new {@link CovarianceSamp}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link CovarianceSamp}.
+		 */
+		public static CovarianceSamp covarianceSampOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new CovarianceSamp(asFields(fieldReference));
+		}
+
+		/**
+		 * Creates new {@link CovarianceSamp}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link CovarianceSamp}.
+		 */
+		public static CovarianceSamp covarianceSampOf(AggregationExpression expression) {
+			return new CovarianceSamp(Collections.singletonList(expression));
+		}
+
+		/**
+		 * Creates new {@link CovarianceSamp} with all previously added arguments appending the given one.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link CovarianceSamp}.
+		 */
+		public CovarianceSamp and(String fieldReference) {
+			return new CovarianceSamp(append(asFields(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link CovarianceSamp} with all previously added arguments appending the given one.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link CovarianceSamp}.
+		 */
+		public CovarianceSamp and(AggregationExpression expression) {
+			return new CovarianceSamp(append(expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$covarianceSamp";
+		}
+	}
+
+	/**
+	 * {@link ExpMovingAvg} calculates the exponential moving average of numeric values.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class ExpMovingAvg extends AbstractAggregationExpression {
+
+		private ExpMovingAvg(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Create a new {@link ExpMovingAvg} by defining the field holding the value to be used as input.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public static ExpMovingAvg expMovingAvgOf(String fieldReference) {
+			return new ExpMovingAvg(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Create a new {@link ExpMovingAvg} by defining the {@link AggregationExpression expression} to compute the value
+		 * to be used as input.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public static ExpMovingAvg expMovingAvgOf(AggregationExpression expression) {
+			return new ExpMovingAvg(Collections.singletonMap("input", expression));
+		}
+
+		/**
+		 * Define the number of historical documents with significant mathematical weight. <br />
+		 * Specify either {@link #n(int) N} or {@link #alpha(double) aplha}. Not both!
+		 *
+		 * @param numberOfHistoricalDocuments
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public ExpMovingAvg n/*umber of historical documents*/(int numberOfHistoricalDocuments) {
+			return new ExpMovingAvg(append("N", numberOfHistoricalDocuments));
+		}
+
+		/**
+		 * Define the exponential decay value. <br />
+		 * Specify either {@link #alpha(double) aplha} or {@link #n(int) N}. Not both!
+		 *
+		 * @param exponentialDecayValue
+		 * @return new instance of {@link ExpMovingAvg}.
+		 */
+		public ExpMovingAvg alpha(double exponentialDecayValue) {
+			return new ExpMovingAvg(append("alpha", exponentialDecayValue));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$expMovingAvg";
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AddFieldsOperation.java

@@ -201,4 +201,5 @@ public class AddFieldsOperation extends DocumentEnhancingOperation {
 			AddFieldsOperationBuilder withValueOfExpression(String operation, Object... values);
 		}
 	}
+
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Aggregation.java

@@ -227,7 +227,7 @@ public class Aggregation {
 
 	/**
 	 * Obtain an {@link AddFieldsOperationBuilder builder} instance to create a new {@link AddFieldsOperation}.
-	 * <p/>
+	 * <br />
 	 * Starting in version 4.2, MongoDB adds a new aggregation pipeline stage {@link AggregationUpdate#set $set} that is
 	 * an alias for {@code $addFields}.
 	 *
@@ -499,6 +499,17 @@ public class Aggregation {
 		return new MatchOperation(criteria);
 	}
 
+	/**
+	 * Creates a new {@link MatchOperation} using the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link MatchOperation}.
+	 * @since 3.3
+	 */
+	public static MatchOperation match(AggregationExpression expression) {
+		return new MatchOperation(expression);
+	}
+
 	/**
 	 * Creates a new {@link GeoNearOperation} instance from the given {@link NearQuery} and the {@code distanceField}. The
 	 * {@code distanceField} defines output field that contains the calculated distance.
@@ -715,7 +726,7 @@ public class Aggregation {
 
 	/**
 	 * Converts this {@link Aggregation} specification to a {@link Document}.
-	 * <p/>
+	 * <br />
 	 * MongoDB requires as of 3.6 cursor-based aggregation. Use {@link #toPipeline(AggregationOperationContext)} to render
 	 * an aggregation pipeline.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationSpELExpression.java

@@ -24,15 +24,15 @@ import org.springframework.util.Assert;
  * expression</a>. <br />
  * <br />
  * <strong>Samples:</strong> <br />
- * <code>
  * <pre>
+ * <code>
  * // { $and: [ { $gt: [ "$qty", 100 ] }, { $lt: [ "$qty", 250 ] } ] }
  * expressionOf("qty > 100 && qty < 250);
  *
  * // { $cond : { if : { $gte : [ "$a", 42 ]}, then : "answer", else : "no-answer" } }
  * expressionOf("cond(a >= 42, 'answer', 'no-answer')");
- * </pre>
  * </code>
+ * </pre>
  *
  * @author Christoph Strobl
  * @author Mark Paluch

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationUpdate.java

@@ -71,8 +71,7 @@ import org.springframework.util.Assert;
  *
  * @author Christoph Strobl
  * @author Mark Paluch
- * @see <a href=
- *      "https://docs.mongodb.com/manual/reference/method/db.collection.update/#update-with-aggregation-pipeline">MongoDB
+ * @see <a href="https://docs.mongodb.com/manual/reference/method/db.collection.update/#update-with-aggregation-pipeline">MongoDB
  *      Reference Documentation</a>
  * @since 3.0
  */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BucketAutoOperation.java

@@ -28,8 +28,7 @@ import org.springframework.util.Assert;
  * We recommend to use the static factory method {@link Aggregation#bucketAuto(String, int)} instead of creating
  * instances of this class directly.
  *
- * @see <a href=
- *      "https://docs.mongodb.org/manual/reference/aggregation/bucketAuto/">https://docs.mongodb.org/manual/reference/aggregation/bucketAuto/</a>
+ * @see <a href="https://docs.mongodb.org/manual/reference/aggregation/bucketAuto/">https://docs.mongodb.org/manual/reference/aggregation/bucketAuto/</a>
  * @see BucketOperationSupport
  * @author Mark Paluch
  * @author Christoph Strobl
@@ -248,8 +247,7 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 	/**
 	 * Supported MongoDB granularities.
 	 *
-	 * @see <a
-	 *      href="https://docs.mongodb.com/manual/reference/operator/aggregation/bucketAuto/#granularity>https://docs.mongodb.com/manual/reference/operator/aggregation/bucketAuto/#granularity</a>
+	 * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/bucketAuto/#granularity">https://docs.mongodb.com/manual/reference/operator/aggregation/bucketAuto/#granularity</a>
 	 * @author Mark Paluch
 	 */
 	public enum Granularities implements Granularity {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BucketOperation.java

@@ -31,8 +31,7 @@ import org.springframework.util.Assert;
  * We recommend to use the static factory method {@link Aggregation#bucket(String)} instead of creating instances of
  * this class directly.
  *
- * @see <a href=
- *      "https://docs.mongodb.org/manual/reference/aggregation/bucket/">https://docs.mongodb.org/manual/reference/aggregation/bucket/</a>
+ * @see <a href="https://docs.mongodb.org/manual/reference/aggregation/bucket/">https://docs.mongodb.org/manual/reference/aggregation/bucket/</a>
  * @see BucketOperationSupport
  * @author Mark Paluch
  * @since 1.10

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ConditionalOperators.java

@@ -17,6 +17,7 @@ package org.springframework.data.mongodb.core.aggregation;
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
@@ -235,7 +236,7 @@ public class ConditionalOperators {
 	 *
 	 * @author Mark Paluch
 	 * @see <a href=
-	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/">https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/</a>
+	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/">https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/</a>
 	 */
 	public static class IfNull implements AggregationExpression {
 
@@ -251,7 +252,8 @@ public class ConditionalOperators {
 		/**
 		 * Creates new {@link IfNull}.
 		 *
-		 * @param fieldReference the field to check for a {@literal null} value, field reference must not be {@literal null}.
+		 * @param fieldReference the field to check for a {@literal null} value, field reference must not be
+		 *          {@literal null}.
 		 * @return never {@literal null}.
 		 */
 		public static ThenBuilder ifNull(String fieldReference) {
@@ -264,7 +266,7 @@ public class ConditionalOperators {
 		 * Creates new {@link IfNull}.
 		 *
 		 * @param expression the expression to check for a {@literal null} value, field reference must not be
-		 * {@literal null}.
+		 *          {@literal null}.
 		 * @return never {@literal null}.
 		 */
 		public static ThenBuilder ifNull(AggregationExpression expression) {
@@ -282,19 +284,29 @@ public class ConditionalOperators {
 
 			List<Object> list = new ArrayList<Object>();
 
-			if (condition instanceof Field) {
-				list.add(context.getReference((Field) condition).toString());
-			} else if (condition instanceof AggregationExpression) {
-				list.add(((AggregationExpression) condition).toDocument(context));
+			if (condition instanceof Collection) {
+				for (Object val : ((Collection) this.condition)) {
+					list.add(mapCondition(val, context));
+				}
 			} else {
-				list.add(condition);
+				list.add(mapCondition(condition, context));
 			}
 
 			list.add(resolve(value, context));
-
 			return new Document("$ifNull", list);
 		}
 
+		private Object mapCondition(Object condition, AggregationOperationContext context) {
+
+			if (condition instanceof Field) {
+				return context.getReference((Field) condition).toString();
+			} else if (condition instanceof AggregationExpression) {
+				return ((AggregationExpression) condition).toDocument(context);
+			} else {
+				return condition;
+			}
+		}
+
 		private Object resolve(Object value, AggregationOperationContext context) {
 
 			if (value instanceof Field) {
@@ -315,28 +327,48 @@ public class ConditionalOperators {
 
 			/**
 			 * @param fieldReference the field to check for a {@literal null} value, field reference must not be
-			 * {@literal null}.
+			 *          {@literal null}.
 			 * @return the {@link ThenBuilder}
 			 */
 			ThenBuilder ifNull(String fieldReference);
 
 			/**
 			 * @param expression the expression to check for a {@literal null} value, field name must not be {@literal null}
-			 * or empty.
-			 * @return the {@link ThenBuilder}
+			 *          or empty.
+			 * @return the {@link ThenBuilder}.
 			 */
 			ThenBuilder ifNull(AggregationExpression expression);
 		}
 
+		/**
+		 * @author Christoph Strobl
+		 * @since 3.3
+		 */
+		public interface OrBuilder {
+
+			/**
+			 * @param fieldReference the field to check for a {@literal null} value, field reference must not be
+			 *          {@literal null}.
+			 * @return the {@link ThenBuilder}
+			 */
+			ThenBuilder orIfNull(String fieldReference);
+
+			/**
+			 * @param expression the expression to check for a {@literal null} value,
+			 * @return the {@link ThenBuilder}.
+			 */
+			ThenBuilder orIfNull(AggregationExpression expression);
+		}
+
 		/**
 		 * @author Mark Paluch
 		 */
-		public interface ThenBuilder {
+		public interface ThenBuilder extends OrBuilder {
 
 			/**
 			 * @param value the value to be used if the {@code $ifNull} condition evaluates {@literal true}. Can be a
-			 * {@link Document}, a value that is supported by MongoDB or a value that can be converted to a MongoDB
-			 * representation but must not be {@literal null}.
+			 *          {@link Document}, a value that is supported by MongoDB or a value that can be converted to a MongoDB
+			 *          representation but must not be {@literal null}.
 			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull then(Object value);
@@ -361,9 +393,10 @@ public class ConditionalOperators {
 		 */
 		static final class IfNullOperatorBuilder implements IfNullBuilder, ThenBuilder {
 
-			private @Nullable Object condition;
+			private @Nullable List<Object> conditions;
 
 			private IfNullOperatorBuilder() {
+				conditions = new ArrayList<>();
 			}
 
 			/**
@@ -381,7 +414,7 @@ public class ConditionalOperators {
 			public ThenBuilder ifNull(String fieldReference) {
 
 				Assert.hasText(fieldReference, "FieldReference name must not be null or empty!");
-				this.condition = Fields.field(fieldReference);
+				this.conditions.add(Fields.field(fieldReference));
 				return this;
 			}
 
@@ -392,15 +425,25 @@ public class ConditionalOperators {
 			public ThenBuilder ifNull(AggregationExpression expression) {
 
 				Assert.notNull(expression, "AggregationExpression name must not be null or empty!");
-				this.condition = expression;
+				this.conditions.add(expression);
 				return this;
 			}
 
+			@Override
+			public ThenBuilder orIfNull(String fieldReference) {
+				return ifNull(fieldReference);
+			}
+
+			@Override
+			public ThenBuilder orIfNull(AggregationExpression expression) {
+				return ifNull(expression);
+			}
+
 			/* (non-Javadoc)
 			 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperators.IfNull.ThenBuilder#then(java.lang.Object)
 			 */
 			public IfNull then(Object value) {
-				return new IfNull(condition, value);
+				return new IfNull(conditions, value);
 			}
 
 			/* (non-Javadoc)
@@ -409,7 +452,7 @@ public class ConditionalOperators {
 			public IfNull thenValueOf(String fieldReference) {
 
 				Assert.notNull(fieldReference, "FieldReference must not be null!");
-				return new IfNull(condition, Fields.field(fieldReference));
+				return new IfNull(conditions, Fields.field(fieldReference));
 			}
 
 			/* (non-Javadoc)
@@ -418,7 +461,7 @@ public class ConditionalOperators {
 			public IfNull thenValueOf(AggregationExpression expression) {
 
 				Assert.notNull(expression, "Expression must not be null!");
-				return new IfNull(condition, expression);
+				return new IfNull(conditions, expression);
 			}
 		}
 	}
@@ -458,7 +501,7 @@ public class ConditionalOperators {
 		public static Switch switchCases(List<CaseOperator> conditions) {
 
 			Assert.notNull(conditions, "Conditions must not be null!");
-			return new Switch(Collections.<String, Object>singletonMap("branches", new ArrayList<CaseOperator>(conditions)));
+			return new Switch(Collections.<String, Object> singletonMap("branches", new ArrayList<CaseOperator>(conditions)));
 		}
 
 		/**
@@ -545,7 +588,7 @@ public class ConditionalOperators {
 	 * @author Mark Paluch
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/cond/">https://docs.mongodb.com/manual/reference/operator/aggregation/cond/</a>
+	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/cond/">https://docs.mongodb.com/manual/reference/operator/aggregation/cond/</a>
 	 */
 	public static class Cond implements AggregationExpression {
 
@@ -806,8 +849,8 @@ public class ConditionalOperators {
 
 			/**
 			 * @param value the value to be used if the condition evaluates {@literal true}. Can be a {@link Document}, a
-			 * value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
-			 * must not be {@literal null}.
+			 *          value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
+			 *          must not be {@literal null}.
 			 * @return the {@link OtherwiseBuilder}
 			 */
 			OtherwiseBuilder then(Object value);
@@ -832,8 +875,8 @@ public class ConditionalOperators {
 
 			/**
 			 * @param value the value to be used if the condition evaluates {@literal false}. Can be a {@link Document}, a
-			 * value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
-			 * must not be {@literal null}.
+			 *          value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
+			 *          must not be {@literal null}.
 			 * @return the {@link Cond}
 			 */
 			Cond otherwise(Object value);
@@ -861,8 +904,7 @@ public class ConditionalOperators {
 			private @Nullable Object condition;
 			private @Nullable Object thenValue;
 
-			private ConditionalExpressionBuilder() {
-			}
+			private ConditionalExpressionBuilder() {}
 
 			/**
 			 * Creates a new builder for {@link Cond}.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ConvertOperators.java

@@ -231,6 +231,17 @@ public class ConvertOperators {
 			return ToString.toString(valueObject());
 		}
 
+		/**
+		 * {@link AggregationExpression} for {@code $degreesToRadians} that converts an input value measured in degrees to
+		 * radians.
+		 *
+		 * @return new instance of {@link DegreesToRadians}.
+		 * @since 3.3
+		 */
+		public DegreesToRadians convertDegreesToRadians() {
+			return DegreesToRadians.degreesToRadians(valueObject());
+		}
+
 		private Convert createConvert() {
 			return usesFieldRef() ? Convert.convertValueOf(fieldReference) : Convert.convertValueOf(expression);
 		}
@@ -317,9 +328,9 @@ public class ConvertOperators {
 		 * <dt>1</dt>
 		 * <dd>double</dd>
 		 * <dt>2</dt>
-		 * <dd>string</li>
+		 * <dd>string</dd>
 		 * <dt>7</dt>
-		 * <dd>objectId</li>
+		 * <dd>objectId</dd>
 		 * <dt>8</dt>
 		 * <dd>bool</dd>
 		 * <dt>9</dt>
@@ -692,4 +703,52 @@ public class ConvertOperators {
 			return "$toString";
 		}
 	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $degreesToRadians} that converts an input value measured in degrees to radians.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DegreesToRadians extends AbstractAggregationExpression {
+
+		private DegreesToRadians(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Create a new instance of {@link DegreesToRadians} that converts the value of the given field, measured in degrees, to radians.
+		 *
+		 * @param fieldName must not be {@literal null}.
+		 * @return new instance of {@link DegreesToRadians}.
+		 */
+		public static DegreesToRadians degreesToRadiansOf(String fieldName) {
+			return degreesToRadians(Fields.field(fieldName));
+		}
+
+		/**
+		 * Create a new instance of {@link DegreesToRadians} that converts the result of the given {@link AggregationExpression expression}, measured in degrees, to radians.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link DegreesToRadians}.
+		 */
+		public static DegreesToRadians degreesToRadiansOf(AggregationExpression expression) {
+			return degreesToRadians(expression);
+		}
+
+		/**
+		 * Create a new instance of {@link DegreesToRadians} that converts the given value, measured in degrees, to radians.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link DegreesToRadians}.
+		 */
+		public static DegreesToRadians degreesToRadians(Object value) {
+			return new DegreesToRadians(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$degreesToRadians";
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/CountOperation.java

@@ -24,8 +24,7 @@ import org.springframework.util.Assert;
  * We recommend to use the static factory method {@link Aggregation#count()} instead of creating instances of this class
  * directly.
  *
- * @see <a href=
- *      "https://docs.mongodb.com/manual/reference/operator/aggregation/count/#pipe._S_count">https://docs.mongodb.com/manual/reference/operator/aggregation/count/</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/count/#pipe._S_count">https://docs.mongodb.com/manual/reference/operator/aggregation/count/</a>
  * @author Mark Paluch
  * @since 1.10
  */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DateOperators.java

@@ -15,9 +15,16 @@
  */
 package org.springframework.data.mongodb.core.aggregation;
 
+import java.time.ZoneId;
+import java.time.ZoneOffset;
+import java.time.temporal.ChronoUnit;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.LinkedHashMap;
+import java.util.Locale;
 import java.util.Map;
+import java.util.TimeZone;
+import java.util.concurrent.TimeUnit;
 
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -45,6 +52,19 @@ public class DateOperators {
 		return new DateOperatorFactory(fieldReference);
 	}
 
+	/**
+	 * Take the date referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return new instance of {@link DateOperatorFactory}.
+	 * @since 3.3
+	 */
+	public static DateOperatorFactory zonedDateOf(String fieldReference, Timezone timezone) {
+
+		Assert.notNull(fieldReference, "FieldReference must not be null!");
+		return new DateOperatorFactory(fieldReference).withTimezone(timezone);
+	}
+
 	/**
 	 * Take the date resulting from the given {@link AggregationExpression}.
 	 *
@@ -57,9 +77,22 @@ public class DateOperators {
 		return new DateOperatorFactory(expression);
 	}
 
+	/**
+	 * Take the date resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link DateOperatorFactory}.
+	 * @since 3.3
+	 */
+	public static DateOperatorFactory zonedDateOf(AggregationExpression expression, Timezone timezone) {
+
+		Assert.notNull(expression, "Expression must not be null!");
+		return new DateOperatorFactory(expression).withTimezone(timezone);
+	}
+
 	/**
 	 * Take the given value as date.
-	 * <p/>
+	 * <br />
 	 * This can be one of:
 	 * <ul>
 	 * <li>{@link java.util.Date}</li>
@@ -109,7 +142,7 @@ public class DateOperators {
 	 * Timezone represents a MongoDB timezone abstraction which can be represented with a timezone ID or offset as a
 	 * {@link String}. Also accepts a {@link AggregationExpression} or {@link Field} that resolves to a {@link String} of
 	 * either Olson Timezone Identifier or a UTC Offset.<br />
-	 * <table valign="top">
+	 * <table>
 	 * <tr>
 	 * <th>Format</th>
 	 * <th>Example</th>
@@ -130,6 +163,7 @@ public class DateOperators {
 	 * <strong>NOTE: </strong>Support for timezones in aggregations Requires MongoDB 3.6 or later.
 	 *
 	 * @author Christoph Strobl
+	 * @author Mark Paluch
 	 * @since 2.1
 	 */
 	public static class Timezone {
@@ -156,7 +190,7 @@ public class DateOperators {
 		 * representing an Olson Timezone Identifier or UTC Offset.
 		 *
 		 * @param value the plain timezone {@link String}, a {@link Field} holding the timezone or an
-		 *          {@link AggregationExpression} resulting in the timezone.
+		 * {@link AggregationExpression} resulting in the timezone.
 		 * @return new instance of {@link Timezone}.
 		 */
 		public static Timezone valueOf(Object value) {
@@ -165,6 +199,61 @@ public class DateOperators {
 			return new Timezone(value);
 		}
 
+		/**
+		 * Create a {@link Timezone} for the given {@link TimeZone} rendering the offset as UTC offset.
+		 *
+		 * @param timeZone {@link TimeZone} rendering the offset as UTC offset.
+		 * @return new instance of {@link Timezone}.
+		 * @since 3.3
+		 */
+		public static Timezone fromOffset(TimeZone timeZone) {
+
+			Assert.notNull(timeZone, "TimeZone must not be null!");
+
+			return fromOffset(
+					ZoneOffset.ofTotalSeconds(Math.toIntExact(TimeUnit.MILLISECONDS.toSeconds(timeZone.getRawOffset()))));
+		}
+
+		/**
+		 * Create a {@link Timezone} for the given {@link ZoneOffset} rendering the offset as UTC offset.
+		 *
+		 * @param offset {@link ZoneOffset} rendering the offset as UTC offset.
+		 * @return new instance of {@link Timezone}.
+		 * @since 3.3
+		 */
+		public static Timezone fromOffset(ZoneOffset offset) {
+
+			Assert.notNull(offset, "ZoneOffset must not be null!");
+			return new Timezone(offset.toString());
+		}
+
+		/**
+		 * Create a {@link Timezone} for the given {@link TimeZone} rendering the offset as UTC offset.
+		 *
+		 * @param timeZone {@link Timezone} rendering the offset as zone identifier.
+		 * @return new instance of {@link Timezone}.
+		 * @since 3.3
+		 */
+		public static Timezone fromZone(TimeZone timeZone) {
+
+			Assert.notNull(timeZone, "TimeZone must not be null!");
+
+			return valueOf(timeZone.getID());
+		}
+
+		/**
+		 * Create a {@link Timezone} for the given {@link java.time.ZoneId} rendering the offset as UTC offset.
+		 *
+		 * @param zoneId {@link ZoneId} rendering the offset as zone identifier.
+		 * @return new instance of {@link Timezone}.
+		 * @since 3.3
+		 */
+		public static Timezone fromZone(ZoneId zoneId) {
+
+			Assert.notNull(zoneId, "ZoneId must not be null!");
+			return new Timezone(zoneId.toString());
+		}
+
 		/**
 		 * Create a {@link Timezone} for the {@link Field} reference holding the Olson Timezone Identifier or UTC Offset.
 		 *
@@ -185,6 +274,11 @@ public class DateOperators {
 		public static Timezone ofExpression(AggregationExpression expression) {
 			return valueOf(expression);
 		}
+
+		@Nullable
+		Object getValue() {
+			return value;
+		}
 	}
 
 	/**
@@ -240,7 +334,7 @@ public class DateOperators {
 
 		/**
 		 * Creates new {@link DateOperatorFactory} for given {@code value} that resolves to a Date.
-		 * <p/>
+		 * <br />
 		 * <ul>
 		 * <li>{@link java.util.Date}</li>
 		 * <li>{@link java.util.Calendar}</li>
@@ -274,6 +368,89 @@ public class DateOperators {
 			return new DateOperatorFactory(fieldReference, expression, dateValue, timezone);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that adds the value of the given {@link AggregationExpression
+		 * expression} (in {@literal units}).
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateAdd addValueOf(AggregationExpression expression, String unit) {
+			return applyTimezone(DateAdd.addValueOf(expression, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that adds the value of the given {@link AggregationExpression
+		 * expression} (in {@literal units}).
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateAdd addValueOf(AggregationExpression expression, TemporalUnit unit) {
+
+			Assert.notNull(unit, "TemporalUnit must not be null");
+			return applyTimezone(DateAdd.addValueOf(expression, unit.name().toLowerCase(Locale.ROOT)).toDate(dateReference()),
+					timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that adds the value stored at the given {@literal field} (in
+		 * {@literal units}).
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateAdd addValueOf(String fieldReference, String unit) {
+			return applyTimezone(DateAdd.addValueOf(fieldReference, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that adds the value stored at the given {@literal field} (in
+		 * {@literal units}).
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateAdd addValueOf(String fieldReference, TemporalUnit unit) {
+
+			Assert.notNull(unit, "TemporalUnit must not be null");
+
+			return applyTimezone(
+					DateAdd.addValueOf(fieldReference, unit.name().toLowerCase(Locale.ROOT)).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that adds the given value (in {@literal units}).
+		 *
+		 * @param value must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return
+		 * @since 3.3 new instance of {@link DateAdd}.
+		 */
+		public DateAdd add(Object value, String unit) {
+			return applyTimezone(DateAdd.addValue(value, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that adds the given value (in {@literal units}).
+		 *
+		 * @param value must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return
+		 * @since 3.3 new instance of {@link DateAdd}.
+		 */
+		public DateAdd add(Object value, TemporalUnit unit) {
+
+			Assert.notNull(unit, "TemporalUnit must not be null");
+
+			return applyTimezone(DateAdd.addValue(value, unit.name().toLowerCase(Locale.ROOT)).toDate(dateReference()),
+					timezone);
+		}
+
 		/**
 		 * Creates new {@link AggregationExpression} that returns the day of the year for a date as a number between 1 and
 		 * 366.
@@ -304,6 +481,90 @@ public class DateOperators {
 			return applyTimezone(DayOfWeek.dayOfWeek(dateReference()), timezone);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units}) to the date
+		 * computed by the given {@link AggregationExpression expression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateDiff diffValueOf(AggregationExpression expression, String unit) {
+			return applyTimezone(DateDiff.diffValueOf(expression, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units}) to the date
+		 * computed by the given {@link AggregationExpression expression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateDiff diffValueOf(AggregationExpression expression, TemporalUnit unit) {
+
+			Assert.notNull(unit, "TemporalUnit must not be null");
+
+			return applyTimezone(
+					DateDiff.diffValueOf(expression, unit.name().toLowerCase(Locale.ROOT)).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units}) to the date stored
+		 * at the given {@literal field}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateDiff diffValueOf(String fieldReference, String unit) {
+			return applyTimezone(DateDiff.diffValueOf(fieldReference, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units}) to the date stored
+		 * at the given {@literal field}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateDiff diffValueOf(String fieldReference, TemporalUnit unit) {
+
+			Assert.notNull(unit, "TemporalUnit must not be null");
+
+			return applyTimezone(
+					DateDiff.diffValueOf(fieldReference, unit.name().toLowerCase(Locale.ROOT)).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units}) to the date given
+		 * {@literal value}.
+		 *
+		 * @param value anything the resolves to a valid date. Must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateDiff diff(Object value, String unit) {
+			return applyTimezone(DateDiff.diffValue(value, unit).toDate(dateReference()), timezone);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that calculates the difference (in {@literal units}) to the date given
+		 * {@literal value}.
+		 *
+		 * @param value anything the resolves to a valid date. Must not be {@literal null}.
+		 * @param unit the unit of measure. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}. @since 3.3
+		 */
+		public DateDiff diff(Object value, TemporalUnit unit) {
+
+			Assert.notNull(unit, "TemporalUnit must not be null");
+
+			return applyTimezone(DateDiff.diffValue(value, unit.name().toLowerCase(Locale.ROOT)).toDate(dateReference()),
+					timezone);
+		}
+
 		/**
 		 * Creates new {@link AggregationExpression} that returns the year portion of a date.
 		 *
@@ -1480,7 +1741,6 @@ public class DateOperators {
 				} else {
 					clone.put("timezone", ((Timezone) value).value);
 				}
-
 			} else {
 				clone.put(key, value);
 			}
@@ -1911,7 +2171,7 @@ public class DateOperators {
 	 * @author Matt Morrissette
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/</a>
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/</a>
 	 * @since 2.1
 	 */
 	public static class DateFromParts extends TimezonedDateAggregationExpression implements DateParts<DateFromParts> {
@@ -2086,7 +2346,7 @@ public class DateOperators {
 	 * @author Matt Morrissette
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/</a>
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromParts/</a>
 	 * @since 2.1
 	 */
 	public static class IsoDateFromParts extends TimezonedDateAggregationExpression
@@ -2262,7 +2522,7 @@ public class DateOperators {
 	 * @author Matt Morrissette
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/dateToParts/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateToParts/</a>
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/dateToParts/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateToParts/</a>
 	 * @since 2.1
 	 */
 	public static class DateToParts extends TimezonedDateAggregationExpression {
@@ -2343,7 +2603,7 @@ public class DateOperators {
 	 * @author Matt Morrissette
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromString/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromString/</a>
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromString/">https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromString/</a>
 	 * @since 2.1
 	 */
 	public static class DateFromString extends TimezonedDateAggregationExpression {
@@ -2418,6 +2678,290 @@ public class DateOperators {
 		}
 	}
 
+	/**
+	 * {@link AggregationExpression} for {@code $dateAdd}.<br />
+	 * <strong>NOTE:</strong> Requires MongoDB 5.0 or later.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DateAdd extends TimezonedDateAggregationExpression {
+
+		private DateAdd(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Add the number of {@literal units} of the result of the given {@link AggregationExpression expression} to a
+		 * {@link #toDate(Object) start date}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateAdd addValueOf(AggregationExpression expression, String unit) {
+			return addValue(expression, unit);
+		}
+
+		/**
+		 * Add the number of {@literal units} from a {@literal field} to a {@link #toDate(Object) start date}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateAdd addValueOf(String fieldReference, String unit) {
+			return addValue(Fields.field(fieldReference), unit);
+		}
+
+		/**
+		 * Add the number of {@literal units} to a {@link #toDate(Object) start date}.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateAdd addValue(Object value, String unit) {
+
+			Map<String, Object> args = new HashMap<>();
+			args.put("unit", unit);
+			args.put("amount", value);
+			return new DateAdd(args);
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateAdd toDateOf(AggregationExpression expression) {
+			return toDate(expression);
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateAdd toDateOf(String fieldReference) {
+			return toDate(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param dateExpression anything that evaluates to a valid date. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateAdd toDate(Object dateExpression) {
+			return new DateAdd(append("startDate", dateExpression));
+		}
+
+		/**
+		 * Optionally set the {@link Timezone} to use. If not specified {@literal UTC} is used.
+		 *
+		 * @param timezone must not be {@literal null}. Consider {@link Timezone#none()} instead.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateAdd withTimezone(Timezone timezone) {
+			return new DateAdd(appendTimezone(argumentMap(), timezone));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dateAdd";
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $dateDiff}.<br />
+	 * <strong>NOTE:</strong> Requires MongoDB 5.0 or later.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DateDiff extends TimezonedDateAggregationExpression {
+
+		private DateDiff(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Add the number of {@literal units} of the result of the given {@link AggregationExpression expression} to a
+		 * {@link #toDate(Object) start date}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateDiff diffValueOf(AggregationExpression expression, String unit) {
+			return diffValue(expression, unit);
+		}
+
+		/**
+		 * Add the number of {@literal units} from a {@literal field} to a {@link #toDate(Object) start date}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateDiff diffValueOf(String fieldReference, String unit) {
+			return diffValue(Fields.field(fieldReference), unit);
+		}
+
+		/**
+		 * Add the number of {@literal units} to a {@link #toDate(Object) start date}.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @param unit must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public static DateDiff diffValue(Object value, String unit) {
+
+			Map<String, Object> args = new HashMap<>();
+			args.put("unit", unit);
+			args.put("endDate", value);
+			return new DateDiff(args);
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff toDateOf(AggregationExpression expression) {
+			return toDate(expression);
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff toDateOf(String fieldReference) {
+			return toDate(Fields.field(fieldReference));
+		}
+
+		/**
+		 * Define the start date, in UTC, for the addition operation.
+		 *
+		 * @param dateExpression anything that evaluates to a valid date. Must not be {@literal null}.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff toDate(Object dateExpression) {
+			return new DateDiff(append("startDate", dateExpression));
+		}
+
+		/**
+		 * Optionally set the {@link Timezone} to use. If not specified {@literal UTC} is used.
+		 *
+		 * @param timezone must not be {@literal null}. Consider {@link Timezone#none()} instead.
+		 * @return new instance of {@link DateAdd}.
+		 */
+		public DateDiff withTimezone(Timezone timezone) {
+			return new DateDiff(appendTimezone(argumentMap(), timezone));
+		}
+
+		/**
+		 * Set the start day of the week if the unit if measure is set to {@literal week}. Uses {@literal Sunday} by
+		 * default.
+		 *
+		 * @param day must not be {@literal null}.
+		 * @return new instance of {@link DateDiff}.
+		 */
+		public DateDiff startOfWeek(Object day) {
+			return new DateDiff(append("startOfWeek", day));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$dateDiff";
+		}
+	}
+
+	/**
+	 * Interface defining a temporal unit for date operators.
+	 *
+	 * @author Mark Paluch
+	 * @since 3.3
+	 */
+	public interface TemporalUnit {
+
+		String name();
+
+		/**
+		 * Converts the given time unit into a {@link TemporalUnit}. Supported units are: days, hours, minutes, seconds, and
+		 * milliseconds.
+		 *
+		 * @param timeUnit the time unit to convert, must not be {@literal null}.
+		 * @return
+		 * @throws IllegalArgumentException if the {@link TimeUnit} is {@literal null} or not supported for conversion.
+		 */
+		static TemporalUnit from(TimeUnit timeUnit) {
+
+			Assert.notNull(timeUnit, "TimeUnit must not be null");
+
+			switch (timeUnit) {
+				case DAYS:
+					return TemporalUnits.DAY;
+				case HOURS:
+					return TemporalUnits.HOUR;
+				case MINUTES:
+					return TemporalUnits.MINUTE;
+				case SECONDS:
+					return TemporalUnits.SECOND;
+				case MILLISECONDS:
+					return TemporalUnits.MILLISECOND;
+			}
+
+			throw new IllegalArgumentException(String.format("Cannot create TemporalUnit from %s", timeUnit));
+		}
+
+		/**
+		 * Converts the given chrono unit into a {@link TemporalUnit}. Supported units are: years, weeks, months, days,
+		 * hours, minutes, seconds, and millis.
+		 *
+		 * @param chronoUnit the chrono unit to convert, must not be {@literal null}.
+		 * @return
+		 * @throws IllegalArgumentException if the {@link TimeUnit} is {@literal null} or not supported for conversion.
+		 */
+		static TemporalUnit from(ChronoUnit chronoUnit) {
+
+			switch (chronoUnit) {
+				case YEARS:
+					return TemporalUnits.YEAR;
+				case WEEKS:
+					return TemporalUnits.WEEK;
+				case MONTHS:
+					return TemporalUnits.MONTH;
+				case DAYS:
+					return TemporalUnits.DAY;
+				case HOURS:
+					return TemporalUnits.HOUR;
+				case MINUTES:
+					return TemporalUnits.MINUTE;
+				case SECONDS:
+					return TemporalUnits.SECOND;
+				case MILLIS:
+					return TemporalUnits.MILLISECOND;
+			}
+
+			throw new IllegalArgumentException(String.format("Cannot create TemporalUnit from %s", chronoUnit));
+		}
+	}
+
+	/**
+	 * Supported temporal units.
+	 */
+	enum TemporalUnits implements TemporalUnit {
+		YEAR, QUARTER, WEEK, MONTH, DAY, HOUR, MINUTE, SECOND, MILLISECOND
+
+	}
+
 	@SuppressWarnings("unchecked")
 	private static <T extends TimezonedDateAggregationExpression> T applyTimezone(T instance, Timezone timezone) {
 		return !ObjectUtils.nullSafeEquals(Timezone.none(), timezone) && !instance.hasTimezone()

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DocumentOperators.java

@@ -0,0 +1,222 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.Collections;
+
+import org.bson.Document;
+
+/**
+ * Gateway to {@literal document expressions} such as {@literal $rank, $documentNumber, etc.}
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public class DocumentOperators {
+
+	/**
+	 * Obtain the document position (including gaps) relative to others (rank).
+	 *
+	 * @return new instance of {@link Rank}.
+	 * @since 3.3
+	 */
+	public static Rank rank() {
+		return new Rank();
+	}
+
+	/**
+	 * Obtain the document position (without gaps) relative to others (rank).
+	 *
+	 * @return new instance of {@link DenseRank}.
+	 * @since 3.3
+	 */
+	public static DenseRank denseRank() {
+		return new DenseRank();
+	}
+
+	/**
+	 * Take the field referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return new instance of {@link DocumentOperatorsFactory}.
+	 */
+	public static DocumentOperatorsFactory valueOf(String fieldReference) {
+		return new DocumentOperatorsFactory(fieldReference);
+	}
+
+	/**
+	 * Take the value resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link DocumentOperatorsFactory}.
+	 */
+	public static DocumentOperatorsFactory valueOf(AggregationExpression expression) {
+		return new DocumentOperatorsFactory(expression);
+	}
+
+	/**
+	 * Obtain the current document position.
+	 *
+	 * @return new instance of {@link DocumentNumber}.
+	 * @since 3.3
+	 */
+	public static DocumentNumber documentNumber() {
+		return new DocumentNumber();
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class DocumentOperatorsFactory {
+
+		private final Object target;
+
+		public DocumentOperatorsFactory(Object target) {
+			this.target = target;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that applies the expression to a document at specified position
+		 * relative to the current document.
+		 *
+		 * @param by the value to add to the current position.
+		 * @return new instance of {@link Shift}.
+		 */
+		public Shift shift(int by) {
+
+			Shift shift = usesExpression() ? Shift.shift((AggregationExpression) target) : Shift.shift(target.toString());
+			return shift.by(by);
+		}
+
+		private boolean usesExpression() {
+			return target instanceof AggregationExpression;
+		}
+	}
+
+	/**
+	 * {@link Rank} resolves the current document position (the rank) relative to other documents. If multiple documents
+	 * occupy the same rank, {@literal $rank} places the document with the subsequent value at a rank with a gap.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class Rank implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$rank", new Document());
+		}
+	}
+
+	/**
+	 * {@link DenseRank} resolves the current document position (the rank) relative to other documents. If multiple
+	 * documents occupy the same rank, {@literal $denseRank} places the document with the subsequent value at the next
+	 * rank without any gaps.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DenseRank implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$denseRank", new Document());
+		}
+	}
+
+	/**
+	 * {@link DocumentNumber} resolves the current document position.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class DocumentNumber implements AggregationExpression {
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return new Document("$documentNumber", new Document());
+		}
+	}
+
+	/**
+	 * Shift applies an expression to a document in a specified position relative to the current document.
+	 *
+	 * @author Christoph Strobl
+	 * @since 3.3
+	 */
+	public static class Shift extends AbstractAggregationExpression {
+
+		private Shift(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Specifies the field to evaluate and return.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Shift}.
+		 */
+		public static Shift shift(String fieldReference) {
+			return new Shift(Collections.singletonMap("output", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Specifies the {@link AggregationExpression expression} to evaluate and return.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Shift}.
+		 */
+		public static Shift shift(AggregationExpression expression) {
+			return new Shift(Collections.singletonMap("output", expression));
+		}
+
+		/**
+		 * Shift the document position relative to the current. Use a positive value for follow up documents (eg. 1 for the
+		 * next) or a negative value for the predecessor documents (eg. -1 for the previous).
+		 *
+		 * @param shiftBy value to add to the current position.
+		 * @return new instance of {@link Shift}.
+		 */
+		public Shift by(int shiftBy) {
+			return new Shift(append("by", shiftBy));
+		}
+
+		/**
+		 * Define the default value if the target document is out of range.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Shift}.
+		 */
+		public Shift defaultTo(Object value) {
+			return new Shift(append("default", value));
+		}
+
+		/**
+		 * Define the {@link AggregationExpression expression} to evaluate if the target document is out of range.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Shift}.
+		 */
+		public Shift defaultToValueOf(AggregationExpression expression) {
+			return defaultTo(expression);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$shift";
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/EvaluationOperators.java

@@ -0,0 +1,155 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.bson.Document;
+
+import org.springframework.data.mongodb.core.query.CriteriaDefinition;
+import org.springframework.util.Assert;
+
+/**
+ * Gateway to {@literal evaluation operators} such as {@literal $expr}.
+ *
+ * @author Divya Srivastava
+ * @since 3.3
+ */
+public class EvaluationOperators {
+
+	/**
+	 * Take the value resulting from the given fieldReference.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return new instance of {@link EvaluationOperatorFactory}.
+	 */
+	public static EvaluationOperatorFactory valueOf(String fieldReference) {
+		return new EvaluationOperatorFactory(fieldReference);
+	}
+
+	/**
+	 * Take the value resulting from the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link EvaluationOperatorFactory}.
+	 */
+	public static EvaluationOperatorFactory valueOf(AggregationExpression expression) {
+		return new EvaluationOperatorFactory(expression);
+	}
+
+	public static class EvaluationOperatorFactory {
+
+		private final String fieldReference;
+		private final AggregationExpression expression;
+
+		/**
+		 * Creates new {@link EvaluationOperatorFactory} for given {@literal fieldReference}.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 */
+		public EvaluationOperatorFactory(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			this.fieldReference = fieldReference;
+			this.expression = null;
+		}
+
+		/**
+		 * Creates new {@link EvaluationOperatorFactory} for given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 */
+		public EvaluationOperatorFactory(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+			this.fieldReference = null;
+			this.expression = expression;
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that is a valid aggregation expression.
+		 *
+		 * @return new instance of {@link Expr}.
+		 */
+		public Expr expr() {
+			return usesFieldRef() ? Expr.valueOf(fieldReference) : Expr.valueOf(expression);
+		}
+
+		/**
+		 * Allows the use of aggregation expressions within the query language.
+		 */
+		public static class Expr extends AbstractAggregationExpression {
+
+			private Expr(Object value) {
+				super(value);
+			}
+
+			@Override
+			protected String getMongoMethod() {
+				return "$expr";
+			}
+
+			/**
+			 * Creates new {@link Expr}.
+			 *
+			 * @param fieldReference must not be {@literal null}.
+			 * @return new instance of {@link Expr}.
+			 */
+			public static Expr valueOf(String fieldReference) {
+
+				Assert.notNull(fieldReference, "FieldReference must not be null!");
+				return new Expr(Fields.field(fieldReference));
+			}
+
+			/**
+			 * Creates new {@link Expr}.
+			 *
+			 * @param expression must not be {@literal null}.
+			 * @return new instance of {@link Expr}.
+			 */
+			public static Expr valueOf(AggregationExpression expression) {
+
+				Assert.notNull(expression, "Expression must not be null!");
+				return new Expr(expression);
+			}
+
+			/**
+			 * Creates {@code $expr} as {@link CriteriaDefinition}.
+			 *
+			 * @return the {@link CriteriaDefinition} from this expression.
+			 */
+			public CriteriaDefinition toCriteriaDefinition(AggregationOperationContext context) {
+
+				Document criteriaObject = toDocument(context);
+
+				return new CriteriaDefinition() {
+					@Override
+					public Document getCriteriaObject() {
+						return criteriaObject;
+					}
+
+					@Override
+					public String getKey() {
+						return getMongoMethod();
+					}
+				};
+			}
+		}
+
+		private boolean usesFieldRef() {
+			return fieldReference != null;
+		}
+	}
+
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GraphLookupOperation.java

@@ -36,8 +36,7 @@ import org.springframework.util.ClassUtils;
  * We recommend to use the static factory method {@link Aggregation#graphLookup(String)} instead of creating instances
  * of this class directly.
  *
- * @see <a href=
- *      "https://docs.mongodb.org/manual/reference/aggregation/graphLookup/">https://docs.mongodb.org/manual/reference/aggregation/graphLookup/</a>
+ * @see <a href="https://docs.mongodb.org/manual/reference/aggregation/graphLookup/">https://docs.mongodb.org/manual/reference/aggregation/graphLookup/</a>
  * @author Mark Paluch
  * @author Christoph Strobl
  * @since 1.10

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java

@@ -139,7 +139,7 @@ public class GroupOperation implements FieldsExposingAggregationOperation {
 	 * Generates an {@link GroupOperationBuilder} for a {@code $sum}-expression.
 	 * <p>
 	 * Count expressions are emulated via {@code $sum: 1}.
-	 * <p>
+	 * </p>
 	 *
 	 * @return
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/MatchOperation.java

@@ -16,6 +16,7 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import org.bson.Document;
+
 import org.springframework.data.mongodb.core.query.CriteriaDefinition;
 import org.springframework.util.Assert;
 
@@ -29,6 +30,7 @@ import org.springframework.util.Assert;
  * @author Sebastian Herold
  * @author Thomas Darimont
  * @author Oliver Gierke
+ * @author Divya Srivastava
  * @since 1.3
  * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/match/">MongoDB Aggregation Framework:
  *      $match</a>
@@ -36,6 +38,7 @@ import org.springframework.util.Assert;
 public class MatchOperation implements AggregationOperation {
 
 	private final CriteriaDefinition criteriaDefinition;
+	private final AggregationExpression expression;
 
 	/**
 	 * Creates a new {@link MatchOperation} for the given {@link CriteriaDefinition}.
@@ -45,7 +48,23 @@ public class MatchOperation implements AggregationOperation {
 	public MatchOperation(CriteriaDefinition criteriaDefinition) {
 
 		Assert.notNull(criteriaDefinition, "Criteria must not be null!");
+
 		this.criteriaDefinition = criteriaDefinition;
+		this.expression = null;
+	}
+
+	/**
+	 * Creates a new {@link MatchOperation} for the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @since 3.3
+	 */
+	public MatchOperation(AggregationExpression expression) {
+
+		Assert.notNull(expression, "Expression must not be null!");
+
+		this.criteriaDefinition = null;
+		this.expression = expression;
 	}
 
 	/*
@@ -54,7 +73,9 @@ public class MatchOperation implements AggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document(getOperator(), context.getMappedObject(criteriaDefinition.getCriteriaObject()));
+
+		return new Document(getOperator(),
+				context.getMappedObject(expression != null ? expression.toDocument() : criteriaDefinition.getCriteriaObject()));
 	}
 
 	/*

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/PrefixingDelegatingAggregationOperationContext.java

@@ -30,7 +30,7 @@ import org.springframework.lang.Nullable;
 /**
  * {@link AggregationOperationContext} implementation prefixing non-command keys on root level with the given prefix.
  * Useful when mapping fields to domain specific types while having to prefix keys for query purpose.
- * <p />
+ * <br />
  * Fields to be excluded from prefixing my be added to a {@literal denylist}.
  *
  * @author Christoph Strobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/RedactOperation.java

@@ -33,8 +33,7 @@ import org.springframework.util.Assert;
  * </pre>
  *
  * @author Christoph Strobl
- * @see <a href=
- *      "https://docs.mongodb.com/manual/reference/operator/aggregation/redact/">https://docs.mongodb.com/manual/reference/operator/aggregation/redact/</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/redact/">https://docs.mongodb.com/manual/reference/operator/aggregation/redact/</a>
  * @since 3.0
  */
 public class RedactOperation implements AggregationOperation {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ReplaceRootOperation.java

@@ -21,6 +21,7 @@ import java.util.Collections;
 import java.util.List;
 
 import org.bson.Document;
+
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.expression.spel.ast.Projection;
 import org.springframework.util.Assert;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ScriptOperators.java

@@ -30,7 +30,7 @@ import org.springframework.util.CollectionUtils;
 
 /**
  * Gateway to {@literal $function} and {@literal $accumulator} aggregation operations.
- * <p />
+ * <br />
  * Using {@link ScriptOperators} as part of the {@link Aggregation} requires MongoDB server to have
  * <a href="https://docs.mongodb.com/master/core/server-side-javascript/">server-side JavaScript</a> execution
  * <a href="https://docs.mongodb.com/master/reference/configuration-options/#security.javascriptEnabled">enabled</a>.
@@ -43,7 +43,7 @@ public class ScriptOperators {
 
 	/**
 	 * Create a custom aggregation
-	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/function/">$function<a /> in JavaScript.
+	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/function/">$function</a> in JavaScript.
 	 *
 	 * @param body The function definition. Must not be {@literal null}.
 	 * @return new instance of {@link Function}.
@@ -53,8 +53,8 @@ public class ScriptOperators {
 	}
 
 	/**
-	 * Create a custom <a href="https://docs.mongodb.com/master/reference/operator/aggregation/accumulator/">$accumulator
-	 * operator</a> in Javascript.
+	 * Create a custom <a href="https://docs.mongodb.com/master/reference/operator/aggregation/accumulator/">$accumulator operator</a>
+	 * in Javascript.
 	 *
 	 * @return new instance of {@link AccumulatorInitBuilder}.
 	 */
@@ -65,7 +65,7 @@ public class ScriptOperators {
 	/**
 	 * {@link Function} defines a custom aggregation
 	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/function/">$function</a> in JavaScript.
-	 * <p />
+	 * <br />
 	 * <code class="java">
 	 * {
 	 *   $function: {
@@ -75,7 +75,7 @@ public class ScriptOperators {
 	 *   }
 	 * }
 	 * </code>
-	 * <p />
+	 * <br />
 	 * {@link Function} cannot be used as part of {@link org.springframework.data.mongodb.core.schema.MongoJsonSchema
 	 * schema} validation query expression. <br />
 	 * <b>NOTE:</b> <a href="https://docs.mongodb.com/master/core/server-side-javascript/">Server-Side JavaScript</a>
@@ -179,7 +179,7 @@ public class ScriptOperators {
 	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/accumulator/">$accumulator operator</a>,
 	 * one that maintains its state (e.g. totals, maximums, minimums, and related data) as documents progress through the
 	 * pipeline, in JavaScript.
-	 * <p />
+	 * <br />
 	 * <code class="java">
 	 * {
 	 *   $accumulator: {
@@ -193,7 +193,7 @@ public class ScriptOperators {
 	 *   }
 	 * }
 	 * </code>
-	 * <p />
+	 * <br />
 	 * {@link Accumulator} can be used as part of {@link GroupOperation $group}, {@link BucketOperation $bucket} and
 	 * {@link BucketAutoOperation $bucketAuto} pipeline stages. <br />
 	 * <b>NOTE:</b> <a href="https://docs.mongodb.com/master/core/server-side-javascript/">Server-Side JavaScript</a>
@@ -241,7 +241,7 @@ public class ScriptOperators {
 			/**
 			 * Define the {@code init} {@link Function} for the {@link Accumulator accumulators} initial state. The function
 			 * receives its arguments from the {@link Function#args(Object...) initArgs} array expression.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(initArg1, initArg2, ...) {
 			 *   ...
@@ -259,7 +259,7 @@ public class ScriptOperators {
 			/**
 			 * Define the {@code init} function for the {@link Accumulator accumulators} initial state. The function receives
 			 * its arguments from the {@link AccumulatorInitArgsBuilder#initArgs(Object...)} array expression.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(initArg1, initArg2, ...) {
 			 *   ...
@@ -308,7 +308,7 @@ public class ScriptOperators {
 			 * Set the {@code accumulate} {@link Function} that updates the state for each document. The functions first
 			 * argument is the current {@code state}, additional arguments can be defined via {@link Function#args(Object...)
 			 * accumulateArgs}.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state, accumArg1, accumArg2, ...) {
 			 *   ...
@@ -327,7 +327,7 @@ public class ScriptOperators {
 			 * Set the {@code accumulate} function that updates the state for each document. The functions first argument is
 			 * the current {@code state}, additional arguments can be defined via
 			 * {@link AccumulatorAccumulateArgsBuilder#accumulateArgs(Object...)}.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state, accumArg1, accumArg2, ...) {
 			 *   ...
@@ -370,7 +370,7 @@ public class ScriptOperators {
 			 * Set the {@code merge} function used to merge two internal states. <br />
 			 * This might be required because the operation is run on a sharded cluster or when the operator exceeds its
 			 * memory limit.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state1, state2) {
 			 *   ...
@@ -389,7 +389,7 @@ public class ScriptOperators {
 			/**
 			 * Set the {@code finalize} function used to update the result of the accumulation when all documents have been
 			 * processed.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state) {
 			 *   ...
@@ -425,7 +425,7 @@ public class ScriptOperators {
 			/**
 			 * Define the {@code init} function for the {@link Accumulator accumulators} initial state. The function receives
 			 * its arguments from the {@link #initArgs(Object...)} array expression.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(initArg1, initArg2, ...) {
 			 *   ...
@@ -461,7 +461,7 @@ public class ScriptOperators {
 			/**
 			 * Set the {@code accumulate} function that updates the state for each document. The functions first argument is
 			 * the current {@code state}, additional arguments can be defined via {@link #accumulateArgs(Object...)}.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state, accumArg1, accumArg2, ...) {
 			 *   ...
@@ -500,7 +500,7 @@ public class ScriptOperators {
 			 * Set the {@code merge} function used to merge two internal states. <br />
 			 * This might be required because the operation is run on a sharded cluster or when the operator exceeds its
 			 * memory limit.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state1, state2) {
 			 *   ...
@@ -537,7 +537,7 @@ public class ScriptOperators {
 			/**
 			 * Set the {@code finalize} function used to update the result of the accumulation when all documents have been
 			 * processed.
-			 * <p />
+			 * <br />
 			 * <code class="java">
 			 * function(state) {
 			 *   ...

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SetOperation.java

@@ -193,5 +193,6 @@ public class SetOperation extends DocumentEnhancingOperation {
 			 */
 			SetOperation withValueOfExpression(String operation, Object... values);
 		}
+
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SetWindowFieldsOperation.java

@@ -0,0 +1,873 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.time.temporal.ChronoUnit;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+
+import org.bson.Document;
+import org.springframework.data.domain.Sort;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+/**
+ * Encapsulates the {@code setWindowFields}-operation.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/setWindowFields/">https://docs.mongodb.com/manual/reference/operator/aggregation/setWindowFields/</a>
+ */
+public class SetWindowFieldsOperation
+		implements AggregationOperation, FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation {
+
+	private static final String CURRENT = "current";
+	private static final String UNBOUNDED = "unbounded";
+
+	private final @Nullable Object partitionBy;
+	private final @Nullable AggregationOperation sortBy;
+	private final WindowOutput output;
+
+	/**
+	 * Create a new {@link SetWindowFieldsOperation} with given args.
+	 *
+	 * @param partitionBy The field or {@link AggregationExpression} to group by.
+	 * @param sortBy the {@link SortOperation operation} to sort the documents by in the partition.
+	 * @param output the {@link WindowOutput} containing the fields to add and the rules to calculate their respective
+	 *          values.
+	 */
+	protected SetWindowFieldsOperation(@Nullable Object partitionBy, @Nullable AggregationOperation sortBy,
+			WindowOutput output) {
+
+		this.partitionBy = partitionBy;
+		this.sortBy = sortBy;
+		this.output = output;
+	}
+
+	/**
+	 * Obtain a {@link SetWindowFieldsOperationBuilder builder} to create a {@link SetWindowFieldsOperation}.
+	 *
+	 * @return new instance of {@link SetWindowFieldsOperationBuilder}.
+	 */
+	public static SetWindowFieldsOperationBuilder builder() {
+		return new SetWindowFieldsOperationBuilder();
+	}
+
+	@Override
+	public ExposedFields getFields() {
+		return ExposedFields.nonSynthetic(Fields.from(output.fields.toArray(new Field[0])));
+	}
+
+	@Override
+	public Document toDocument(AggregationOperationContext context) {
+
+		Document $setWindowFields = new Document();
+		if (partitionBy != null) {
+			if (partitionBy instanceof AggregationExpression) {
+				$setWindowFields.append("partitionBy", ((AggregationExpression) partitionBy).toDocument(context));
+			} else if (partitionBy instanceof Field) {
+				$setWindowFields.append("partitionBy", context.getReference((Field) partitionBy).toString());
+			} else {
+				$setWindowFields.append("partitionBy", partitionBy);
+			}
+		}
+
+		if (sortBy != null) {
+			$setWindowFields.append("sortBy", sortBy.toDocument(context).get(sortBy.getOperator()));
+		}
+
+		Document output = new Document();
+		for (ComputedField field : this.output.fields) {
+
+			Document fieldOperation = field.getWindowOperator().toDocument(context);
+			if (field.window != null) {
+				fieldOperation.put("window", field.window.toDocument(context));
+			}
+			output.append(field.getName(), fieldOperation);
+		}
+		$setWindowFields.append("output", output);
+
+		return new Document(getOperator(), $setWindowFields);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$setWindowFields";
+	}
+
+	/**
+	 * {@link WindowOutput} defines output of {@literal $setWindowFields} stage by defining the {@link ComputedField
+	 * field(s)} to append to the documents in the output.
+	 */
+	public static class WindowOutput {
+
+		private final List<ComputedField> fields;
+
+		/**
+		 * Create a new output containing the single given {@link ComputedField field}.
+		 *
+		 * @param outputField must not be {@literal null}.
+		 */
+		public WindowOutput(ComputedField outputField) {
+
+			Assert.notNull(outputField, "OutputField must not be null!");
+
+			this.fields = new ArrayList<>();
+			this.fields.add(outputField);
+		}
+
+		/**
+		 * Append the given {@link ComputedField field} to the outptut.
+		 *
+		 * @param field must not be {@literal null}.
+		 * @return this.
+		 */
+		public WindowOutput append(ComputedField field) {
+
+			Assert.notNull(field, "Field must not be null!");
+
+			fields.add(field);
+			return this;
+		}
+
+		/**
+		 * Append the given {@link AggregationExpression} as a {@link ComputedField field} in a fluent way.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ComputedFieldAppender}.
+		 * @see #append(ComputedField)
+		 */
+		public ComputedFieldAppender append(AggregationExpression expression) {
+
+			return new ComputedFieldAppender() {
+
+				@Nullable private Window window;
+
+				@Override
+				public WindowOutput as(String fieldname) {
+
+					return WindowOutput.this.append(new ComputedField(fieldname, expression, window));
+				}
+
+				@Override
+				public ComputedFieldAppender within(Window window) {
+					this.window = window;
+					return this;
+				}
+			};
+		}
+
+		/**
+		 * Tiny little helper to allow fluent API usage for {@link #append(ComputedField)}.
+		 */
+		interface ComputedFieldAppender {
+
+			/**
+			 * Specify the target field name.
+			 *
+			 * @param fieldname the name of field to add to the target document.
+			 * @return the {@link WindowOutput} that started the append operation.
+			 */
+			WindowOutput as(String fieldname);
+
+			/**
+			 * Specify the window boundaries.
+			 *
+			 * @param window must not be {@literal null}.
+			 * @return this.
+			 */
+			ComputedFieldAppender within(Window window);
+		}
+	}
+
+	/**
+	 * A {@link Field} that the result of a computation done via an {@link AggregationExpression}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class ComputedField implements Field {
+
+		private final String name;
+		private final AggregationExpression windowOperator;
+		private final @Nullable Window window;
+
+		/**
+		 * Create a new {@link ComputedField}.
+		 *
+		 * @param name the target field name.
+		 * @param windowOperator the expression to calculate the field value.
+		 */
+		public ComputedField(String name, AggregationExpression windowOperator) {
+			this(name, windowOperator, null);
+		}
+
+		/**
+		 * Create a new {@link ComputedField}.
+		 *
+		 * @param name the target field name.
+		 * @param windowOperator the expression to calculate the field value.
+		 * @param window the boundaries to operate within. Can be {@literal null}.
+		 */
+		public ComputedField(String name, AggregationExpression windowOperator, @Nullable Window window) {
+
+			this.name = name;
+			this.windowOperator = windowOperator;
+			this.window = window;
+		}
+
+		@Override
+		public String getName() {
+			return name;
+		}
+
+		@Override
+		public String getTarget() {
+			return getName();
+		}
+
+		@Override
+		public boolean isAliased() {
+			return false;
+		}
+
+		public AggregationExpression getWindowOperator() {
+			return windowOperator;
+		}
+
+		public Window getWindow() {
+			return window;
+		}
+	}
+
+	/**
+	 * Quick access to {@link DocumentWindow documents} and {@literal RangeWindow range} {@link Window windows}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public interface Windows {
+
+		/**
+		 * Create a document window relative to the position of the current document.
+		 *
+		 * @param lower an integer for a position relative to the current document, {@literal current} or
+		 *          {@literal unbounded}.
+		 * @param upper an integer for a position relative to the current document, {@literal current} or
+		 *          {@literal unbounded}.
+		 * @return new instance of {@link DocumentWindow}.
+		 */
+		static DocumentWindow documents(Object lower, Object upper) {
+			return new DocumentWindow(lower, upper);
+		}
+
+		/**
+		 * Create a range window defined based on sort expression.
+		 *
+		 * @param lower a numeric value to add the sort by field value of the current document, {@literal current} or
+		 *          {@literal unbounded}.
+		 * @param upper a numeric value to add the sort by field value of the current document, {@literal current} or
+		 *          {@literal unbounded}.
+		 * @return new instance of {@link RangeWindow}.
+		 */
+		static RangeWindow range(Object lower, Object upper, @Nullable WindowUnit unit) {
+			return new RangeWindow(lower, upper, unit == null ? WindowUnits.DEFAULT : unit);
+		}
+
+		/**
+		 * Create a range window based on the {@link Sort sort value} of the current document via a fluent API.
+		 *
+		 * @return new instance of {@link RangeWindowBuilder}.
+		 */
+		static RangeWindowBuilder range() {
+			return new RangeWindowBuilder();
+		}
+
+		/**
+		 * Create a document window relative to the position of the current document via a fluent API.
+		 *
+		 * @return new instance of {@link DocumentWindowBuilder}.
+		 */
+		static DocumentWindowBuilder documents() {
+			return new DocumentWindowBuilder();
+		}
+	}
+
+	/**
+	 * A {@link Window} to be used for {@link ComputedField#getWindow() ComputedField}.
+	 */
+	public interface Window {
+
+		/**
+		 * The lower (inclusive) boundary.
+		 *
+		 * @return
+		 */
+		Object getLower();
+
+		/**
+		 * The upper (inclusive) boundary.
+		 *
+		 * @return
+		 */
+		Object getUpper();
+
+		/**
+		 * Obtain the document representation of the window in a default {@link AggregationOperationContext context}.
+		 *
+		 * @return never {@literal null}.
+		 */
+		default Document toDocument() {
+			return toDocument(Aggregation.DEFAULT_CONTEXT);
+		}
+
+		/**
+		 * Obtain the document representation of the window in the given {@link AggregationOperationContext context}.
+		 *
+		 * @return never {@literal null}.
+		 */
+		Document toDocument(AggregationOperationContext ctx);
+	}
+
+	/**
+	 * Builder API for a {@link RangeWindow}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class RangeWindowBuilder {
+
+		private @Nullable Object lower;
+		private @Nullable Object upper;
+		private @Nullable WindowUnit unit;
+
+		/**
+		 * The lower (inclusive) range limit based on the sortBy field.
+		 *
+		 * @param lower eg. {@literal current} or {@literal unbounded}.
+		 * @return this.
+		 */
+		public RangeWindowBuilder from(String lower) {
+
+			this.lower = lower;
+			return this;
+		}
+
+		/**
+		 * The upper (inclusive) range limit based on the sortBy field.
+		 *
+		 * @param upper eg. {@literal current} or {@literal unbounded}.
+		 * @return this.
+		 */
+		public RangeWindowBuilder to(String upper) {
+
+			this.upper = upper;
+			return this;
+		}
+
+		/**
+		 * The lower (inclusive) range limit value to add to the value based on the sortBy field. Use a negative integer for
+		 * a position before the current document. Use a positive integer for a position after the current document.
+		 * {@code 0} is the current document position.
+		 *
+		 * @param lower
+		 * @return this.
+		 */
+		public RangeWindowBuilder from(Number lower) {
+
+			this.lower = lower;
+			return this;
+		}
+
+		/**
+		 * The upper (inclusive) range limit value to add to the value based on the sortBy field. Use a negative integer for
+		 * a position before the current document. Use a positive integer for a position after the current document.
+		 * {@code 0} is the current document position.
+		 *
+		 * @param upper
+		 * @return this.
+		 */
+		public RangeWindowBuilder to(Number upper) {
+
+			this.upper = upper;
+			return this;
+		}
+
+		/**
+		 * Use {@literal current} as {@link #from(String) lower} limit.
+		 *
+		 * @return this.
+		 */
+		public RangeWindowBuilder fromCurrent() {
+			return from(CURRENT);
+		}
+
+		/**
+		 * Use {@literal unbounded} as {@link #from(String) lower} limit.
+		 *
+		 * @return this.
+		 */
+		public RangeWindowBuilder fromUnbounded() {
+			return from(UNBOUNDED);
+		}
+
+		/**
+		 * Use {@literal current} as {@link #to(String) upper} limit.
+		 *
+		 * @return this.
+		 */
+		public RangeWindowBuilder toCurrent() {
+			return to(CURRENT);
+		}
+
+		/**
+		 * Use {@literal unbounded} as {@link #to(String) upper} limit.
+		 *
+		 * @return this.
+		 */
+		public RangeWindowBuilder toUnbounded() {
+			return to(UNBOUNDED);
+		}
+
+		/**
+		 * Set the {@link WindowUnit unit} or measure for the given {@link Window}.
+		 *
+		 * @param windowUnit must not be {@literal null}. Can be on of {@link Windows}.
+		 * @return this.
+		 */
+		public RangeWindowBuilder unit(WindowUnit windowUnit) {
+
+			Assert.notNull(windowUnit, "WindowUnit must not be null");
+			this.unit = windowUnit;
+			return this;
+		}
+
+		/**
+		 * Build the {@link RangeWindow}.
+		 *
+		 * @return new instance of {@link RangeWindow}.
+		 */
+		public RangeWindow build() {
+
+			Assert.notNull(lower, "Lower bound must not be null");
+			Assert.notNull(upper, "Upper bound must not be null");
+			Assert.notNull(unit, "WindowUnit bound must not be null");
+
+			return new RangeWindow(lower, upper, unit);
+		}
+	}
+
+	/**
+	 * Builder API for a {@link RangeWindow}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class DocumentWindowBuilder {
+
+		private @Nullable Object lower;
+		private @Nullable Object upper;
+
+		/**
+		 * The lower (inclusive) range limit based on current document. Use a negative integer for a position before the
+		 * current document. Use a positive integer for a position after the current document. {@code 0} is the current
+		 * document position.
+		 *
+		 * @param lower
+		 * @return this.
+		 */
+		public DocumentWindowBuilder from(Number lower) {
+
+			this.lower = lower;
+			return this;
+		}
+
+		public DocumentWindowBuilder fromCurrent() {
+			return from(CURRENT);
+		}
+
+		public DocumentWindowBuilder fromUnbounded() {
+			return from(UNBOUNDED);
+		}
+
+		public DocumentWindowBuilder to(String upper) {
+
+			this.upper = upper;
+			return this;
+		}
+
+		/**
+		 * The lower (inclusive) range limit based on current document.
+		 *
+		 * @param lower eg. {@literal current} or {@literal unbounded}.
+		 * @return this.
+		 */
+		public DocumentWindowBuilder from(String lower) {
+
+			this.lower = lower;
+			return this;
+		}
+
+		/**
+		 * The upper (inclusive) range limit based on current document. Use a negative integer for a position before the
+		 * current document. Use a positive integer for a position after the current document. {@code 0} is the current
+		 * document position.
+		 *
+		 * @param upper
+		 * @return this.
+		 */
+		public DocumentWindowBuilder to(Number upper) {
+
+			this.upper = upper;
+			return this;
+		}
+
+		public DocumentWindowBuilder toCurrent() {
+			return to(CURRENT);
+		}
+
+		public DocumentWindowBuilder toUnbounded() {
+			return to(UNBOUNDED);
+		}
+
+		public DocumentWindow build() {
+
+			Assert.notNull(lower, "Lower bound must not be null");
+			Assert.notNull(upper, "Upper bound must not be null");
+
+			return new DocumentWindow(lower, upper);
+		}
+	}
+
+	/**
+	 * Common base class for {@link Window} implementation.
+	 *
+	 * @author Christoph Strobl
+	 */
+	static abstract class WindowImpl implements Window {
+
+		private final Object lower;
+		private final Object upper;
+
+		protected WindowImpl(Object lower, Object upper) {
+			this.lower = lower;
+			this.upper = upper;
+		}
+
+		@Override
+		public Object getLower() {
+			return lower;
+		}
+
+		@Override
+		public Object getUpper() {
+			return upper;
+		}
+	}
+
+	/**
+	 * {@link Window} implementation based on the current document.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class DocumentWindow extends WindowImpl {
+
+		DocumentWindow(Object lower, Object upper) {
+			super(lower, upper);
+		}
+
+		@Override
+		public Document toDocument(AggregationOperationContext ctx) {
+			return new Document("documents", Arrays.asList(getLower(), getUpper()));
+		}
+	}
+
+	/**
+	 * {@link Window} implementation based on the sort fields.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class RangeWindow extends WindowImpl {
+
+		private final WindowUnit unit;
+
+		protected RangeWindow(Object lower, Object upper, WindowUnit unit) {
+
+			super(lower, upper);
+			this.unit = unit;
+		}
+
+		@Override
+		public Document toDocument(AggregationOperationContext ctx) {
+
+			Document range = new Document("range", new Object[] { getLower(), getUpper() });
+			if (unit != null && !WindowUnits.DEFAULT.equals(unit)) {
+				range.append("unit", unit.name().toLowerCase());
+			}
+			return range;
+		}
+	}
+
+	/**
+	 * The actual time unit to apply to a {@link Window}.
+	 */
+	public interface WindowUnit {
+
+		String name();
+
+		/**
+		 * Converts the given time unit into a {@link WindowUnit}. Supported units are: days, hours, minutes, seconds, and
+		 * milliseconds.
+		 *
+		 * @param timeUnit the time unit to convert, must not be {@literal null}.
+		 * @return
+		 * @throws IllegalArgumentException if the {@link TimeUnit} is {@literal null} or not supported for conversion.
+		 */
+		static WindowUnit from(TimeUnit timeUnit) {
+
+			Assert.notNull(timeUnit, "TimeUnit must not be null");
+
+			switch (timeUnit) {
+				case DAYS:
+					return WindowUnits.DAY;
+				case HOURS:
+					return WindowUnits.HOUR;
+				case MINUTES:
+					return WindowUnits.MINUTE;
+				case SECONDS:
+					return WindowUnits.SECOND;
+				case MILLISECONDS:
+					return WindowUnits.MILLISECOND;
+			}
+
+			throw new IllegalArgumentException(String.format("Cannot create WindowUnit from %s", timeUnit));
+		}
+
+		/**
+		 * Converts the given chrono unit into a {@link WindowUnit}. Supported units are: years, weeks, months, days, hours,
+		 * minutes, seconds, and millis.
+		 *
+		 * @param chronoUnit the chrono unit to convert, must not be {@literal null}.
+		 * @return
+		 * @throws IllegalArgumentException if the {@link TimeUnit} is {@literal null} or not supported for conversion.
+		 */
+		static WindowUnit from(ChronoUnit chronoUnit) {
+
+			switch (chronoUnit) {
+				case YEARS:
+					return WindowUnits.YEAR;
+				case WEEKS:
+					return WindowUnits.WEEK;
+				case MONTHS:
+					return WindowUnits.MONTH;
+				case DAYS:
+					return WindowUnits.DAY;
+				case HOURS:
+					return WindowUnits.HOUR;
+				case MINUTES:
+					return WindowUnits.MINUTE;
+				case SECONDS:
+					return WindowUnits.SECOND;
+				case MILLIS:
+					return WindowUnits.MILLISECOND;
+			}
+
+			throw new IllegalArgumentException(String.format("Cannot create WindowUnit from %s", chronoUnit));
+		}
+	}
+
+	/**
+	 * Quick access to available {@link WindowUnit units}.
+	 */
+	public enum WindowUnits implements WindowUnit {
+		DEFAULT, YEAR, QUARTER, MONTH, WEEK, DAY, HOUR, MINUTE, SECOND, MILLISECOND
+	}
+
+	/**
+	 * A fluent builder to create a {@link SetWindowFieldsOperation}.
+	 *
+	 * @author Christoph Strobl
+	 */
+	public static class SetWindowFieldsOperationBuilder {
+
+		private Object partitionBy;
+		private SortOperation sortOperation;
+		private WindowOutput output;
+
+		/**
+		 * Specify the field to group by.
+		 *
+		 * @param fieldName must not be {@literal null} or null.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder partitionByField(String fieldName) {
+
+			Assert.hasText(fieldName, "Field name must not be empty or null");
+			return partitionBy(Fields.field("$" + fieldName, fieldName));
+		}
+
+		/**
+		 * Specify the {@link AggregationExpression expression} to group by.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder partitionByExpression(AggregationExpression expression) {
+			return partitionBy(expression);
+		}
+
+		/**
+		 * Sort {@link Sort.Direction#ASC ascending} by the given fields.
+		 *
+		 * @param fields must not be {@literal null}.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder sortBy(String... fields) {
+			return sortBy(Sort.by(fields));
+		}
+
+		/**
+		 * Set the sort order.
+		 *
+		 * @param sort must not be {@literal null}.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder sortBy(Sort sort) {
+			return sortBy(new SortOperation(sort));
+		}
+
+		/**
+		 * Set the {@link SortOperation} to use.
+		 *
+		 * @param sort must not be {@literal null}.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder sortBy(SortOperation sort) {
+
+			Assert.notNull(sort, "SortOperation must not be null");
+
+			this.sortOperation = sort;
+			return this;
+		}
+
+		/**
+		 * Define the actual output computation.
+		 *
+		 * @param output must not be {@literal null}.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder output(WindowOutput output) {
+
+			Assert.notNull(output, "WindowOutput must not be null");
+
+			this.output = output;
+			return this;
+		}
+
+		/**
+		 * Add a field capturing the result of the given {@link AggregationExpression expression} to the output.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link WindowChoice}.
+		 */
+		public WindowChoice output(AggregationExpression expression) {
+
+			return new WindowChoice() {
+
+				@Nullable private Window window;
+
+				@Override
+				public As within(Window window) {
+
+					Assert.notNull(window, "Window must not be null");
+
+					this.window = window;
+					return this;
+				}
+
+				@Override
+				public SetWindowFieldsOperationBuilder as(String targetFieldName) {
+
+					Assert.hasText(targetFieldName, "Target field name must not be empty or null");
+
+					ComputedField computedField = new ComputedField(targetFieldName, expression, window);
+
+					if (SetWindowFieldsOperationBuilder.this.output == null) {
+						SetWindowFieldsOperationBuilder.this.output = new WindowOutput(computedField);
+					} else {
+						SetWindowFieldsOperationBuilder.this.output.append(computedField);
+					}
+
+					return SetWindowFieldsOperationBuilder.this;
+				}
+			};
+		}
+
+		/**
+		 * Interface to capture field name used to capture the computation result.
+		 */
+		public interface As {
+
+			/**
+			 * Define the target name field name to hold the computation result.
+			 *
+			 * @param targetFieldName must not be {@literal null} or empty.
+			 * @return the starting point {@link SetWindowFieldsOperationBuilder builder} instance.
+			 */
+			SetWindowFieldsOperationBuilder as(String targetFieldName);
+		}
+
+		/**
+		 * Interface to capture an optional {@link Window} applicable to the field computation.
+		 */
+		public interface WindowChoice extends As {
+
+			/**
+			 * Specify calculation boundaries.
+			 *
+			 * @param window must not be {@literal null}.
+			 * @return never {@literal null}.
+			 */
+			As within(Window window);
+
+		}
+
+		/**
+		 * Partition by a value that translates to a valid mongodb expression.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return this.
+		 */
+		public SetWindowFieldsOperationBuilder partitionBy(Object value) {
+
+			Assert.notNull(value, "Partition By must not be null");
+
+			partitionBy = value;
+			return this;
+		}
+
+		/**
+		 * Obtain a new instance of {@link SetWindowFieldsOperation} with previously set arguments.
+		 *
+		 * @return new instance of {@link SetWindowFieldsOperation}.
+		 */
+		public SetWindowFieldsOperation build() {
+			return new SetWindowFieldsOperation(partitionBy, sortOperation, output);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SortByCountOperation.java

@@ -21,17 +21,16 @@ import org.springframework.util.Assert;
 
 /**
  * Encapsulates the aggregation framework {@code $sortByCount}-operation.
- * <p/>
+ * <br />
  * {@code $sortByCount} stage is typically used with {@link Aggregation} and {@code $facet}. Groups incoming documents
  * based on the value of a specified expression and computes the count of documents in each distinct group.
  * {@link SortByCountOperation} is equivalent to {@code { $group: { _id: <expression>, count: { $sum: 1 } } }, { $sort:
  * { count: -1 } }}.
- * <p/>
+ * <br />
  * We recommend to use the static factory method {@link Aggregation#sortByCount(String)} instead of creating instances
  * of this class directly.
  *
- * @see <a href=
- *      "https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/">https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/">https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/</a>
  * @author Jérôme Guyon
  * @author Mark Paluch
  * @since 2.1

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformer.java

@@ -102,7 +102,7 @@ class SpelExpressionTransformer implements AggregationExpressionTransformer {
 		ExpressionState state = new ExpressionState(new StandardEvaluationContext(params), CONFIG);
 		ExpressionNode node = ExpressionNode.from(spelExpression.getAST(), state);
 
-		return transform(new AggregationExpressionTransformationContext<ExpressionNode>(node, null, null, context));
+		return transform(new AggregationExpressionTransformationContext<>(node, null, null, context));
 	}
 
 	/*
@@ -500,7 +500,10 @@ class SpelExpressionTransformer implements AggregationExpressionTransformer {
 					dbo.put(methodReference.getArgumentMap()[i++], transform(child, context));
 				}
 				args = dbo;
-			} else {
+			} else if (ObjectUtils.nullSafeEquals(methodReference.getArgumentType(), ArgumentType.EMPTY_DOCUMENT)) {
+				args = new Document();
+			}
+			else {
 
 				List<Object> argList = new ArrayList<Object>();
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/StringOperators.java

@@ -18,8 +18,11 @@ package org.springframework.data.mongodb.core.aggregation;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
+import java.util.regex.Pattern;
 
 import org.springframework.data.domain.Range;
+import org.springframework.data.mongodb.util.RegexFlags;
 import org.springframework.util.Assert;
 
 /**
@@ -27,6 +30,7 @@ import org.springframework.util.Assert;
  *
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Divya Srivastava
  * @since 1.10
  */
 public class StringOperators {
@@ -516,6 +520,173 @@ public class StringOperators {
 			return usesFieldRef() ? RTrim.valueOf(fieldReference) : RTrim.valueOf(expression);
 		}
 
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the given
+		 * regular expression to find the document with the first match.<br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @param regex must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 * @since 3.3
+		 */
+		public RegexFind regexFind(String regex) {
+			return createRegexFind().regex(regex);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the regular
+		 * expression resulting from the given {@link AggregationExpression} to find the document with the first
+		 * match.<br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 * @since 3.3
+		 */
+		public RegexFind regexFind(AggregationExpression expression) {
+			return createRegexFind().regexOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the {@link Pattern} and applies the regular expression with
+		 * the options specified in the argument to find the document with the first match.
+		 *
+		 * @param pattern the pattern object to apply.
+		 * @return new instance of {@link RegexFind}.
+		 * @since 3.3
+		 */
+		public RegexFind regexFind(Pattern pattern) {
+			return createRegexFind().pattern(pattern);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the regular
+		 * expression with the options specified in the argument to find the document with the first match.
+		 *
+		 * @param regex the regular expression to apply.
+		 * @param options the options to use.
+		 * @return new instance of {@link RegexFind}.
+		 * @since 3.3
+		 */
+		public RegexFind regexFind(String regex, String options) {
+			return createRegexFind().regex(regex).options(options);
+		}
+
+		private RegexFind createRegexFind() {
+			return usesFieldRef() ? RegexFind.valueOf(fieldReference) : RegexFind.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the given
+		 * regular expression to find all the documents with the match.<br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @param regex must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 * @since 3.3
+		 */
+		public RegexFindAll regexFindAll(String regex) {
+			return createRegexFindAll().regex(regex);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the regular
+		 * expression resulting from the given {@link AggregationExpression} to find all the documents with the
+		 * match..<br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 * @since 3.3
+		 */
+		public RegexFindAll regexFindAll(AggregationExpression expression) {
+			return createRegexFindAll().regexOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes a {@link Pattern} and applies the regular expression with
+		 * the options specified in the argument to find all the documents with the match.
+		 *
+		 * @param pattern the pattern object to apply.
+		 * @return new instance of {@link RegexFindAll}.
+		 * @since 3.3
+		 */
+		public RegexFindAll regexFindAll(Pattern pattern) {
+			return createRegexFindAll().pattern(pattern);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the regular
+		 * expression with the options specified in the argument to find all the documents with the match.
+		 *
+		 * @param regex the regular expression to apply.
+		 * @param options the options to use.
+		 * @return new instance of {@link RegexFindAll}.
+		 * @since 3.3
+		 */
+		public RegexFindAll regexFindAll(String regex, String options) {
+			return createRegexFindAll().regex(regex).options(options);
+		}
+
+		private RegexFindAll createRegexFindAll() {
+			return usesFieldRef() ? RegexFindAll.valueOf(fieldReference) : RegexFindAll.valueOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the given
+		 * regular expression to find if a match is found or not.<br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @param regex must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 * @since 3.3
+		 */
+		public RegexMatch regexMatch(String regex) {
+			return createRegexMatch().regex(regex);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the regular
+		 * expression resulting from the given {@link AggregationExpression} to find if a match is found or not.<br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 * @since 3.3
+		 */
+		public RegexMatch regexMatch(AggregationExpression expression) {
+			return createRegexMatch().regexOf(expression);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes a {@link Pattern} and applies the regular expression with
+		 * the options specified in the argument to find if a match is found or not.
+		 *
+		 * @param pattern the pattern object to apply.
+		 * @return new instance of {@link RegexMatch}.
+		 * @since 3.3
+		 */
+		public RegexMatch regexMatch(Pattern pattern) {
+			return createRegexMatch().pattern(pattern);
+		}
+
+		/**
+		 * Creates new {@link AggregationExpression} that takes the associated string representation and applies the regular
+		 * expression with the options specified in the argument to find if a match is found or not.
+		 *
+		 * @param regex the regular expression to apply.
+		 * @param options the options to use.
+		 * @return new instance of {@link RegexMatch}.
+		 * @since 3.3
+		 */
+		public RegexMatch regexMatch(String regex, String options) {
+			return createRegexMatch().regex(regex).options(options);
+		}
+
+		private RegexMatch createRegexMatch() {
+			return usesFieldRef() ? RegexMatch.valueOf(fieldReference) : RegexMatch.valueOf(expression);
+		}
+
 		private boolean usesFieldRef() {
 			return fieldReference != null;
 		}
@@ -1477,4 +1648,434 @@ public class StringOperators {
 			return "$rtrim";
 		}
 	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $regexFind} which applies a regular expression (regex) to a string and
+	 * returns information on the first matched substring. <br />
+	 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+	 *
+	 * @author Divya Srivastava
+	 * @since 3.3
+	 */
+	public static class RegexFind extends AbstractAggregationExpression {
+
+		protected RegexFind(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Creates new {@link RegexFind} using the value of the provided {@link Field fieldReference} as {@literal input}
+		 * value.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public static RegexFind valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new RegexFind(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link RegexFind} using the result of the provided {@link AggregationExpression} as {@literal input}
+		 * value.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public static RegexFind valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexFind(Collections.singletonMap("input", expression));
+		}
+
+		/**
+		 * Optional specify the options to use with the regular expression.
+		 *
+		 * @param options must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind options(String options) {
+
+			Assert.notNull(options, "Options must not be null!");
+
+			return new RegexFind(append("options", options));
+		}
+
+		/**
+		 * Optional specify the reference to the {@link Field field} holding the options values to use with the regular
+		 * expression.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind optionsOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new RegexFind(append("options", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Optional specify the {@link AggregationExpression} evaluating to the options values to use with the regular
+		 * expression.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind optionsOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexFind(append("options", expression));
+		}
+
+		/**
+		 * Specify the regular expression to apply.
+		 *
+		 * @param regex must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind regex(String regex) {
+
+			Assert.notNull(regex, "Regex must not be null!");
+
+			return new RegexFind(append("regex", regex));
+		}
+
+		/**
+		 * Apply a {@link Pattern} into {@code regex} and {@code options} fields.
+		 *
+		 * @param pattern must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind pattern(Pattern pattern) {
+
+			Assert.notNull(pattern, "Pattern must not be null!");
+
+			Map<String, Object> regex = append("regex", pattern.pattern());
+			regex.put("options", RegexFlags.toRegexOptions(pattern.flags()));
+
+			return new RegexFind(regex);
+		}
+
+		/**
+		 * Specify the reference to the {@link Field field} holding the regular expression to apply.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind regexOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "fieldReference must not be null!");
+
+			return new RegexFind(append("regex", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Specify the {@link AggregationExpression} evaluating to the regular expression to apply.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFind}.
+		 */
+		public RegexFind regexOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexFind(append("regex", expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$regexFind";
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $regexFindAll} which applies a regular expression (regex) to a string and
+	 * returns information on all the matched substrings. <br />
+	 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+	 *
+	 * @author Divya Srivastava
+	 * @since 3.3
+	 */
+	public static class RegexFindAll extends AbstractAggregationExpression {
+
+		protected RegexFindAll(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Creates new {@link RegexFindAll} using the value of the provided {@link Field fieldReference} as {@literal input}
+		 * value.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public static RegexFindAll valueOf(String fieldReference) {
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+			return new RegexFindAll(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link RegexFindAll} using the result of the provided {@link AggregationExpression} as
+		 * {@literal input} value.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public static RegexFindAll valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexFindAll(Collections.singletonMap("input", expression));
+		}
+
+		/**
+		 * Optional specify the options to use with the regular expression.
+		 *
+		 * @param options must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll options(String options) {
+
+			Assert.notNull(options, "Options must not be null!");
+
+			return new RegexFindAll(append("options", options));
+		}
+
+		/**
+		 * Optional specify the reference to the {@link Field field} holding the options values to use with the regular
+		 * expression.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll optionsOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "fieldReference must not be null!");
+
+			return new RegexFindAll(append("options", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Optional specify the {@link AggregationExpression} evaluating to the options values to use with the regular
+		 * expression.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll optionsOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexFindAll(append("options", expression));
+		}
+
+		/**
+		 * Apply a {@link Pattern} into {@code regex} and {@code options} fields.
+		 *
+		 * @param pattern must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll pattern(Pattern pattern) {
+
+			Assert.notNull(pattern, "Pattern must not be null!");
+
+			Map<String, Object> regex = append("regex", pattern.pattern());
+			regex.put("options", RegexFlags.toRegexOptions(pattern.flags()));
+
+			return new RegexFindAll(regex);
+		}
+
+		/**
+		 * Specify the regular expression to apply.
+		 *
+		 * @param regex must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll regex(String regex) {
+
+			Assert.notNull(regex, "Regex must not be null!");
+
+			return new RegexFindAll(append("regex", regex));
+		}
+
+		/**
+		 * Specify the reference to the {@link Field field} holding the regular expression to apply.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll regexOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "fieldReference must not be null!");
+
+			return new RegexFindAll(append("regex", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Specify the {@link AggregationExpression} evaluating to the regular expression to apply.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexFindAll}.
+		 */
+		public RegexFindAll regexOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexFindAll(append("regex", expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$regexFindAll";
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $regexMatch} which applies a regular expression (regex) to a string and
+	 * returns a boolean that indicates if a match is found or not. <br />
+	 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+	 *
+	 * @author Divya Srivastava
+	 * @since 3.3
+	 */
+	public static class RegexMatch extends AbstractAggregationExpression {
+
+		protected RegexMatch(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Creates new {@link RegexMatch} using the value of the provided {@link Field fieldReference} as {@literal input}
+		 * value.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public static RegexMatch valueOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new RegexMatch(Collections.singletonMap("input", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Creates new {@link RegexMatch} using the result of the provided {@link AggregationExpression} as {@literal input}
+		 * value.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public static RegexMatch valueOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexMatch(Collections.singletonMap("input", expression));
+		}
+
+		/**
+		 * Optional specify the options to use with the regular expression.
+		 *
+		 * @param options must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch options(String options) {
+
+			Assert.notNull(options, "Options must not be null!");
+
+			return new RegexMatch(append("options", options));
+		}
+
+		/**
+		 * Optional specify the reference to the {@link Field field} holding the options values to use with the regular
+		 * expression.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch optionsOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new RegexMatch(append("options", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Optional specify the {@link AggregationExpression} evaluating to the options values to use with the regular
+		 * expression.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch optionsOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexMatch(append("options", expression));
+		}
+
+		/**
+		 * Apply a {@link Pattern} into {@code regex} and {@code options} fields.
+		 *
+		 * @param pattern must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch pattern(Pattern pattern) {
+
+			Assert.notNull(pattern, "Pattern must not be null!");
+
+			Map<String, Object> regex = append("regex", pattern.pattern());
+			regex.put("options", RegexFlags.toRegexOptions(pattern.flags()));
+
+			return new RegexMatch(regex);
+		}
+
+		/**
+		 * Specify the regular expression to apply.
+		 *
+		 * @param regex must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch regex(String regex) {
+
+			Assert.notNull(regex, "Regex must not be null!");
+
+			return new RegexMatch(append("regex", regex));
+		}
+
+		/**
+		 * Specify the reference to the {@link Field field} holding the regular expression to apply.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch regexOf(String fieldReference) {
+
+			Assert.notNull(fieldReference, "FieldReference must not be null!");
+
+			return new RegexMatch(append("regex", Fields.field(fieldReference)));
+		}
+
+		/**
+		 * Optional specify the {@link AggregationExpression} evaluating to the regular expression to apply.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link RegexMatch}.
+		 */
+		public RegexMatch regexOf(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Expression must not be null!");
+
+			return new RegexMatch(append("regex", expression));
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$regexMatch";
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/UnionWithOperation.java

@@ -28,7 +28,7 @@ import org.springframework.util.Assert;
  * containing duplicates, into a single result set that is handed over to the next stage. <br />
  * In order to remove duplicates it is possible to append a {@link GroupOperation} right after
  * {@link UnionWithOperation}.
- * <p />
+ * <br />
  * If the {@link UnionWithOperation} uses a
  * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/unionWith/#unionwith-pipeline">pipeline</a>
  * to process documents, field names within the pipeline will be treated as is. In order to map domain type property

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DbRefResolver.java

@@ -35,7 +35,7 @@ import com.mongodb.DBRef;
  * @author Mark Paluch
  * @since 1.4
  */
-public interface DbRefResolver {
+public interface DbRefResolver extends ReferenceResolver {
 
 	/**
 	 * Resolves the given {@link DBRef} into an object of the given {@link MongoPersistentProperty}'s type. The method

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DefaultDbRefResolver.java

@@ -15,13 +15,6 @@
  */
 package org.springframework.data.mongodb.core.convert;
 
-import static org.springframework.util.ReflectionUtils.*;
-
-import java.io.IOException;
-import java.io.ObjectInputStream;
-import java.io.ObjectOutputStream;
-import java.io.Serializable;
-import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -29,28 +22,18 @@ import java.util.List;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-import org.aopalliance.intercept.MethodInterceptor;
-import org.aopalliance.intercept.MethodInvocation;
 import org.bson.Document;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-import org.springframework.aop.framework.ProxyFactory;
-import org.springframework.cglib.proxy.Callback;
-import org.springframework.cglib.proxy.Enhancer;
-import org.springframework.cglib.proxy.Factory;
-import org.springframework.cglib.proxy.MethodProxy;
-import org.springframework.dao.DataAccessException;
+
 import org.springframework.dao.InvalidDataAccessApiUsageException;
-import org.springframework.dao.support.PersistenceExceptionTranslator;
-import org.springframework.data.mongodb.ClientSessionException;
-import org.springframework.data.mongodb.LazyLoadingException;
 import org.springframework.data.mongodb.MongoDatabaseFactory;
 import org.springframework.data.mongodb.MongoDatabaseUtils;
+import org.springframework.data.mongodb.core.convert.ReferenceLoader.DocumentReferenceQuery;
+import org.springframework.data.mongodb.core.mapping.BasicMongoPersistentProperty;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.lang.Nullable;
-import org.springframework.objenesis.ObjenesisStd;
 import org.springframework.util.Assert;
-import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
 
 import com.mongodb.DBRef;
@@ -67,13 +50,11 @@ import com.mongodb.client.model.Filters;
  * @author Mark Paluch
  * @since 1.4
  */
-public class DefaultDbRefResolver implements DbRefResolver {
+public class DefaultDbRefResolver extends DefaultReferenceResolver implements DbRefResolver, ReferenceResolver {
 
 	private static final Logger LOGGER = LoggerFactory.getLogger(DefaultDbRefResolver.class);
 
 	private final MongoDatabaseFactory mongoDbFactory;
-	private final PersistenceExceptionTranslator exceptionTranslator;
-	private final ObjenesisStd objenesis;
 
 	/**
 	 * Creates a new {@link DefaultDbRefResolver} with the given {@link MongoDatabaseFactory}.
@@ -82,11 +63,11 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	 */
 	public DefaultDbRefResolver(MongoDatabaseFactory mongoDbFactory) {
 
+		super(new MongoDatabaseFactoryReferenceLoader(mongoDbFactory), mongoDbFactory.getExceptionTranslator());
+
 		Assert.notNull(mongoDbFactory, "MongoDbFactory translator must not be null!");
 
 		this.mongoDbFactory = mongoDbFactory;
-		this.exceptionTranslator = mongoDbFactory.getExceptionTranslator();
-		this.objenesis = new ObjenesisStd(true);
 	}
 
 	/*
@@ -114,17 +95,8 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	 */
 	@Override
 	public Document fetch(DBRef dbRef) {
-
-		MongoCollection<Document> mongoCollection = getCollection(dbRef);
-
-		if (LOGGER.isTraceEnabled()) {
-			LOGGER.trace("Fetching DBRef '{}' from {}.{}.", dbRef.getId(),
-					StringUtils.hasText(dbRef.getDatabaseName()) ? dbRef.getDatabaseName()
-							: mongoCollection.getNamespace().getDatabaseName(),
-					dbRef.getCollectionName());
-		}
-
-		return mongoCollection.find(Filters.eq("_id", dbRef.getId())).first();
+		return getReferenceLoader().fetchOne(DocumentReferenceQuery.forSingleDocument(Filters.eq("_id", dbRef.getId())),
+				ReferenceCollection.fromDBRef(dbRef));
 	}
 
 	/*
@@ -165,7 +137,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		}
 
 		List<Document> result = mongoCollection //
-				.find(new Document("_id", new Document("$in", ids))) //
+				.find(new Document(BasicMongoPersistentProperty.ID_FIELD_NAME, new Document("$in", ids))) //
 				.into(new ArrayList<>());
 
 		return ids.stream() //
@@ -185,44 +157,9 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	private Object createLazyLoadingProxy(MongoPersistentProperty property, @Nullable DBRef dbref,
 			DbRefResolverCallback callback, DbRefProxyHandler handler) {
 
-		Class<?> propertyType = property.getType();
-		LazyLoadingInterceptor interceptor = new LazyLoadingInterceptor(property, dbref, exceptionTranslator, callback);
+		Object lazyLoadingProxy = getProxyFactory().createLazyLoadingProxy(property, callback, dbref);
 
-		if (!propertyType.isInterface()) {
-
-			Factory factory = (Factory) objenesis.newInstance(getEnhancedTypeFor(propertyType));
-			factory.setCallbacks(new Callback[] { interceptor });
-
-			return handler.populateId(property, dbref, factory);
-		}
-
-		ProxyFactory proxyFactory = new ProxyFactory();
-
-		for (Class<?> type : propertyType.getInterfaces()) {
-			proxyFactory.addInterface(type);
-		}
-
-		proxyFactory.addInterface(LazyLoadingProxy.class);
-		proxyFactory.addInterface(propertyType);
-		proxyFactory.addAdvice(interceptor);
-
-		return handler.populateId(property, dbref, proxyFactory.getProxy(LazyLoadingProxy.class.getClassLoader()));
-	}
-
-	/**
-	 * Returns the CGLib enhanced type for the given source type.
-	 *
-	 * @param type
-	 * @return
-	 */
-	private Class<?> getEnhancedTypeFor(Class<?> type) {
-
-		Enhancer enhancer = new Enhancer();
-		enhancer.setSuperclass(type);
-		enhancer.setCallbackType(org.springframework.cglib.proxy.MethodInterceptor.class);
-		enhancer.setInterfaces(new Class[] { LazyLoadingProxy.class });
-
-		return enhancer.createClass();
+		return handler.populateId(property, dbref, lazyLoadingProxy);
 	}
 
 	/**
@@ -245,253 +182,10 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	private static Stream<Document> documentWithId(Object identifier, Collection<Document> documents) {
 
 		return documents.stream() //
-				.filter(it -> it.get("_id").equals(identifier)) //
+				.filter(it -> it.get(BasicMongoPersistentProperty.ID_FIELD_NAME).equals(identifier)) //
 				.limit(1);
 	}
 
-	/**
-	 * A {@link MethodInterceptor} that is used within a lazy loading proxy. The property resolving is delegated to a
-	 * {@link DbRefResolverCallback}. The resolving process is triggered by a method invocation on the proxy and is
-	 * guaranteed to be performed only once.
-	 *
-	 * @author Thomas Darimont
-	 * @author Oliver Gierke
-	 * @author Christoph Strobl
-	 */
-	static class LazyLoadingInterceptor
-			implements MethodInterceptor, org.springframework.cglib.proxy.MethodInterceptor, Serializable {
-
-		private static final Method INITIALIZE_METHOD, TO_DBREF_METHOD, FINALIZE_METHOD;
-
-		private final DbRefResolverCallback callback;
-		private final MongoPersistentProperty property;
-		private final PersistenceExceptionTranslator exceptionTranslator;
-
-		private volatile boolean resolved;
-		private final @Nullable DBRef dbref;
-		private @Nullable Object result;
-
-		static {
-			try {
-				INITIALIZE_METHOD = LazyLoadingProxy.class.getMethod("getTarget");
-				TO_DBREF_METHOD = LazyLoadingProxy.class.getMethod("toDBRef");
-				FINALIZE_METHOD = Object.class.getDeclaredMethod("finalize");
-			} catch (Exception e) {
-				throw new RuntimeException(e);
-			}
-		}
-
-		/**
-		 * Creates a new {@link LazyLoadingInterceptor} for the given {@link MongoPersistentProperty},
-		 * {@link PersistenceExceptionTranslator} and {@link DbRefResolverCallback}.
-		 *
-		 * @param property must not be {@literal null}.
-		 * @param dbref can be {@literal null}.
-		 * @param callback must not be {@literal null}.
-		 */
-		public LazyLoadingInterceptor(MongoPersistentProperty property, @Nullable DBRef dbref,
-				PersistenceExceptionTranslator exceptionTranslator, DbRefResolverCallback callback) {
-
-			Assert.notNull(property, "Property must not be null!");
-			Assert.notNull(exceptionTranslator, "Exception translator must not be null!");
-			Assert.notNull(callback, "Callback must not be null!");
-
-			this.dbref = dbref;
-			this.callback = callback;
-			this.exceptionTranslator = exceptionTranslator;
-			this.property = property;
-		}
-
-		/*
-		 * (non-Javadoc)
-		 * @see org.aopalliance.intercept.MethodInterceptor#invoke(org.aopalliance.intercept.MethodInvocation)
-		 */
-		@Override
-		public Object invoke(@Nullable MethodInvocation invocation) throws Throwable {
-			return intercept(invocation.getThis(), invocation.getMethod(), invocation.getArguments(), null);
-		}
-
-		/*
-		 * (non-Javadoc)
-		 * @see org.springframework.cglib.proxy.MethodInterceptor#intercept(java.lang.Object, java.lang.reflect.Method, java.lang.Object[], org.springframework.cglib.proxy.MethodProxy)
-		 */
-		@Nullable
-		@Override
-		public Object intercept(Object obj, Method method, Object[] args, @Nullable MethodProxy proxy) throws Throwable {
-
-			if (INITIALIZE_METHOD.equals(method)) {
-				return ensureResolved();
-			}
-
-			if (TO_DBREF_METHOD.equals(method)) {
-				return this.dbref;
-			}
-
-			if (isObjectMethod(method) && Object.class.equals(method.getDeclaringClass())) {
-
-				if (ReflectionUtils.isToStringMethod(method)) {
-					return proxyToString(proxy);
-				}
-
-				if (ReflectionUtils.isEqualsMethod(method)) {
-					return proxyEquals(proxy, args[0]);
-				}
-
-				if (ReflectionUtils.isHashCodeMethod(method)) {
-					return proxyHashCode(proxy);
-				}
-
-				// DATAMONGO-1076 - finalize methods should not trigger proxy initialization
-				if (FINALIZE_METHOD.equals(method)) {
-					return null;
-				}
-			}
-
-			Object target = ensureResolved();
-
-			if (target == null) {
-				return null;
-			}
-
-			ReflectionUtils.makeAccessible(method);
-
-			return method.invoke(target, args);
-		}
-
-		/**
-		 * Returns a to string representation for the given {@code proxy}.
-		 *
-		 * @param proxy
-		 * @return
-		 */
-		private String proxyToString(@Nullable Object proxy) {
-
-			StringBuilder description = new StringBuilder();
-			if (dbref != null) {
-				description.append(dbref.getCollectionName());
-				description.append(":");
-				description.append(dbref.getId());
-			} else {
-				description.append(System.identityHashCode(proxy));
-			}
-			description.append("$").append(LazyLoadingProxy.class.getSimpleName());
-
-			return description.toString();
-		}
-
-		/**
-		 * Returns the hashcode for the given {@code proxy}.
-		 *
-		 * @param proxy
-		 * @return
-		 */
-		private int proxyHashCode(@Nullable Object proxy) {
-			return proxyToString(proxy).hashCode();
-		}
-
-		/**
-		 * Performs an equality check for the given {@code proxy}.
-		 *
-		 * @param proxy
-		 * @param that
-		 * @return
-		 */
-		private boolean proxyEquals(@Nullable Object proxy, Object that) {
-
-			if (!(that instanceof LazyLoadingProxy)) {
-				return false;
-			}
-
-			if (that == proxy) {
-				return true;
-			}
-
-			return proxyToString(proxy).equals(that.toString());
-		}
-
-		/**
-		 * Will trigger the resolution if the proxy is not resolved already or return a previously resolved result.
-		 *
-		 * @return
-		 */
-		@Nullable
-		private Object ensureResolved() {
-
-			if (!resolved) {
-				this.result = resolve();
-				this.resolved = true;
-			}
-
-			return this.result;
-		}
-
-		/**
-		 * Callback method for serialization.
-		 *
-		 * @param out
-		 * @throws IOException
-		 */
-		private void writeObject(ObjectOutputStream out) throws IOException {
-
-			ensureResolved();
-			out.writeObject(this.result);
-		}
-
-		/**
-		 * Callback method for deserialization.
-		 *
-		 * @param in
-		 * @throws IOException
-		 */
-		private void readObject(ObjectInputStream in) throws IOException {
-
-			try {
-				this.resolved = true;
-				this.result = in.readObject();
-			} catch (ClassNotFoundException e) {
-				throw new LazyLoadingException("Could not deserialize result", e);
-			}
-		}
-
-		/**
-		 * Resolves the proxy into its backing object.
-		 *
-		 * @return
-		 */
-		@Nullable
-		private synchronized Object resolve() {
-
-			if (resolved) {
-
-				if (LOGGER.isTraceEnabled()) {
-					LOGGER.trace("Accessing already resolved lazy loading property {}.{}",
-							property.getOwner() != null ? property.getOwner().getName() : "unknown", property.getName());
-				}
-				return result;
-			}
-
-			try {
-				if (LOGGER.isTraceEnabled()) {
-					LOGGER.trace("Resolving lazy loading property {}.{}",
-							property.getOwner() != null ? property.getOwner().getName() : "unknown", property.getName());
-				}
-
-				return callback.resolve(property);
-
-			} catch (RuntimeException ex) {
-
-				DataAccessException translatedException = this.exceptionTranslator.translateExceptionIfPossible(ex);
-
-				if (translatedException instanceof ClientSessionException) {
-					throw new LazyLoadingException("Unable to lazily resolve DBRef! Invalid session state.", ex);
-				}
-
-				throw new LazyLoadingException("Unable to lazily resolve DBRef!",
-						translatedException != null ? translatedException : ex);
-			}
-		}
-	}
-
 	/**
 	 * Customization hook for obtaining the {@link MongoCollection} for a given {@link DBRef}.
 	 *
@@ -504,4 +198,10 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		return MongoDatabaseUtils.getDatabase(dbref.getDatabaseName(), mongoDbFactory)
 				.getCollection(dbref.getCollectionName(), Document.class);
 	}
+
+	protected MongoCollection<Document> getCollection(ReferenceCollection context) {
+
+		return MongoDatabaseUtils.getDatabase(context.getDatabase(), mongoDbFactory).getCollection(context.getCollection(),
+				Document.class);
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DefaultReferenceResolver.java

@@ -0,0 +1,113 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import static org.springframework.data.mongodb.core.convert.ReferenceLookupDelegate.*;
+
+import java.util.Collections;
+
+import org.springframework.dao.support.PersistenceExceptionTranslator;
+import org.springframework.data.mongodb.core.mapping.DBRef;
+import org.springframework.data.mongodb.core.mapping.DocumentReference;
+import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+import org.springframework.util.Assert;
+
+/**
+ * {@link ReferenceResolver} implementation that uses a given {@link ReferenceLookupDelegate} to load and convert entity
+ * associations expressed via a {@link MongoPersistentProperty persitent property}. Creates {@link LazyLoadingProxy
+ * proxies} for associations that should be lazily loaded.
+ *
+ * @author Christoph Strobl
+ * @author Mark Paluch
+ * @since 3.3
+ */
+public class DefaultReferenceResolver implements ReferenceResolver {
+
+	private final ReferenceLoader referenceLoader;
+	private final LazyLoadingProxyFactory proxyFactory;
+
+	private final LookupFunction collectionLookupFunction = (filter, ctx) -> getReferenceLoader().fetchMany(filter, ctx);
+	private final LookupFunction singleValueLookupFunction = (filter, ctx) -> {
+		Object target = getReferenceLoader().fetchOne(filter, ctx);
+		return target == null ? Collections.emptyList() : Collections.singleton(getReferenceLoader().fetchOne(filter, ctx));
+	};
+
+	/**
+	 * Create a new instance of {@link DefaultReferenceResolver}.
+	 *
+	 * @param referenceLoader must not be {@literal null}.
+	 * @param exceptionTranslator must not be {@literal null}.
+	 */
+	public DefaultReferenceResolver(ReferenceLoader referenceLoader, PersistenceExceptionTranslator exceptionTranslator) {
+
+		Assert.notNull(referenceLoader, "ReferenceLoader must not be null!");
+		Assert.notNull(exceptionTranslator, "ExceptionTranslator must not be null!");
+
+		this.referenceLoader = referenceLoader;
+		this.proxyFactory = new LazyLoadingProxyFactory(exceptionTranslator);
+	}
+
+	@Override
+	public Object resolveReference(MongoPersistentProperty property, Object source,
+			ReferenceLookupDelegate referenceLookupDelegate, MongoEntityReader entityReader) {
+
+		LookupFunction lookupFunction = (property.isCollectionLike() || property.isMap()) ? collectionLookupFunction
+				: singleValueLookupFunction;
+
+		if (isLazyReference(property)) {
+			return createLazyLoadingProxy(property, source, referenceLookupDelegate, lookupFunction, entityReader);
+		}
+
+		return referenceLookupDelegate.readReference(property, source, lookupFunction, entityReader);
+	}
+
+	/**
+	 * Check if the association expressed by the given {@link MongoPersistentProperty property} should be resolved lazily.
+	 *
+	 * @param property
+	 * @return return {@literal true} if the defined association is lazy.
+	 * @see DBRef#lazy()
+	 * @see DocumentReference#lazy()
+	 */
+	protected boolean isLazyReference(MongoPersistentProperty property) {
+
+		if (property.isDocumentReference()) {
+			return property.getDocumentReference().lazy();
+		}
+
+		return property.getDBRef() != null && property.getDBRef().lazy();
+	}
+
+	/**
+	 * The {@link ReferenceLoader} executing the lookup.
+	 *
+	 * @return never {@literal null}.
+	 */
+	protected ReferenceLoader getReferenceLoader() {
+		return referenceLoader;
+	}
+
+	LazyLoadingProxyFactory getProxyFactory() {
+		return proxyFactory;
+	}
+
+	private Object createLazyLoadingProxy(MongoPersistentProperty property, Object source,
+			ReferenceLookupDelegate referenceLookupDelegate, LookupFunction lookupFunction, MongoEntityReader entityReader) {
+		return proxyFactory.createLazyLoadingProxy(property, it -> {
+			return referenceLookupDelegate.readReference(it, source, lookupFunction, entityReader);
+		}, source instanceof DocumentReferenceSource ? ((DocumentReferenceSource)source).getTargetSource() : source);
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DocumentAccessor.java

@@ -135,7 +135,7 @@ class DocumentAccessor {
 	 */
 	@Nullable
 	public Object getRawId(MongoPersistentEntity<?> entity) {
-		return entity.hasIdProperty() ? get(entity.getRequiredIdProperty()) : BsonUtils.asMap(document).get("_id");
+		return entity.hasIdProperty() ? get(entity.getRequiredIdProperty()) : BsonUtils.get(document, "_id");
 	}
 
 	/**

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DocumentPointerFactory.java

@@ -0,0 +1,259 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.WeakHashMap;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.bson.Document;
+import org.bson.types.ObjectId;
+import org.springframework.core.convert.ConversionService;
+import org.springframework.dao.InvalidDataAccessApiUsageException;
+import org.springframework.data.annotation.Reference;
+import org.springframework.data.mapping.PersistentPropertyAccessor;
+import org.springframework.data.mapping.PersistentPropertyPath;
+import org.springframework.data.mapping.PropertyPath;
+import org.springframework.data.mapping.context.MappingContext;
+import org.springframework.data.mapping.model.BeanWrapperPropertyAccessorFactory;
+import org.springframework.data.mongodb.core.mapping.DocumentPointer;
+import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
+import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+
+/**
+ * Internal API to construct {@link DocumentPointer} for a given property. Considers {@link LazyLoadingProxy},
+ * registered {@link Object} to {@link DocumentPointer} {@link org.springframework.core.convert.converter.Converter},
+ * simple {@literal _id} lookups and cases where the {@link DocumentPointer} needs to be computed via a lookup query.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+class DocumentPointerFactory {
+
+	private final ConversionService conversionService;
+	private final MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
+	private final Map<String, LinkageDocument> cache;
+
+	/**
+	 * A {@link Pattern} matching quoted and unquoted variants (with/out whitespaces) of
+	 * <code>{'_id' : ?#{#target} }</code>.
+	 */
+	private static final Pattern DEFAULT_LOOKUP_PATTERN = Pattern.compile("\\{\\s?" + // document start (whitespace opt)
+			"['\"]?_id['\"]?" + // followed by an optionally quoted _id. Like: _id, '_id' or "_id"
+			"?\\s?:\\s?" + // then a colon optionally wrapped inside whitespaces
+			"['\"]?\\?#\\{#target\\}['\"]?" + // leading to the potentially quoted ?#{#target} expression
+			"\\s*}"); // some optional whitespaces and document close
+
+	DocumentPointerFactory(ConversionService conversionService,
+			MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {
+
+		this.conversionService = conversionService;
+		this.mappingContext = mappingContext;
+		this.cache = new WeakHashMap<>();
+	}
+
+	DocumentPointer<?> computePointer(
+			MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext,
+			MongoPersistentProperty property, Object value, Class<?> typeHint) {
+
+		if (value instanceof LazyLoadingProxy) {
+			return () -> ((LazyLoadingProxy) value).getSource();
+		}
+
+		if (conversionService.canConvert(typeHint, DocumentPointer.class)) {
+			return conversionService.convert(value, DocumentPointer.class);
+		}
+
+		MongoPersistentEntity<?> persistentEntity = mappingContext
+				.getRequiredPersistentEntity(property.getAssociationTargetType());
+
+		if (usesDefaultLookup(property)) {
+
+			MongoPersistentProperty idProperty = persistentEntity.getIdProperty();
+			Object idValue = persistentEntity.getIdentifierAccessor(value).getIdentifier();
+
+			if (idProperty.hasExplicitWriteTarget()
+					&& conversionService.canConvert(idValue.getClass(), idProperty.getFieldType())) {
+				return () -> conversionService.convert(idValue, idProperty.getFieldType());
+			}
+
+			if (idValue instanceof String && ObjectId.isValid((String) idValue)) {
+				return () -> new ObjectId((String) idValue);
+			}
+
+			return () -> idValue;
+		}
+
+		MongoPersistentEntity<?> valueEntity = mappingContext.getPersistentEntity(value.getClass());
+		PersistentPropertyAccessor<Object> propertyAccessor;
+		if (valueEntity == null) {
+			propertyAccessor = BeanWrapperPropertyAccessorFactory.INSTANCE.getPropertyAccessor(property.getOwner(), value);
+		} else {
+			propertyAccessor = valueEntity.getPropertyPathAccessor(value);
+		}
+
+		return cache.computeIfAbsent(property.getDocumentReference().lookup(), LinkageDocument::from)
+				.getDocumentPointer(mappingContext, persistentEntity, propertyAccessor);
+	}
+
+	private boolean usesDefaultLookup(MongoPersistentProperty property) {
+
+		if (property.isDocumentReference()) {
+			return DEFAULT_LOOKUP_PATTERN.matcher(property.getDocumentReference().lookup()).matches();
+		}
+
+		Reference atReference = property.findAnnotation(Reference.class);
+		if (atReference != null) {
+			return true;
+		}
+
+		throw new IllegalStateException(String.format("%s does not seem to be define Reference", property));
+	}
+
+	/**
+	 * Value object that computes a document pointer from a given lookup query by identifying SpEL expressions and
+	 * inverting it.
+	 *
+	 * <pre class="code">
+	 * // source
+	 * { 'firstname' : ?#{fn}, 'lastname' : '?#{ln} }
+	 *
+	 * // target
+	 * { 'fn' : ..., 'ln' : ... }
+	 * </pre>
+	 *
+	 * The actual pointer is the computed via
+	 * {@link #getDocumentPointer(MappingContext, MongoPersistentEntity, PersistentPropertyAccessor)} applying values from
+	 * the provided {@link PersistentPropertyAccessor} to the target document by looking at the keys of the expressions
+	 * from the source.
+	 */
+	static class LinkageDocument {
+
+		static final Pattern EXPRESSION_PATTERN = Pattern.compile("\\?#\\{#?(?<fieldName>[\\w\\d\\.\\-)]*)\\}");
+		static final Pattern PLACEHOLDER_PATTERN = Pattern.compile("###_(?<index>\\d*)_###");
+
+		private final String lookup;
+		private final org.bson.Document documentPointer;
+		private final Map<String, String> placeholderMap;
+		private final boolean isSimpleTargetPointer;
+
+		static LinkageDocument from(String lookup) {
+			return new LinkageDocument(lookup);
+		}
+
+		private LinkageDocument(String lookup) {
+
+			this.lookup = lookup;
+			this.placeholderMap = new LinkedHashMap<>();
+
+			int index = 0;
+			Matcher matcher = EXPRESSION_PATTERN.matcher(lookup);
+			String targetLookup = lookup;
+
+			while (matcher.find()) {
+
+				String expression = matcher.group();
+				String fieldName = matcher.group("fieldName").replace("target.", "");
+
+				String placeholder = placeholder(index);
+				placeholderMap.put(placeholder, fieldName);
+				targetLookup = targetLookup.replace(expression, "'" + placeholder + "'");
+				index++;
+			}
+
+			this.documentPointer = org.bson.Document.parse(targetLookup);
+			this.isSimpleTargetPointer = placeholderMap.size() == 1 && placeholderMap.containsValue("target")
+					&& lookup.contains("#target");
+		}
+
+		private String placeholder(int index) {
+			return "###_" + index + "_###";
+		}
+
+		private boolean isPlaceholder(String key) {
+			return PLACEHOLDER_PATTERN.matcher(key).matches();
+		}
+
+		DocumentPointer<Object> getDocumentPointer(
+				MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext,
+				MongoPersistentEntity<?> persistentEntity, PersistentPropertyAccessor<?> propertyAccessor) {
+			return () -> updatePlaceholders(documentPointer, new Document(), mappingContext, persistentEntity,
+					propertyAccessor);
+		}
+
+		Object updatePlaceholders(org.bson.Document source, org.bson.Document target,
+				MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext,
+				MongoPersistentEntity<?> persistentEntity, PersistentPropertyAccessor<?> propertyAccessor) {
+
+			for (Entry<String, Object> entry : source.entrySet()) {
+
+				if (entry.getKey().startsWith("$")) {
+					throw new InvalidDataAccessApiUsageException(String.format(
+							"Cannot derive document pointer from lookup '%s' using query operator (%s). Please consider registering a custom converter.",
+							lookup, entry.getKey()));
+				}
+
+				if (entry.getValue() instanceof Document) {
+
+					MongoPersistentProperty persistentProperty = persistentEntity.getPersistentProperty(entry.getKey());
+					if (persistentProperty != null && persistentProperty.isEntity()) {
+
+						MongoPersistentEntity<?> nestedEntity = mappingContext.getPersistentEntity(persistentProperty.getType());
+						target.put(entry.getKey(), updatePlaceholders((Document) entry.getValue(), new Document(), mappingContext,
+								nestedEntity, nestedEntity.getPropertyAccessor(propertyAccessor.getProperty(persistentProperty))));
+					} else {
+						target.put(entry.getKey(), updatePlaceholders((Document) entry.getValue(), new Document(), mappingContext,
+								persistentEntity, propertyAccessor));
+					}
+					continue;
+				}
+
+				if (placeholderMap.containsKey(entry.getValue())) {
+
+					String attribute = placeholderMap.get(entry.getValue());
+					if (attribute.contains(".")) {
+						attribute = attribute.substring(attribute.lastIndexOf('.') + 1);
+					}
+
+					String fieldName = entry.getKey().equals("_id") ? "id" : entry.getKey();
+					if (!fieldName.contains(".")) {
+
+						Object targetValue = propertyAccessor.getProperty(persistentEntity.getPersistentProperty(fieldName));
+						target.put(attribute, targetValue);
+						continue;
+					}
+
+					PersistentPropertyPath<?> path = mappingContext
+							.getPersistentPropertyPath(PropertyPath.from(fieldName, persistentEntity.getTypeInformation()));
+					Object targetValue = propertyAccessor.getProperty(path);
+					target.put(attribute, targetValue);
+					continue;
+				}
+
+				target.put(entry.getKey(), entry.getValue());
+			}
+
+			if (target.size() == 1 && isSimpleTargetPointer) {
+				return target.values().iterator().next();
+			}
+
+			return target;
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DocumentReferenceSource.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import org.springframework.lang.Nullable;
+
+/**
+ * The source object to resolve document references upon. Encapsulates the actual source and the reference specific
+ * values.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public class DocumentReferenceSource {
+
+	private final Object self;
+
+	private final @Nullable Object targetSource;
+
+	/**
+	 * Create a new instance of {@link DocumentReferenceSource}.
+	 *
+	 * @param self the entire wrapper object holding references. Must not be {@literal null}.
+	 * @param targetSource the reference value source.
+	 */
+	DocumentReferenceSource(Object self, @Nullable Object targetSource) {
+
+		this.self = self;
+		this.targetSource = targetSource;
+	}
+
+	/**
+	 * Get the outer document.
+	 *
+	 * @return never {@literal null}.
+	 */
+	public Object getSelf() {
+		return self;
+	}
+
+	/**
+	 * Get the actual (property specific) reference value.
+	 *
+	 * @return can be {@literal null}.
+	 */
+	@Nullable
+	public Object getTargetSource() {
+		return targetSource;
+	}
+
+	/**
+	 * Dereference a {@code targetSource} if it is a {@link DocumentReferenceSource} or return {@code source} otherwise.
+	 *
+	 * @param source
+	 * @return
+	 */
+	@Nullable
+	static Object getTargetSource(Object source) {
+		return source instanceof DocumentReferenceSource ? ((DocumentReferenceSource) source).getTargetSource() : source;
+	}
+
+	/**
+	 * Dereference a {@code self} object if it is a {@link DocumentReferenceSource} or return {@code self} otherwise.
+	 *
+	 * @param self
+	 * @return
+	 */
+	static Object getSelf(Object self) {
+		return self instanceof DocumentReferenceSource ? ((DocumentReferenceSource) self).getSelf() : self;
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/LazyLoadingProxy.java

@@ -15,18 +15,18 @@
  */
 package org.springframework.data.mongodb.core.convert;
 
-import org.springframework.data.mongodb.core.convert.DefaultDbRefResolver.LazyLoadingInterceptor;
 import org.springframework.lang.Nullable;
 
 import com.mongodb.DBRef;
 
 /**
- * Allows direct interaction with the underlying {@link LazyLoadingInterceptor}.
+ * Allows direct interaction with the underlying {@code LazyLoadingInterceptor}.
  *
  * @author Thomas Darimont
  * @author Christoph Strobl
  * @author Mark Paluch
  * @since 1.5
+ * @see LazyLoadingProxyFactory
  */
 public interface LazyLoadingProxy {
 
@@ -46,4 +46,15 @@ public interface LazyLoadingProxy {
 	 */
 	@Nullable
 	DBRef toDBRef();
+
+	/**
+	 * Returns the raw {@literal source} object that defines the reference.
+	 *
+	 * @return can be {@literal null}.
+	 * @since 3.3
+	 */
+	@Nullable
+	default Object getSource() {
+		return toDBRef();
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/LazyLoadingProxyFactory.java

@@ -0,0 +1,303 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import static org.springframework.util.ReflectionUtils.*;
+
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.Serializable;
+import java.lang.reflect.Method;
+
+import org.aopalliance.intercept.MethodInterceptor;
+import org.aopalliance.intercept.MethodInvocation;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import org.springframework.aop.framework.ProxyFactory;
+import org.springframework.cglib.proxy.Callback;
+import org.springframework.cglib.proxy.Enhancer;
+import org.springframework.cglib.proxy.Factory;
+import org.springframework.cglib.proxy.MethodProxy;
+import org.springframework.dao.DataAccessException;
+import org.springframework.dao.support.PersistenceExceptionTranslator;
+import org.springframework.data.mongodb.ClientSessionException;
+import org.springframework.data.mongodb.LazyLoadingException;
+import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+import org.springframework.lang.Nullable;
+import org.springframework.objenesis.ObjenesisStd;
+import org.springframework.util.ReflectionUtils;
+
+import com.mongodb.DBRef;
+
+/**
+ * {@link ProxyFactory} to create a proxy for {@link MongoPersistentProperty#getType()} to resolve a reference lazily.
+ *
+ * @author Christoph Strobl
+ * @author Mark Paluch
+ */
+class LazyLoadingProxyFactory {
+
+	private static final Logger LOGGER = LoggerFactory.getLogger(LazyLoadingProxyFactory.class);
+
+	private final ObjenesisStd objenesis;
+
+	private final PersistenceExceptionTranslator exceptionTranslator;
+
+	public LazyLoadingProxyFactory(PersistenceExceptionTranslator exceptionTranslator) {
+		this.exceptionTranslator = exceptionTranslator;
+		this.objenesis = new ObjenesisStd(true);
+	}
+
+	public Object createLazyLoadingProxy(MongoPersistentProperty property, DbRefResolverCallback callback,
+			Object source) {
+
+		Class<?> propertyType = property.getType();
+		LazyLoadingInterceptor interceptor = new LazyLoadingInterceptor(property, callback, source, exceptionTranslator);
+
+		if (!propertyType.isInterface()) {
+
+			Factory factory = (Factory) objenesis.newInstance(getEnhancedTypeFor(propertyType));
+			factory.setCallbacks(new Callback[] { interceptor });
+
+			return factory;
+		}
+
+		ProxyFactory proxyFactory = new ProxyFactory();
+
+		for (Class<?> type : propertyType.getInterfaces()) {
+			proxyFactory.addInterface(type);
+		}
+
+		proxyFactory.addInterface(LazyLoadingProxy.class);
+		proxyFactory.addInterface(propertyType);
+		proxyFactory.addAdvice(interceptor);
+
+		return proxyFactory.getProxy(LazyLoadingProxy.class.getClassLoader());
+	}
+
+	/**
+	 * Returns the CGLib enhanced type for the given source type.
+	 *
+	 * @param type
+	 * @return
+	 */
+	private Class<?> getEnhancedTypeFor(Class<?> type) {
+
+		Enhancer enhancer = new Enhancer();
+		enhancer.setSuperclass(type);
+		enhancer.setCallbackType(org.springframework.cglib.proxy.MethodInterceptor.class);
+		enhancer.setInterfaces(new Class[] { LazyLoadingProxy.class });
+
+		return enhancer.createClass();
+	}
+
+	public static class LazyLoadingInterceptor
+			implements MethodInterceptor, org.springframework.cglib.proxy.MethodInterceptor, Serializable {
+
+		private static final Method INITIALIZE_METHOD, TO_DBREF_METHOD, FINALIZE_METHOD, GET_SOURCE_METHOD;
+
+		static {
+			try {
+				INITIALIZE_METHOD = LazyLoadingProxy.class.getMethod("getTarget");
+				TO_DBREF_METHOD = LazyLoadingProxy.class.getMethod("toDBRef");
+				FINALIZE_METHOD = Object.class.getDeclaredMethod("finalize");
+				GET_SOURCE_METHOD = LazyLoadingProxy.class.getMethod("getSource");
+			} catch (Exception e) {
+				throw new RuntimeException(e);
+			}
+		}
+
+		private final MongoPersistentProperty property;
+		private final DbRefResolverCallback callback;
+		private final Object source;
+		private final PersistenceExceptionTranslator exceptionTranslator;
+		private volatile boolean resolved;
+		private @Nullable Object result;
+
+		public LazyLoadingInterceptor(MongoPersistentProperty property, DbRefResolverCallback callback, Object source,
+				PersistenceExceptionTranslator exceptionTranslator) {
+
+			this.property = property;
+			this.callback = callback;
+			this.source = source;
+			this.exceptionTranslator = exceptionTranslator;
+		}
+
+		@Nullable
+		@Override
+		public Object invoke(MethodInvocation invocation) throws Throwable {
+			return intercept(invocation.getThis(), invocation.getMethod(), invocation.getArguments(), null);
+		}
+
+		@Nullable
+		@Override
+		public Object intercept(Object o, Method method, Object[] args, MethodProxy proxy) throws Throwable {
+
+			if (INITIALIZE_METHOD.equals(method)) {
+				return ensureResolved();
+			}
+
+			if (TO_DBREF_METHOD.equals(method)) {
+				return source instanceof DBRef ? source : null;
+			}
+
+			if (GET_SOURCE_METHOD.equals(method)) {
+				return source;
+			}
+
+			if (isObjectMethod(method) && Object.class.equals(method.getDeclaringClass())) {
+
+				if (ReflectionUtils.isToStringMethod(method)) {
+					return proxyToString(source);
+				}
+
+				if (ReflectionUtils.isEqualsMethod(method)) {
+					return proxyEquals(o, args[0]);
+				}
+
+				if (ReflectionUtils.isHashCodeMethod(method)) {
+					return proxyHashCode();
+				}
+
+				// DATAMONGO-1076 - finalize methods should not trigger proxy initialization
+				if (FINALIZE_METHOD.equals(method)) {
+					return null;
+				}
+			}
+
+			Object target = ensureResolved();
+
+			if (target == null) {
+				return null;
+			}
+
+			ReflectionUtils.makeAccessible(method);
+
+			return method.invoke(target, args);
+		}
+
+		@Nullable
+		private Object ensureResolved() {
+
+			if (!resolved) {
+				this.result = resolve();
+				this.resolved = true;
+			}
+
+			return this.result;
+		}
+
+		private String proxyToString(@Nullable Object source) {
+
+			StringBuilder description = new StringBuilder();
+			if (source != null) {
+				if (source instanceof DBRef) {
+					description.append(((DBRef) source).getCollectionName());
+					description.append(":");
+					description.append(((DBRef) source).getId());
+				} else {
+					description.append(source);
+				}
+			} else {
+				description.append(System.identityHashCode(source));
+			}
+			description.append("$").append(LazyLoadingProxy.class.getSimpleName());
+
+			return description.toString();
+		}
+
+		private boolean proxyEquals(@Nullable Object proxy, Object that) {
+
+			if (!(that instanceof LazyLoadingProxy)) {
+				return false;
+			}
+
+			if (that == proxy) {
+				return true;
+			}
+
+			return proxyToString(proxy).equals(that.toString());
+		}
+
+		private int proxyHashCode() {
+			return proxyToString(source).hashCode();
+		}
+
+		/**
+		 * Callback method for serialization.
+		 *
+		 * @param out
+		 * @throws IOException
+		 */
+		private void writeObject(ObjectOutputStream out) throws IOException {
+
+			ensureResolved();
+			out.writeObject(this.result);
+		}
+
+		/**
+		 * Callback method for deserialization.
+		 *
+		 * @param in
+		 * @throws IOException
+		 */
+		private void readObject(ObjectInputStream in) throws IOException {
+
+			try {
+				this.resolved = true;
+				this.result = in.readObject();
+			} catch (ClassNotFoundException e) {
+				throw new LazyLoadingException("Could not deserialize result", e);
+			}
+		}
+
+		@Nullable
+		private synchronized Object resolve() {
+
+			if (resolved) {
+
+				if (LOGGER.isTraceEnabled()) {
+					LOGGER.trace("Accessing already resolved lazy loading property {}.{}",
+							property.getOwner() != null ? property.getOwner().getName() : "unknown", property.getName());
+				}
+				return result;
+			}
+
+			try {
+				if (LOGGER.isTraceEnabled()) {
+					LOGGER.trace("Resolving lazy loading property {}.{}",
+							property.getOwner() != null ? property.getOwner().getName() : "unknown", property.getName());
+				}
+
+				return callback.resolve(property);
+
+			} catch (RuntimeException ex) {
+
+				DataAccessException translatedException = exceptionTranslator.translateExceptionIfPossible(ex);
+
+				if (translatedException instanceof ClientSessionException) {
+					throw new LazyLoadingException("Unable to lazily resolve DBRef! Invalid session state.", ex);
+				}
+
+				throw new LazyLoadingException("Unable to lazily resolve DBRef!",
+						translatedException != null ? translatedException : ex);
+			}
+		}
+	}
+
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java

@@ -25,9 +25,9 @@ import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.Map.Entry;
 import java.util.Optional;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 import org.bson.Document;
 import org.bson.codecs.Codec;
@@ -43,6 +43,7 @@ import org.springframework.context.ApplicationContextAware;
 import org.springframework.core.CollectionFactory;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.data.annotation.Reference;
 import org.springframework.data.convert.TypeMapper;
 import org.springframework.data.mapping.Association;
 import org.springframework.data.mapping.MappingException;
@@ -62,6 +63,8 @@ import org.springframework.data.mapping.model.SpELExpressionEvaluator;
 import org.springframework.data.mapping.model.SpELExpressionParameterValueProvider;
 import org.springframework.data.mongodb.CodecRegistryProvider;
 import org.springframework.data.mongodb.MongoDatabaseFactory;
+import org.springframework.data.mongodb.core.mapping.BasicMongoPersistentProperty;
+import org.springframework.data.mongodb.core.mapping.DocumentPointer;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.data.mongodb.core.mapping.Unwrapped;
@@ -98,6 +101,7 @@ import com.mongodb.DBRef;
  * @author Mark Paluch
  * @author Roman Puchkovskiy
  * @author Heesu Jung
+ * @author Divya Srivastava
  */
 public class MappingMongoConverter extends AbstractMongoConverter implements ApplicationContextAware {
 
@@ -112,6 +116,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	protected final QueryMapper idMapper;
 	protected final DbRefResolver dbRefResolver;
 	protected final DefaultDbRefProxyHandler dbRefProxyHandler;
+	protected final ReferenceLookupDelegate referenceLookupDelegate;
 
 	protected @Nullable ApplicationContext applicationContext;
 	protected MongoTypeMapper typeMapper;
@@ -120,6 +125,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 	private SpELContext spELContext;
 	private @Nullable EntityCallbacks entityCallbacks;
+	private final DocumentPointerFactory documentPointerFactory;
 
 	/**
 	 * Creates a new {@link MappingMongoConverter} given the new {@link DbRefResolver} and {@link MappingContext}.
@@ -136,12 +142,12 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		Assert.notNull(mappingContext, "MappingContext must not be null!");
 
 		this.dbRefResolver = dbRefResolver;
+
 		this.mappingContext = mappingContext;
 		this.typeMapper = new DefaultMongoTypeMapper(DefaultMongoTypeMapper.DEFAULT_TYPE_KEY, mappingContext,
 				this::getWriteTarget);
 		this.idMapper = new QueryMapper(this);
 
-
 		this.spELContext = new SpELContext(DocumentPropertyAccessor.INSTANCE);
 		this.dbRefProxyHandler = new DefaultDbRefProxyHandler(spELContext, mappingContext,
 				(prop, bson, evaluator, path) -> {
@@ -149,6 +155,9 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 					ConversionContext context = getConversionContext(path);
 					return MappingMongoConverter.this.getValueInternal(context, prop, bson, evaluator);
 				});
+
+		this.referenceLookupDelegate = new ReferenceLookupDelegate(mappingContext, spELContext);
+		this.documentPointerFactory = new DocumentPointerFactory(conversionService, mappingContext);
 	}
 
 	/**
@@ -161,8 +170,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 		Assert.notNull(path, "ObjectPath must not be null");
 
-		return new ConversionContext(path, this::readDocument, this::readCollectionOrArray, this::readMap, this::readDBRef,
-				this::getPotentiallyConvertedSimpleRead);
+		return new ConversionContext(conversions, path, this::readDocument, this::readCollectionOrArray, this::readMap,
+				this::readDBRef, this::getPotentiallyConvertedSimpleRead);
 	}
 
 	/**
@@ -253,7 +262,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	/**
 	 * Set the {@link EntityCallbacks} instance to use when invoking
 	 * {@link org.springframework.data.mapping.callback.EntityCallback callbacks} like the {@link AfterConvertCallback}.
-	 * <p />
+	 * <br />
 	 * Overrides potentially existing {@link EntityCallbacks}.
 	 *
 	 * @param entityCallbacks must not be {@literal null}.
@@ -270,7 +279,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.core.MongoReader#read(java.lang.Class, com.mongodb.Document)
 	 */
-	public <S extends Object> S read(Class<S> clazz, final Bson bson) {
+	public <S extends Object> S read(Class<S> clazz, Bson bson) {
 		return read(ClassTypeInformation.from(clazz), bson);
 	}
 
@@ -354,11 +363,18 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				parameterProvider);
 	}
 
-	private <S extends Object> S read(ConversionContext context, MongoPersistentEntity<S> entity, Document bson) {
+	private <S> S read(ConversionContext context, MongoPersistentEntity<S> entity, Document bson) {
 
 		SpELExpressionEvaluator evaluator = new DefaultSpELExpressionEvaluator(bson, spELContext);
 		DocumentAccessor documentAccessor = new DocumentAccessor(bson);
 
+		if (hasIdentifier(bson)) {
+			S existing = findContextualEntity(context, entity, bson);
+			if (existing != null) {
+				return existing;
+			}
+		}
+
 		PreferredConstructor<S, MongoPersistentProperty> persistenceConstructor = entity.getPersistenceConstructor();
 
 		ParameterValueProvider<MongoPersistentProperty> provider = persistenceConstructor != null
@@ -369,15 +385,25 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		S instance = instantiator.createInstance(entity, provider);
 
 		if (entity.requiresPropertyPopulation()) {
+
 			return populateProperties(context, entity, documentAccessor, evaluator, instance);
 		}
 
 		return instance;
 	}
 
+	private boolean hasIdentifier(Document bson) {
+		return bson.get(BasicMongoPersistentProperty.ID_FIELD_NAME) != null;
+	}
+
+	@Nullable
+	private <S> S findContextualEntity(ConversionContext context, MongoPersistentEntity<S> entity, Document bson) {
+		return context.getPath().getPathItem(bson.get(BasicMongoPersistentProperty.ID_FIELD_NAME), entity.getCollection(),
+				entity.getType());
+	}
+
 	private <S> S populateProperties(ConversionContext context, MongoPersistentEntity<S> entity,
-			DocumentAccessor documentAccessor,
-			SpELExpressionEvaluator evaluator, S instance) {
+			DocumentAccessor documentAccessor, SpELExpressionEvaluator evaluator, S instance) {
 
 		PersistentPropertyAccessor<S> accessor = new ConvertingPropertyAccessor<>(entity.getPropertyAccessor(instance),
 				conversionService);
@@ -423,8 +449,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 	@Nullable
 	private Object readIdValue(ConversionContext context, SpELExpressionEvaluator evaluator,
-			MongoPersistentProperty idProperty,
-			Object rawId) {
+			MongoPersistentProperty idProperty, Object rawId) {
 
 		String expression = idProperty.getSpelExpression();
 		Object resolvedValue = expression != null ? evaluator.evaluate(expression) : rawId;
@@ -434,8 +459,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 	private void readProperties(ConversionContext context, MongoPersistentEntity<?> entity,
 			PersistentPropertyAccessor<?> accessor, DocumentAccessor documentAccessor,
-			MongoDbPropertyValueProvider valueProvider,
-			SpELExpressionEvaluator evaluator) {
+			MongoDbPropertyValueProvider valueProvider, SpELExpressionEvaluator evaluator) {
 
 		DbRefResolverCallback callback = null;
 
@@ -447,7 +471,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 					callback = getDbRefResolverCallback(context, documentAccessor, evaluator);
 				}
 
-				readAssociation(prop.getRequiredAssociation(), accessor, documentAccessor, dbRefProxyHandler, callback);
+				readAssociation(prop.getRequiredAssociation(), accessor, documentAccessor, dbRefProxyHandler, callback, context,
+						evaluator);
 				continue;
 			}
 
@@ -474,7 +499,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 					callback = getDbRefResolverCallback(context, documentAccessor, evaluator);
 				}
 
-				readAssociation(prop.getRequiredAssociation(), accessor, documentAccessor, dbRefProxyHandler, callback);
+				readAssociation(prop.getRequiredAssociation(), accessor, documentAccessor, dbRefProxyHandler, callback, context,
+						evaluator);
 				continue;
 			}
 
@@ -490,23 +516,49 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	}
 
 	private void readAssociation(Association<MongoPersistentProperty> association, PersistentPropertyAccessor<?> accessor,
-			DocumentAccessor documentAccessor, DbRefProxyHandler handler, DbRefResolverCallback callback) {
+			DocumentAccessor documentAccessor, DbRefProxyHandler handler, DbRefResolverCallback callback,
+			ConversionContext context, SpELExpressionEvaluator evaluator) {
 
 		MongoPersistentProperty property = association.getInverse();
 		Object value = documentAccessor.get(property);
 
+		if (property.isDocumentReference()
+				|| (!property.isDbReference() && property.findAnnotation(Reference.class) != null)) {
+
+			// quite unusual but sounds like worth having?
+
+			if (conversionService.canConvert(DocumentPointer.class, property.getActualType())) {
+
+				if (value == null) {
+					return;
+				}
+
+				DocumentPointer<?> pointer = () -> value;
+
+				// collection like special treatment
+				accessor.setProperty(property, conversionService.convert(pointer, property.getActualType()));
+			} else {
+
+				accessor.setProperty(property,
+						dbRefResolver.resolveReference(property,
+								new DocumentReferenceSource(documentAccessor.getDocument(), documentAccessor.get(property)),
+								referenceLookupDelegate, context::convert));
+			}
+			return;
+		}
+
 		if (value == null) {
 			return;
 		}
 
 		DBRef dbref = value instanceof DBRef ? (DBRef) value : null;
+
 		accessor.setProperty(property, dbRefResolver.resolveDbRef(property, dbref, callback, handler));
 	}
 
 	@Nullable
 	private Object readUnwrapped(ConversionContext context, DocumentAccessor documentAccessor,
-			MongoPersistentProperty prop,
-			MongoPersistentEntity<?> unwrappedEntity) {
+			MongoPersistentProperty prop, MongoPersistentEntity<?> unwrappedEntity) {
 
 		if (prop.findAnnotation(Unwrapped.class).onEmpty().equals(OnEmpty.USE_EMPTY)) {
 			return read(context, unwrappedEntity, (Document) documentAccessor.getDocument());
@@ -541,6 +593,49 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		return createDBRef(object, referringProperty);
 	}
 
+	@Override
+	public DocumentPointer toDocumentPointer(Object source, @Nullable MongoPersistentProperty referringProperty) {
+
+		if (source instanceof LazyLoadingProxy) {
+			return () -> ((LazyLoadingProxy) source).getSource();
+		}
+
+		Assert.notNull(referringProperty, "Cannot create DocumentReference. The referringProperty must not be null!");
+
+		if (referringProperty.isDbReference()) {
+			return () -> toDBRef(source, referringProperty);
+		}
+
+		if (referringProperty.isDocumentReference() || referringProperty.findAnnotation(Reference.class) != null) {
+			return createDocumentPointer(source, referringProperty);
+		}
+
+		throw new IllegalArgumentException("The referringProperty is neither a DBRef nor a document reference");
+	}
+
+	DocumentPointer<?> createDocumentPointer(Object source, @Nullable MongoPersistentProperty referringProperty) {
+
+		if (referringProperty == null) {
+			return () -> source;
+		}
+
+		if (source instanceof DocumentPointer) {
+			return (DocumentPointer<?>) source;
+		}
+
+		if (ClassUtils.isAssignableValue(referringProperty.getType(), source)
+				&& conversionService.canConvert(referringProperty.getType(), DocumentPointer.class)) {
+			return conversionService.convert(source, DocumentPointer.class);
+		}
+
+		if (ClassUtils.isAssignableValue(referringProperty.getAssociationTargetType(), source)) {
+			return documentPointerFactory.computePointer(mappingContext, referringProperty, source,
+					referringProperty.getActualType());
+		}
+
+		return () -> source;
+	}
+
 	/**
 	 * Root entry method into write conversion. Adds a type discriminator to the {@link Document}. Shouldn't be called for
 	 * nested conversions.
@@ -648,6 +743,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				continue;
 			}
 			if (prop.isAssociation()) {
+
 				writeAssociation(prop.getRequiredAssociation(), accessor, dbObjectAccessor);
 				continue;
 			}
@@ -655,10 +751,10 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			Object value = accessor.getProperty(prop);
 
 			if (value == null) {
-				continue;
-			}
-
-			if (!conversions.isSimpleType(value.getClass())) {
+				if (prop.writeNullValues()) {
+					dbObjectAccessor.put(prop, null);
+				}
+			} else if (!conversions.isSimpleType(value.getClass())) {
 				writePropertyInternal(value, dbObjectAccessor, prop);
 			} else {
 				writeSimpleInternal(value, bson, prop);
@@ -671,7 +767,14 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 		MongoPersistentProperty inverseProp = association.getInverse();
 
-		writePropertyInternal(accessor.getProperty(inverseProp), dbObjectAccessor, inverseProp);
+		Object value = accessor.getProperty(inverseProp);
+
+		if (value == null && !inverseProp.isUnwrapped() && inverseProp.writeNullValues()) {
+			dbObjectAccessor.put(inverseProp, null);
+			return;
+		}
+
+		writePropertyInternal(value, dbObjectAccessor, inverseProp);
 	}
 
 	@SuppressWarnings({ "unchecked" })
@@ -725,6 +828,13 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			return;
 		}
 
+		if (prop.isAssociation() && prop.isAnnotationPresent(Reference.class)) {
+
+			accessor.put(prop, new DocumentPointerFactory(conversionService, mappingContext)
+					.computePointer(mappingContext, prop, obj, valueType.getType()).getPointer());
+			return;
+		}
+
 		/*
 		 * If we have a LazyLoadingProxy we make sure it is initialized first.
 		 */
@@ -763,10 +873,22 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 		if (!property.isDbReference()) {
 
+			if (property.isAssociation()) {
+
+				List<Object> targetCollection = collection.stream().map(it -> {
+					return documentPointerFactory.computePointer(mappingContext, property, it, property.getActualType())
+							.getPointer();
+				}).collect(Collectors.toList());
+
+				return writeCollectionInternal(targetCollection, ClassTypeInformation.from(DocumentPointer.class),
+						new ArrayList<>());
+			}
+
 			if (property.hasExplicitWriteTarget()) {
 				return writeCollectionInternal(collection, new FieldTypeInformation<>(property), new ArrayList<>());
 			}
-			return writeCollectionInternal(collection, property.getTypeInformation(), new BasicDBList());
+
+			return writeCollectionInternal(collection, property.getTypeInformation(), new ArrayList<>());
 		}
 
 		List<Object> dbList = new ArrayList<>(collection.size());
@@ -795,7 +917,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		Assert.notNull(map, "Given map must not be null!");
 		Assert.notNull(property, "PersistentProperty must not be null!");
 
-		if (!property.isDbReference()) {
+		if (!property.isAssociation()) {
 			return writeMapInternal(map, new Document(), property.getTypeInformation());
 		}
 
@@ -809,7 +931,12 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			if (conversions.isSimpleType(key.getClass())) {
 
 				String simpleKey = prepareMapKey(key.toString());
-				document.put(simpleKey, value != null ? createDBRef(value, property) : null);
+				if (property.isDbReference()) {
+					document.put(simpleKey, value != null ? createDBRef(value, property) : null);
+				} else {
+					document.put(simpleKey, documentPointerFactory
+							.computePointer(mappingContext, property, value, property.getActualType()).getPointer());
+				}
 
 			} else {
 				throw new MappingException("Cannot use a complex object as a key value.");
@@ -846,7 +973,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				collection.add(getPotentiallyConvertedSimpleWrite(element,
 						componentType != null ? componentType.getType() : Object.class));
 			} else if (element instanceof Collection || elementType.isArray()) {
-				collection.add(writeCollectionInternal(BsonUtils.asCollection(element), componentType, new BasicDBList()));
+				collection.add(writeCollectionInternal(BsonUtils.asCollection(element), componentType, new ArrayList<>()));
 			} else {
 				Document document = new Document();
 				writeInternal(element, document, componentType);
@@ -878,7 +1005,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 					writeSimpleInternal(val, bson, simpleKey);
 				} else if (val instanceof Collection || val.getClass().isArray()) {
 					BsonUtils.addToMap(bson, simpleKey,
-							writeCollectionInternal(BsonUtils.asCollection(val), propertyType.getMapValueType(), new BasicDBList()));
+							writeCollectionInternal(BsonUtils.asCollection(val), propertyType.getMapValueType(), new ArrayList<>()));
 				} else {
 					Document document = new Document();
 					TypeInformation<?> valueTypeInfo = propertyType.isMap() ? propertyType.getMapValueType()
@@ -1045,7 +1172,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	@SuppressWarnings({ "rawtypes", "unchecked" })
 	private Object getPotentiallyConvertedSimpleRead(Object value, @Nullable Class<?> target) {
 
-		if (target == null || ClassUtils.isAssignableValue(target, value)) {
+		if (target == null) {
 			return value;
 		}
 
@@ -1053,6 +1180,10 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			return doConvert(value, target);
 		}
 
+		if (ClassUtils.isAssignableValue(target, value)) {
+			return value;
+		}
+
 		if (Enum.class.isAssignableFrom(target)) {
 			return Enum.valueOf((Class<Enum>) target, value.toString());
 		}
@@ -1141,7 +1272,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		for (Object element : source) {
-			items.add(context.convert(element, componentType));
+			items.add(element != null ? context.convert(element, componentType) : element);
 		}
 
 		return getPotentiallyConvertedSimpleRead(items, targetType.getType());
@@ -1193,21 +1324,22 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			return map;
 		}
 
-		for (Entry<String, Object> entry : sourceMap.entrySet()) {
+		sourceMap.forEach((k, v) -> {
 
-			if (typeMapper.isTypeKey(entry.getKey())) {
-				continue;
+			if (typeMapper.isTypeKey(k)) {
+				return;
 			}
 
-			Object key = potentiallyUnescapeMapKey(entry.getKey());
+			Object key = potentiallyUnescapeMapKey(k);
 
 			if (!rawKeyType.isAssignableFrom(key.getClass())) {
 				key = doConvert(key, rawKeyType);
 			}
 
-			Object value = entry.getValue();
-			map.put(key, context.convert(value, valueType));
-		}
+			Object value = v;
+			map.put(key, value == null ? value : context.convert(value, valueType));
+
+		});
 
 		return map;
 	}
@@ -1447,8 +1579,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			T target = null;
 			if (document != null) {
 
-				maybeEmitEvent(
-						new AfterLoadEvent<>(document, (Class<T>) type.getType(), collectionName));
+				maybeEmitEvent(new AfterLoadEvent<>(document, (Class<T>) type.getType(), collectionName));
 				target = (T) readDocument(context, document, type);
 			}
 
@@ -1541,9 +1672,10 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	}
 
 	@SuppressWarnings("ConstantConditions")
-	private <T extends Object> T doConvert(Object value, Class<? extends T> target, @Nullable Class<? extends T> fallback) {
+	private <T extends Object> T doConvert(Object value, Class<? extends T> target,
+			@Nullable Class<? extends T> fallback) {
 
-		if(conversionService.canConvert(value.getClass(), target) || fallback == null) {
+		if (conversionService.canConvert(value.getClass(), target) || fallback == null) {
 			return conversionService.convert(value, target);
 		}
 		return conversionService.convert(value, fallback);
@@ -1687,6 +1819,11 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				return (T) dbRefResolver.resolveDbRef(property, dbref, callback, dbRefProxyHandler);
 			}
 
+			if (property.isDocumentReference()) {
+				return (T) dbRefResolver.resolveReference(property, accessor.get(property), referenceLookupDelegate,
+						context::convert);
+			}
+
 			return super.getPropertyValue(property);
 		}
 	}
@@ -1853,6 +1990,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	 */
 	protected static class ConversionContext {
 
+		private final org.springframework.data.convert.CustomConversions conversions;
 		private final ObjectPath path;
 		private final ContainerValueConverter<Bson> documentConverter;
 		private final ContainerValueConverter<Collection<?>> collectionConverter;
@@ -1860,10 +1998,12 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		private final ContainerValueConverter<DBRef> dbRefConverter;
 		private final ValueConverter<Object> elementConverter;
 
-		ConversionContext(ObjectPath path, ContainerValueConverter<Bson> documentConverter,
-				ContainerValueConverter<Collection<?>> collectionConverter, ContainerValueConverter<Bson> mapConverter,
-				ContainerValueConverter<DBRef> dbRefConverter, ValueConverter<Object> elementConverter) {
+		ConversionContext(org.springframework.data.convert.CustomConversions customConversions, ObjectPath path,
+				ContainerValueConverter<Bson> documentConverter, ContainerValueConverter<Collection<?>> collectionConverter,
+				ContainerValueConverter<Bson> mapConverter, ContainerValueConverter<DBRef> dbRefConverter,
+				ValueConverter<Object> elementConverter) {
 
+			this.conversions = customConversions;
 			this.path = path;
 			this.documentConverter = documentConverter;
 			this.collectionConverter = collectionConverter;
@@ -1882,8 +2022,13 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		@SuppressWarnings("unchecked")
 		public <S extends Object> S convert(Object source, TypeInformation<? extends S> typeHint) {
 
+			Assert.notNull(source, "Source must not be null");
 			Assert.notNull(typeHint, "TypeInformation must not be null");
 
+			if (conversions.hasCustomReadTarget(source.getClass(), typeHint.getType())) {
+				return (S) elementConverter.convert(source, typeHint);
+			}
+
 			if (source instanceof Collection) {
 
 				Class<?> rawType = typeHint.getType();
@@ -1900,7 +2045,17 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			}
 
 			if (typeHint.isMap()) {
-				return (S) mapConverter.convert(this, (Bson) source, typeHint);
+
+				if (ClassUtils.isAssignable(Document.class, typeHint.getType())) {
+					return (S) documentConverter.convert(this, BsonUtils.asBson(source), typeHint);
+				}
+
+				if (BsonUtils.supportsBson(source)) {
+					return (S) mapConverter.convert(this, BsonUtils.asBson(source), typeHint);
+				}
+
+				throw new IllegalArgumentException(
+						String.format("Expected map like structure but found %s", source.getClass()));
 			}
 
 			if (source instanceof DBRef) {
@@ -1912,8 +2067,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 						String.format(INCOMPATIBLE_TYPES, source, BasicDBList.class, typeHint.getType(), getPath()));
 			}
 
-			if (source instanceof Bson) {
-				return (S) documentConverter.convert(this, (Bson) source, typeHint);
+			if (BsonUtils.supportsBson(source)) {
+				return (S) documentConverter.convert(this, BsonUtils.asBson(source), typeHint);
 			}
 
 			return (S) elementConverter.convert(source, typeHint);
@@ -1929,8 +2084,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 			Assert.notNull(currentPath, "ObjectPath must not be null");
 
-			return new ConversionContext(currentPath, documentConverter, collectionConverter, mapConverter, dbRefConverter,
-					elementConverter);
+			return new ConversionContext(conversions, currentPath, documentConverter, collectionConverter, mapConverter,
+					dbRefConverter, elementConverter);
 		}
 
 		public ObjectPath getPath() {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MongoConverter.java

@@ -40,13 +40,14 @@ import com.mongodb.DBRef;
  * @author Thomas Darimont
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Ryan Gibb
  */
 public interface MongoConverter
 		extends EntityConverter<MongoPersistentEntity<?>, MongoPersistentProperty, Object, Bson>, MongoWriter<Object>,
 		EntityReader<Object, Bson> {
 
 	/**
-	 * Returns thw {@link TypeMapper} being used to write type information into {@link Document}s created with that
+	 * Returns the {@link TypeMapper} being used to write type information into {@link Document}s created with that
 	 * converter.
 	 *
 	 * @return will never be {@literal null}.
@@ -139,6 +140,9 @@ public interface MongoConverter
 				if (ObjectId.isValid(id.toString())) {
 					return new ObjectId(id.toString());
 				}
+
+				// avoid ConversionException as convertToMongoType will return String anyways.
+				return id;
 			}
 		}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MongoDatabaseFactoryReferenceLoader.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import org.bson.Document;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.data.mongodb.MongoDatabaseFactory;
+import org.springframework.data.mongodb.MongoDatabaseUtils;
+import org.springframework.data.mongodb.core.convert.ReferenceResolver.ReferenceCollection;
+import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
+
+import com.mongodb.client.MongoCollection;
+
+/**
+ * {@link ReferenceLoader} implementation using a {@link MongoDatabaseFactory} to obtain raw {@link Document documents}
+ * for linked entities via a {@link ReferenceLoader.DocumentReferenceQuery}.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public class MongoDatabaseFactoryReferenceLoader implements ReferenceLoader {
+
+	private static final Logger LOGGER = LoggerFactory.getLogger(MongoDatabaseFactoryReferenceLoader.class);
+
+	private final MongoDatabaseFactory mongoDbFactory;
+
+	/**
+	 * @param mongoDbFactory must not be {@literal null}.
+	 */
+	public MongoDatabaseFactoryReferenceLoader(MongoDatabaseFactory mongoDbFactory) {
+
+		Assert.notNull(mongoDbFactory, "MongoDbFactory translator must not be null!");
+
+		this.mongoDbFactory = mongoDbFactory;
+	}
+
+	@Override
+	public Iterable<Document> fetchMany(DocumentReferenceQuery referenceQuery, ReferenceCollection context) {
+
+		MongoCollection<Document> collection = getCollection(context);
+
+		if (LOGGER.isTraceEnabled()) {
+			LOGGER.trace("Bulk fetching {} from {}.{}.", referenceQuery,
+					StringUtils.hasText(context.getDatabase()) ? context.getDatabase()
+							: collection.getNamespace().getDatabaseName(),
+					context.getCollection());
+		}
+
+		return referenceQuery.apply(collection);
+	}
+
+	/**
+	 * Obtain the {@link MongoCollection} for a given {@link ReferenceCollection} from the underlying
+	 * {@link MongoDatabaseFactory}.
+	 *
+	 * @param context must not be {@literal null}.
+	 * @return the {@link MongoCollection} targeted by the {@link ReferenceCollection}.
+	 */
+	protected MongoCollection<Document> getCollection(ReferenceCollection context) {
+
+		return MongoDatabaseUtils.getDatabase(context.getDatabase(), mongoDbFactory).getCollection(context.getCollection(),
+				Document.class);
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MongoWriter.java

@@ -17,6 +17,8 @@ package org.springframework.data.mongodb.core.convert;
 
 import org.bson.conversions.Bson;
 import org.springframework.data.convert.EntityWriter;
+import org.springframework.data.mongodb.core.mapping.DocumentPointer;
+import org.springframework.data.mongodb.core.mapping.DocumentReference;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.data.util.TypeInformation;
@@ -61,6 +63,7 @@ public interface MongoWriter<T> extends EntityWriter<T, Bson> {
 	default Object convertToMongoType(@Nullable Object obj, MongoPersistentEntity<?> entity) {
 		return convertToMongoType(obj, entity.getTypeInformation());
 	}
+
 	/**
 	 * Creates a {@link DBRef} to refer to the given object.
 	 *
@@ -70,4 +73,17 @@ public interface MongoWriter<T> extends EntityWriter<T, Bson> {
 	 * @return will never be {@literal null}.
 	 */
 	DBRef toDBRef(Object object, @Nullable MongoPersistentProperty referingProperty);
+
+	/**
+	 * Creates a the {@link DocumentPointer} representing the link to another entity.
+	 *
+	 * @param source the object to create a document link to.
+	 * @param referringProperty the client-side property referring to the object which might carry additional metadata for
+	 *          the {@link DBRef} object to create. Can be {@literal null}.
+	 * @return will never be {@literal null}.
+	 * @since 3.3
+	 */
+	default DocumentPointer<?> toDocumentPointer(Object source, @Nullable MongoPersistentProperty referringProperty) {
+		return () -> toDBRef(source, referringProperty);
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/NoOpDbRefResolver.java

@@ -18,6 +18,7 @@ package org.springframework.data.mongodb.core.convert;
 import java.util.List;
 
 import org.bson.Document;
+
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.lang.Nullable;
 
@@ -69,4 +70,11 @@ public enum NoOpDbRefResolver implements DbRefResolver {
 	private <T> T handle() throws UnsupportedOperationException {
 		throw new UnsupportedOperationException("DBRef resolution is not supported!");
 	}
+
+	@Nullable
+	@Override
+	public Object resolveReference(MongoPersistentProperty property, Object source,
+			ReferenceLookupDelegate referenceLookupDelegate, MongoEntityReader entityReader) {
+		return null;
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -19,13 +19,17 @@ import java.util.*;
 import java.util.Map.Entry;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.util.stream.Collectors;
 
 import org.bson.BsonValue;
 import org.bson.Document;
 import org.bson.conversions.Bson;
 import org.bson.types.ObjectId;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.data.annotation.Reference;
 import org.springframework.data.domain.Example;
 import org.springframework.data.mapping.Association;
 import org.springframework.data.mapping.MappingException;
@@ -65,9 +69,13 @@ import com.mongodb.DBRef;
  * @author Thomas Darimont
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author David Julia
+ * @author Divya Srivastava
  */
 public class QueryMapper {
 
+	protected static final Logger LOGGER = LoggerFactory.getLogger(QueryMapper.class);
+
 	private static final List<String> DEFAULT_ID_NAMES = Arrays.asList("id", "_id");
 	private static final Document META_TEXT_SCORE = new Document("$meta", "textScore");
 	static final ClassTypeInformation<?> NESTED_DOCUMENT = ClassTypeInformation.from(NestedDocument.class);
@@ -186,24 +194,11 @@ public class QueryMapper {
 		Assert.notNull(sortObject, "SortObject must not be null!");
 
 		if (sortObject.isEmpty()) {
-			return new Document();
+			return BsonUtils.EMPTY_DOCUMENT;
 		}
 
-		sortObject = filterUnwrappedObjects(sortObject, entity);
-
-		Document mappedSort = new Document();
-		for (Map.Entry<String, Object> entry : BsonUtils.asMap(sortObject).entrySet()) {
-
-			Field field = createPropertyField(entity, entry.getKey(), mappingContext);
-			if (field.getProperty() != null && field.getProperty().isUnwrapped()) {
-				continue;
-			}
-
-			mappedSort.put(field.getMappedKey(), entry.getValue());
-		}
-
-		mapMetaAttributes(mappedSort, entity, MetaMapping.WHEN_PRESENT);
-		return mappedSort;
+		Document mappedSort = mapFieldsToPropertyNames(sortObject, entity);
+		return mapMetaAttributes(mappedSort, entity, MetaMapping.WHEN_PRESENT);
 	}
 
 	/**
@@ -219,26 +214,52 @@ public class QueryMapper {
 
 		Assert.notNull(fieldsObject, "FieldsObject must not be null!");
 
-		fieldsObject = filterUnwrappedObjects(fieldsObject, entity);
-
-		Document mappedFields = getMappedObject(fieldsObject, entity);
-		mapMetaAttributes(mappedFields, entity, MetaMapping.FORCE);
-		return mappedFields;
+		Document mappedFields = mapFieldsToPropertyNames(fieldsObject, entity);
+		return mapMetaAttributes(mappedFields, entity, MetaMapping.FORCE);
 	}
 
-	private void mapMetaAttributes(Document source, @Nullable MongoPersistentEntity<?> entity, MetaMapping metaMapping) {
+	private Document mapFieldsToPropertyNames(Document fields, @Nullable MongoPersistentEntity<?> entity) {
+
+		if (fields.isEmpty()) {
+			return BsonUtils.EMPTY_DOCUMENT;
+
+		}
+		Document target = new Document();
+
+		BsonUtils.asMap(filterUnwrappedObjects(fields, entity)).forEach((k, v) -> {
+
+			Field field = createPropertyField(entity, k, mappingContext);
+			if (field.getProperty() != null && field.getProperty().isUnwrapped()) {
+				return;
+			}
+
+			target.put(field.getMappedKey(), v);
+		});
+
+		return target;
+	}
+
+	private Document mapMetaAttributes(Document source, @Nullable MongoPersistentEntity<?> entity,
+			MetaMapping metaMapping) {
 
 		if (entity == null) {
-			return;
+			return source;
 		}
 
 		if (entity.hasTextScoreProperty() && !MetaMapping.IGNORE.equals(metaMapping)) {
+
+			if (source == BsonUtils.EMPTY_DOCUMENT) {
+				source = new Document();
+			}
+
 			MongoPersistentProperty textScoreProperty = entity.getTextScoreProperty();
 			if (MetaMapping.FORCE.equals(metaMapping)
 					|| (MetaMapping.WHEN_PRESENT.equals(metaMapping) && source.containsKey(textScoreProperty.getFieldName()))) {
 				source.putAll(getMappedTextScoreField(textScoreProperty));
 			}
 		}
+
+		return source;
 	}
 
 	private Document filterUnwrappedObjects(Document fieldsObject, @Nullable MongoPersistentEntity<?> entity) {
@@ -444,6 +465,10 @@ public class QueryMapper {
 			}
 		}
 
+		if (value == null) {
+			return null;
+		}
+
 		if (isNestedKeyword(value)) {
 			return getMappedKeyword(new Keyword((Bson) value), documentField.getPropertyEntity());
 		}
@@ -492,6 +517,10 @@ public class QueryMapper {
 			return true;
 		}
 
+		if (property.isDocumentReference()) {
+			return true;
+		}
+
 		MongoPersistentEntity<?> entity = documentField.getPropertyEntity();
 		return entity.hasIdProperty()
 				&& (type.equals(DBRef.class) || entity.getRequiredIdProperty().getActualType().isAssignableFrom(type));
@@ -605,7 +634,7 @@ public class QueryMapper {
 		if (source instanceof Iterable) {
 			BasicDBList result = new BasicDBList();
 			for (Object element : (Iterable<?>) source) {
-				result.add(createDbRefFor(element, property));
+				result.add(createReferenceFor(element, property));
 			}
 			return result;
 		}
@@ -614,12 +643,12 @@ public class QueryMapper {
 			Document result = new Document();
 			Document dbObject = (Document) source;
 			for (String key : dbObject.keySet()) {
-				result.put(key, createDbRefFor(dbObject.get(key), property));
+				result.put(key, createReferenceFor(dbObject.get(key), property));
 			}
 			return result;
 		}
 
-		return createDbRefFor(source, property);
+		return createReferenceFor(source, property);
 	}
 
 	/**
@@ -663,15 +692,20 @@ public class QueryMapper {
 	private Entry<String, Object> createMapEntry(String key, @Nullable Object value) {
 
 		Assert.hasText(key, "Key must not be null or empty!");
-		return Collections.singletonMap(key, value).entrySet().iterator().next();
+		return new AbstractMap.SimpleEntry<>(key, value);
 	}
 
-	private DBRef createDbRefFor(Object source, MongoPersistentProperty property) {
+	private Object createReferenceFor(Object source, MongoPersistentProperty property) {
 
 		if (source instanceof DBRef) {
 			return (DBRef) source;
 		}
 
+		if (property != null && (property.isDocumentReference()
+				|| (!property.isDbReference() && property.findAnnotation(Reference.class) != null))) {
+			return converter.toDocumentPointer(source, property).getPointer();
+		}
+
 		return converter.toDBRef(source, property);
 	}
 
@@ -706,19 +740,19 @@ public class QueryMapper {
 	 * @param candidate
 	 * @return
 	 */
-	protected boolean isNestedKeyword(Object candidate) {
+	protected boolean isNestedKeyword(@Nullable Object candidate) {
 
 		if (!(candidate instanceof Document)) {
 			return false;
 		}
 
-		Set<String> keys = BsonUtils.asMap((Bson) candidate).keySet();
+		Map<String, Object> map = BsonUtils.asMap((Bson) candidate);
 
-		if (keys.size() != 1) {
+		if (map.size() != 1) {
 			return false;
 		}
 
-		return isKeyword(keys.iterator().next());
+		return isKeyword(map.entrySet().iterator().next().getKey());
 	}
 
 	/**
@@ -735,7 +769,7 @@ public class QueryMapper {
 
 	/**
 	 * Returns whether the given {@link String} is a MongoDB keyword. The default implementation will check against the
-	 * set of registered keywords returned by {@link #getKeywords()}.
+	 * set of registered keywords.
 	 *
 	 * @param candidate
 	 * @return
@@ -751,12 +785,14 @@ public class QueryMapper {
 	 * converted one by one.
 	 *
 	 * @param documentField the field and its meta data
-	 * @param value the actual value
+	 * @param value the actual value. Can be {@literal null}.
 	 * @return the potentially converted target value.
 	 */
-	private Object applyFieldTargetTypeHintToValue(Field documentField, Object value) {
+	@Nullable
+	private Object applyFieldTargetTypeHintToValue(Field documentField, @Nullable Object value) {
 
-		if (documentField.getProperty() == null || !documentField.getProperty().hasExplicitWriteTarget()) {
+		if (value == null || documentField.getProperty() == null || !documentField.getProperty().hasExplicitWriteTarget()
+				|| value instanceof Document || value instanceof DBObject) {
 			return value;
 		}
 
@@ -787,7 +823,6 @@ public class QueryMapper {
 	 */
 	static class Keyword {
 
-		private static final String N_OR_PATTERN = "\\$.*or";
 		private static final Set<String> NON_DBREF_CONVERTING_KEYWORDS = new HashSet<>(
 				Arrays.asList("$", "$size", "$slice", "$gt", "$lt"));
 
@@ -801,11 +836,14 @@ public class QueryMapper {
 
 		public Keyword(Bson bson) {
 
-			Set<String> keys = BsonUtils.asMap(bson).keySet();
-			Assert.isTrue(keys.size() == 1, "Can only use a single value Document!");
+			Map<String, Object> map = BsonUtils.asMap(bson);
+			Assert.isTrue(map.size() == 1, "Can only use a single value Document!");
 
-			this.key = keys.iterator().next();
-			this.value = BsonUtils.get(bson, key);
+			Set<Entry<String, Object>> entries = map.entrySet();
+			Entry<String, Object> entry = entries.iterator().next();
+
+			this.key = entry.getKey();
+			this.value = entry.getValue();
 		}
 
 		/**
@@ -818,7 +856,7 @@ public class QueryMapper {
 		}
 
 		public boolean isOrOrNor() {
-			return key.matches(N_OR_PATTERN);
+			return key.equalsIgnoreCase("$or") || key.equalsIgnoreCase("$nor");
 		}
 
 		/**
@@ -999,8 +1037,8 @@ public class QueryMapper {
 	 */
 	protected static class MetadataBackedField extends Field {
 
-		private static final Pattern POSITIONAL_PARAMETER_PATTERN = Pattern.compile("\\.\\$(\\[.*?\\])?|\\.\\d+");
-		private static final Pattern DOT_POSITIONAL_PATTERN = Pattern.compile("\\.\\d+");
+		private static final Pattern POSITIONAL_PARAMETER_PATTERN = Pattern.compile("\\.\\$(\\[.*?\\])?");
+		private static final Pattern DOT_POSITIONAL_PATTERN = Pattern.compile("\\.\\d+(?!$)");
 		private static final String INVALID_ASSOCIATION_REFERENCE = "Invalid path reference %s! Associations can only be pointed to directly or via their id property!";
 
 		private final MongoPersistentEntity<?> entity;
@@ -1169,8 +1207,8 @@ public class QueryMapper {
 					removePlaceholders(DOT_POSITIONAL_PATTERN, pathExpression));
 
 			if (sourceProperty != null && sourceProperty.getOwner().equals(entity)) {
-				return mappingContext
-						.getPersistentPropertyPath(PropertyPath.from(Pattern.quote(sourceProperty.getName()), entity.getTypeInformation()));
+				return mappingContext.getPersistentPropertyPath(
+						PropertyPath.from(Pattern.quote(sourceProperty.getName()), entity.getTypeInformation()));
 			}
 
 			PropertyPath path = forName(rawPath);
@@ -1178,29 +1216,47 @@ public class QueryMapper {
 				return null;
 			}
 
-			try {
+			PersistentPropertyPath<MongoPersistentProperty> propertyPath = tryToResolvePersistentPropertyPath(path);
 
-				PersistentPropertyPath<MongoPersistentProperty> propertyPath = mappingContext.getPersistentPropertyPath(path);
+			if (propertyPath == null) {
 
-				Iterator<MongoPersistentProperty> iterator = propertyPath.iterator();
-				boolean associationDetected = false;
+				if (QueryMapper.LOGGER.isInfoEnabled()) {
 
-				while (iterator.hasNext()) {
+					String types = StringUtils.collectionToDelimitedString(
+							path.stream().map(it -> it.getType().getSimpleName()).collect(Collectors.toList()), " -> ");
+					QueryMapper.LOGGER.info(
+							"Could not map '{}'. Maybe a fragment in '{}' is considered a simple type. Mapper continues with {}.",
+							path, types, pathExpression);
+				}
+				return null;
+			}
 
-					MongoPersistentProperty property = iterator.next();
+			Iterator<MongoPersistentProperty> iterator = propertyPath.iterator();
+			boolean associationDetected = false;
 
-					if (property.isAssociation()) {
-						associationDetected = true;
-						continue;
-					}
+			while (iterator.hasNext()) {
 
-					if (associationDetected && !property.isIdProperty()) {
-						throw new MappingException(String.format(INVALID_ASSOCIATION_REFERENCE, pathExpression));
-					}
+				MongoPersistentProperty property = iterator.next();
+
+				if (property.isAssociation()) {
+					associationDetected = true;
+					continue;
 				}
 
-				return propertyPath;
-			} catch (InvalidPersistentPropertyPath e) {
+				if (associationDetected && !property.isIdProperty()) {
+					throw new MappingException(String.format(INVALID_ASSOCIATION_REFERENCE, pathExpression));
+				}
+			}
+
+			return propertyPath;
+		}
+
+		@Nullable
+		private PersistentPropertyPath<MongoPersistentProperty> tryToResolvePersistentPropertyPath(PropertyPath path) {
+
+			try {
+				return mappingContext.getPersistentPropertyPath(path);
+			} catch (MappingException e) {
 				return null;
 			}
 		}
@@ -1329,12 +1385,17 @@ public class QueryMapper {
 		static class KeyMapper {
 
 			private final Iterator<String> iterator;
+			private int currentIndex;
+			private String currentPropertyRoot;
+			private final List<String> pathParts;
 
 			public KeyMapper(String key,
 					MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {
 
-				this.iterator = Arrays.asList(key.split("\\.")).iterator();
-				this.iterator.next();
+				this.pathParts = Arrays.asList(key.split("\\."));
+				this.iterator = pathParts.iterator();
+				this.currentPropertyRoot = iterator.next();
+				this.currentIndex = 0;
 			}
 
 			/**
@@ -1346,21 +1407,31 @@ public class QueryMapper {
 			protected String mapPropertyName(MongoPersistentProperty property) {
 
 				StringBuilder mappedName = new StringBuilder(PropertyToFieldNameConverter.INSTANCE.convert(property));
+
 				boolean inspect = iterator.hasNext();
 
 				while (inspect) {
 
 					String partial = iterator.next();
+					currentIndex++;
 
 					boolean isPositional = isPositionalParameter(partial) && property.isCollectionLike();
+					if (property.isMap() && currentPropertyRoot.equals(partial) && iterator.hasNext()) {
+						partial = iterator.next();
+						currentIndex++;
+					}
 
-					if (isPositional || property.isMap()) {
+					if (isPositional || property.isMap() && !currentPropertyRoot.equals(partial)) {
 						mappedName.append(".").append(partial);
 					}
 
 					inspect = isPositional && iterator.hasNext();
 				}
 
+				if (currentIndex + 1 < pathParts.size()) {
+					currentIndex++;
+					currentPropertyRoot = pathParts.get(currentIndex);
+				}
 				return mappedName.toString();
 			}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceLoader.java

@@ -0,0 +1,130 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.convert;
+
+import java.util.Collections;
+import java.util.Iterator;
+
+import org.bson.Document;
+import org.bson.conversions.Bson;
+import org.springframework.data.mongodb.core.convert.ReferenceResolver.ReferenceCollection;
+import org.springframework.lang.Nullable;
+
+import com.mongodb.client.MongoCollection;
+
+/**
+ * The {@link ReferenceLoader} obtains raw {@link Document documents} for linked entities via a
+ * {@link ReferenceLoader.DocumentReferenceQuery}.
+ *
+ * @author Christoph Strobl
+ * @since 3.3
+ */
+public interface ReferenceLoader {
+
+	/**
+	 * Obtain a single {@link Document} matching the given {@literal referenceQuery} in the {@literal context}.
+	 *
+	 * @param referenceQuery must not be {@literal null}.
+	 * @param context must not be {@literal null}.
+	 * @return the matching {@link Document} or {@literal null} if none found.
+	 */
+	@Nullable
+	default Document fetchOne(DocumentReferenceQuery referenceQuery, ReferenceCollection context) {
+
+		Iterator<Document> it = fetchMany(referenceQuery, context).iterator();
+		return it.hasNext() ? it.next() : null;
+	}
+
+	/**
+	 * Obtain multiple {@link Document} matching the given {@literal referenceQuery} in the {@literal context}.
+	 *
+	 * @param referenceQuery must not be {@literal null}.
+	 * @param context must not be {@literal null}.
+	 * @return the matching {@link Document} or {@literal null} if none found.
+	 */
+	Iterable<Document> fetchMany(DocumentReferenceQuery referenceQuery, ReferenceCollection context);
+
+	/**
+	 * The {@link DocumentReferenceQuery} defines the criteria by which {@link Document documents} should be matched
+	 * applying potentially given order criteria.
+	 */
+	interface DocumentReferenceQuery {
+
+		/**
+		 * Get the query to obtain matching {@link Document documents}.
+		 *
+		 * @return never {@literal null}.
+		 */
+		Bson getQuery();
+
+		/**
+		 * Get the sort criteria for ordering results.
+		 *
+		 * @return an empty {@link Document} by default. Never {@literal null}.
+		 */
+		default Bson getSort() {
+			return new Document();
+		}
+
+		default Iterable<Document> apply(MongoCollection<Document> collection) {
+			return restoreOrder(collection.find(getQuery()).sort(getSort()));
+		}
+
+		/**
+		 * Restore the order of fetched documents.
+		 *
+		 * @param documents must not be {@literal null}.
+		 * @return never {@literal null}.
+		 */
+		default Iterable<Document> restoreOrder(Iterable<Document> documents) {
+			return documents;
+		}
+
+		static DocumentReferenceQuery forSingleDocument(Bson bson) {
+
+			return new DocumentReferenceQuery() {
+
+				@Override
+				public Bson getQuery() {
+					return bson;
+				}
+
+				@Override
+				public Iterable<Document> apply(MongoCollection<Document> collection) {
+
+					Document result = collection.find(getQuery()).sort(getSort()).limit(1).first();
+					return result != null ? Collections.singleton(result) : Collections.emptyList();
+				}
+			};
+		}
+
+		static DocumentReferenceQuery forManyDocuments(Bson bson) {
+
+			return new DocumentReferenceQuery() {
+
+				@Override
+				public Bson getQuery() {
+					return bson;
+				}
+
+				@Override
+				public Iterable<Document> apply(MongoCollection<Document> collection) {
+					return collection.find(getQuery()).sort(getSort());
+				}
+			};
+		}
+	}
+}