DATAMONGO-2624 - Release version 2.2.11 (Moore SR11).

Original Pull Request: #885

.travis.yml

@@ -1,38 +0,0 @@
-language: java
-
-jdk:
-  - oraclejdk8
-
-before_install:
-  - mkdir -p downloads
-  - mkdir -p var/db var/log
-  - if [[ ! -d downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION} ]] ; then cd downloads && wget https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}.tgz && tar xzf mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}.tgz && cd ..; fi
-  - downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}/bin/mongod --version
-  - downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}/bin/mongod --dbpath var/db --replSet rs0 --fork --logpath var/log/mongod.log
-  - sleep 10
-  - |-
-    downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}/bin/mongo --eval "rs.initiate({_id: 'rs0', members:[{_id: 0, host: '127.0.0.1:27017'}]});"
-    sleep 15
-
-env:
-  matrix:
-    - MONGO_VERSION=4.1.10
-    - MONGO_VERSION=4.0.4
-    - MONGO_VERSION=3.6.12
-    - MONGO_VERSION=3.4.20
-  global:
-    - PROFILE=ci
-
-addons:
-  apt:
-    packages:
-      - oracle-java8-installer
-
-sudo: false
-
-cache:
-  directories:
-    - $HOME/.m2
-    - downloads
-
-script: "mvn clean dependency:list test -P${PROFILE} -Dsort"

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/2.2.x", threshold: hudson.model.Result.SUCCESS)
 	}
 
 	options {
@@ -68,7 +68,7 @@ pipeline {
 		stage("test: baseline") {
 			when {
 				anyOf {
-					branch 'master'
+					branch '2.2.x'
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -94,7 +94,7 @@ pipeline {
 		stage("Test other configurations") {
 			when {
 				anyOf {
-					branch 'master'
+					branch '2.2.x'
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -143,7 +143,7 @@ pipeline {
 		stage('Release to artifactory') {
 			when {
 				anyOf {
-					branch 'master'
+					branch '2.2.x'
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -175,7 +175,7 @@ pipeline {
 
 		stage('Publish documentation') {
 			when {
-				branch 'master'
+				branch '2.2.x'
 			}
 			agent {
 				docker {

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.0.0.M2</version>
+	<version>2.2.11.RELEASE</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>2.3.0.M2</version>
+		<version>2.2.11.RELEASE</version>
 	</parent>
 
 	<modules>
@@ -26,9 +26,9 @@
 	<properties>
 		<project.type>multi</project.type>
 		<dist.id>spring-data-mongodb</dist.id>
-		<springdata.commons>2.3.0.M2</springdata.commons>
-		<mongo>3.12.0</mongo>
-		<mongo.reactivestreams>1.13.0</mongo.reactivestreams>
+		<springdata.commons>2.2.11.RELEASE</springdata.commons>
+		<mongo>3.11.2</mongo>
+		<mongo.reactivestreams>1.12.0</mongo.reactivestreams>
 		<jmh.version>1.19</jmh.version>
 	</properties>
 
@@ -134,8 +134,8 @@
 
 	<repositories>
 		<repository>
-			<id>spring-libs-milestone</id>
-			<url>https://repo.spring.io/libs-milestone</url>
+			<id>spring-libs-release</id>
+			<url>https://repo.spring.io/libs-release</url>
 		</repository>
 	</repositories>
 

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.0.0.M2</version>
+		<version>2.2.11.RELEASE</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>3.0.0.M2</version>
+		<version>2.2.11.RELEASE</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>3.0.0.M2</version>
+		<version>2.2.11.RELEASE</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

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

@@ -25,8 +25,10 @@ public class LazyLoadingException extends UncategorizedDataAccessException {
 	private static final long serialVersionUID = -7089224903873220037L;
 
 	/**
-	 * @param msg
-	 * @param cause
+	 * Constructor for LazyLoadingException.
+	 *
+	 * @param msg the detail message.
+	 * @param cause the exception thrown by underlying data access API.
 	 */
 	public LazyLoadingException(String msg, Throwable cause) {
 		super(msg, cause);

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

@@ -79,7 +79,7 @@ public class MongoDatabaseUtils {
 	 * @param factory the {@link MongoDbFactory} to get the {@link MongoDatabase} from.
 	 * @return the {@link MongoDatabase} that is potentially associated with a transactional {@link ClientSession}.
 	 */
-	public static MongoDatabase getDatabase(String dbName, MongoDbFactory factory) {
+	public static MongoDatabase getDatabase(@Nullable String dbName, MongoDbFactory factory) {
 		return doGetMongoDatabase(dbName, factory, SessionSynchronization.ON_ACTUAL_TRANSACTION);
 	}
 
@@ -88,13 +88,13 @@ public class MongoDatabaseUtils {
 	 * <p />
 	 * Registers a {@link MongoSessionSynchronization MongoDB specific transaction synchronization} within the current
 	 * {@link Thread} if {@link TransactionSynchronizationManager#isSynchronizationActive() synchronization is active}.
-	 * 
+	 *
 	 * @param dbName the name of the {@link MongoDatabase} to get.
 	 * @param factory the {@link MongoDbFactory} to get the {@link MongoDatabase} from.
 	 * @param sessionSynchronization the synchronization to use. Must not be {@literal null}.
 	 * @return the {@link MongoDatabase} that is potentially associated with a transactional {@link ClientSession}.
 	 */
-	public static MongoDatabase getDatabase(String dbName, MongoDbFactory factory,
+	public static MongoDatabase getDatabase(@Nullable String dbName, MongoDbFactory factory,
 			SessionSynchronization sessionSynchronization) {
 		return doGetMongoDatabase(dbName, factory, sessionSynchronization);
 	}

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

@@ -37,7 +37,7 @@ public interface MongoDbFactory extends CodecRegistryProvider, MongoSessionProvi
 	/**
 	 * Creates a default {@link MongoDatabase} instance.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */
 	MongoDatabase getDb() throws DataAccessException;
@@ -46,7 +46,7 @@ public interface MongoDbFactory extends CodecRegistryProvider, MongoSessionProvi
 	 * Creates a {@link DB} instance to access the database with the given name.
 	 *
 	 * @param dbName must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */
 	MongoDatabase getDb(String dbName) throws DataAccessException;

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

@@ -38,7 +38,7 @@ public interface ReactiveMongoDatabaseFactory extends CodecRegistryProvider {
 	/**
 	 * Creates a default {@link MongoDatabase} instance.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */
 	MongoDatabase getMongoDatabase() throws DataAccessException;
@@ -47,7 +47,7 @@ public interface ReactiveMongoDatabaseFactory extends CodecRegistryProvider {
 	 * Creates a {@link MongoDatabase} instance to access the database with the given name.
 	 *
 	 * @param dbName must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */
 	MongoDatabase getMongoDatabase(String dbName) throws DataAccessException;

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

@@ -42,21 +42,21 @@ public @interface EnableMongoAuditing {
 	/**
 	 * Configures the {@link AuditorAware} bean to be used to lookup the current principal.
 	 *
-	 * @return
+	 * @return empty {@link String} by default.
 	 */
 	String auditorAwareRef() default "";
 
 	/**
 	 * Configures whether the creation and modification dates are set. Defaults to {@literal true}.
 	 *
-	 * @return
+	 * @return {@literal true} by default.
 	 */
 	boolean setDates() default true;
 
 	/**
 	 * Configures whether the entity shall be marked as modified on creation. Defaults to {@literal true}.
 	 *
-	 * @return
+	 * @return {@literal true} by default.
 	 */
 	boolean modifyOnCreate() default true;
 
@@ -64,7 +64,7 @@ public @interface EnableMongoAuditing {
 	 * Configures a {@link DateTimeProvider} bean name that allows customizing the {@link org.joda.time.DateTime} to be
 	 * used for setting creation and modification dates.
 	 *
-	 * @return
+	 * @return empty {@link String} by default.
 	 */
 	String dateTimeProviderRef() default "";
 }

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

@@ -89,7 +89,7 @@ public abstract class MongoConfigurationSupport {
 	/**
 	 * Register custom {@link Converter}s in a {@link CustomConversions} object if required. These
 	 * {@link CustomConversions} will be registered with the {@link #mappingMongoConverter()} and
-	 * {@link #mongoMappingContext()}. Returns an empty {@link MongoCustomConversions} instance by default.
+	 * {@link MongoMappingContext}. Returns an empty {@link MongoCustomConversions} instance by default.
 	 *
 	 * @return must not be {@literal null}.
 	 */

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

@@ -94,7 +94,6 @@ public interface BulkOperations {
 	 * Add a list of updates to the bulk operation. For each update request, all matching documents are updated.
 	 *
 	 * @param updates Update operations to perform.
-	 * @return The bulk operation.
 	 * @return the current {@link BulkOperations} instance with the update added, will never be {@literal null}.
 	 */
 	BulkOperations updateMulti(List<Pair<Query, Update>> updates);
@@ -105,7 +104,6 @@ public interface BulkOperations {
 	 *
 	 * @param query Update criteria.
 	 * @param update Update operation to perform.
-	 * @return The bulk operation.
 	 * @return the current {@link BulkOperations} instance with the update added, will never be {@literal null}.
 	 */
 	BulkOperations upsert(Query query, Update update);
@@ -115,7 +113,6 @@ public interface BulkOperations {
 	 * else an insert.
 	 *
 	 * @param updates Updates/insert operations to perform.
-	 * @return The bulk operation.
 	 * @return the current {@link BulkOperations} instance with the update added, will never be {@literal null}.
 	 */
 	BulkOperations upsert(List<Pair<Query, Update>> updates);

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

@@ -29,7 +29,7 @@ import com.mongodb.client.MongoCollection;
  * @author Grame Rocher
  * @author Oliver Gierke
  * @author John Brisbin
- * @auhtor Christoph Strobl
+ * @author Christoph Strobl
  * @since 1.0
  */
 public interface CollectionCallback<T> {

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

@@ -95,7 +95,7 @@ public class CollectionOptions {
 
 	/**
 	 * Create new {@link CollectionOptions} with already given settings and capped set to {@literal true}. <br />
-	 * <strong>NOTE</strong> Using capped collections requires defining {@link #size(int)}.
+	 * <strong>NOTE</strong> Using capped collections requires defining {@link #size(long)}.
 	 *
 	 * @return new {@link CollectionOptions}.
 	 * @since 2.0

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

@@ -1,227 +0,0 @@
-/*
- * Copyright 2019-2020 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 java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Collection;
-import java.util.Collections;
-import java.util.List;
-import java.util.Map;
-
-import org.bson.Document;
-
-import org.springframework.data.geo.Point;
-import org.springframework.lang.Nullable;
-import org.springframework.util.ObjectUtils;
-
-/**
- * Value object representing a count query. Count queries using {@code $near} or {@code $nearSphere} require a rewrite
- * to {@code $geoWithin}.
- *
- * @author Christoph Strobl
- * @author Mark Paluch
- * @since 3.0
- */
-class CountQuery {
-
-	private Document source;
-
-	private CountQuery(Document source) {
-		this.source = source;
-	}
-
-	public static CountQuery of(Document source) {
-		return new CountQuery(source);
-	}
-
-	/**
-	 * Returns the query {@link Document} that can be used with {@code countDocuments()}. Potentially rewrites the query
-	 * to be usable with {@code countDocuments()}.
-	 *
-	 * @return the query {@link Document} that can be used with {@code countDocuments()}.
-	 */
-	public Document toQueryDocument() {
-
-		if (!requiresRewrite(source)) {
-			return source;
-		}
-
-		Document target = new Document();
-
-		for (Map.Entry<String, Object> entry : source.entrySet()) {
-
-			if (entry.getValue() instanceof Document && requiresRewrite(entry.getValue())) {
-
-				Document theValue = (Document) entry.getValue();
-				target.putAll(createGeoWithin(entry.getKey(), theValue, source.get("$and")));
-				continue;
-			}
-
-			if (entry.getValue() instanceof Collection && requiresRewrite(entry.getValue())) {
-
-				Collection<?> source = (Collection<?>) entry.getValue();
-
-				target.put(entry.getKey(), rewriteCollection(source));
-				continue;
-			}
-
-			if ("$and".equals(entry.getKey()) && target.containsKey("$and")) {
-				// Expect $and to be processed with Document and createGeoWithin.
-				continue;
-			}
-
-			target.put(entry.getKey(), entry.getValue());
-		}
-
-		return target;
-	}
-
-	/**
-	 * @param valueToInspect
-	 * @return {@code true} if the enclosing element needs to be rewritten.
-	 */
-	private boolean requiresRewrite(Object valueToInspect) {
-
-		if (valueToInspect instanceof Document) {
-			return requiresRewrite((Document) valueToInspect);
-		}
-
-		if (valueToInspect instanceof Collection) {
-			return requiresRewrite((Collection) valueToInspect);
-		}
-
-		return false;
-	}
-
-	private boolean requiresRewrite(Collection<?> collection) {
-
-		for (Object o : collection) {
-			if (o instanceof Document && requiresRewrite((Document) o)) {
-				return true;
-			}
-		}
-
-		return false;
-	}
-
-	private boolean requiresRewrite(Document document) {
-
-		if (containsNear(document)) {
-			return true;
-		}
-
-		for (Object entry : document.values()) {
-
-			if (requiresRewrite(entry)) {
-				return true;
-			}
-		}
-
-		return false;
-	}
-
-	private Collection<Object> rewriteCollection(Collection<?> source) {
-
-		Collection<Object> rewrittenCollection = new ArrayList<>(source.size());
-
-		for (Object item : source) {
-			if (item instanceof Document && requiresRewrite(item)) {
-				rewrittenCollection.add(CountQuery.of((Document) item).toQueryDocument());
-			} else {
-				rewrittenCollection.add(item);
-			}
-		}
-
-		return rewrittenCollection;
-	}
-
-	/**
-	 * Rewrite the near query for field {@code key} to {@code $geoWithin}.
-	 *
-	 * @param key the queried field.
-	 * @param source source {@link Document}.
-	 * @param $and potentially existing {@code $and} condition.
-	 * @return the rewritten query {@link Document}.
-	 */
-	private static Document createGeoWithin(String key, Document source, @Nullable Object $and) {
-
-		boolean spheric = source.containsKey("$nearSphere");
-		Object $near = spheric ? source.get("$nearSphere") : source.get("$near");
-
-		Number maxDistance = source.containsKey("$maxDistance") ? (Number) source.get("$maxDistance") : Double.MAX_VALUE;
-		List<Object> $centerMax = Arrays.asList(toCenterCoordinates($near), maxDistance);
-		Document $geoWithinMax = new Document("$geoWithin",
-				new Document(spheric ? "$centerSphere" : "$center", $centerMax));
-
-		if (!containsNearWithMinDistance(source)) {
-			return new Document(key, $geoWithinMax);
-		}
-
-		Number minDistance = (Number) source.get("$minDistance");
-		List<Object> $centerMin = Arrays.asList(toCenterCoordinates($near), minDistance);
-		Document $geoWithinMin = new Document("$geoWithin",
-				new Document(spheric ? "$centerSphere" : "$center", $centerMin));
-
-		List<Document> criteria = new ArrayList<>();
-
-		if ($and != null) {
-			if ($and instanceof Collection) {
-				criteria.addAll((Collection) $and);
-			} else {
-				throw new IllegalArgumentException(
-						"Cannot rewrite query as it contains an '$and' element that is not a Collection!: Offending element: "
-								+ $and);
-			}
-		}
-
-		criteria.add(new Document("$nor", Collections.singletonList(new Document(key, $geoWithinMin))));
-		criteria.add(new Document(key, $geoWithinMax));
-		return new Document("$and", criteria);
-	}
-
-	private static boolean containsNear(Document source) {
-		return source.containsKey("$near") || source.containsKey("$nearSphere");
-	}
-
-	private static boolean containsNearWithMinDistance(Document source) {
-
-		if (!containsNear(source)) {
-			return false;
-		}
-
-		return source.containsKey("$minDistance");
-	}
-
-	private static Object toCenterCoordinates(Object value) {
-
-		if (ObjectUtils.isArray(value)) {
-			return value;
-		}
-
-		if (value instanceof Point) {
-			return Arrays.asList(((Point) value).getX(), ((Point) value).getY());
-		}
-
-		if (value instanceof Document && ((Document) value).containsKey("x")) {
-
-			Document point = (Document) value;
-			return Arrays.asList(point.get("x"), point.get("y"));
-		}
-
-		return value;
-	}
-}

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

@@ -43,9 +43,10 @@ public interface CursorPreparer extends ReadPreferenceAware {
 	/**
 	 * Prepare the given cursor (apply limits, skips and so on). Returns the prepared cursor.
 	 *
-	 * @param cursor
+	 * @param iterable must not be {@literal null}.
+	 * @return never {@literal null}.
 	 */
-	FindIterable<Document> prepare(FindIterable<Document> cursor);
+	FindIterable<Document> prepare(FindIterable<Document> iterable);
 
 	/**
 	 * Apply query specific settings to {@link MongoCollection} and initate a find operation returning a
@@ -53,7 +54,7 @@ public interface CursorPreparer extends ReadPreferenceAware {
 	 *
 	 * @param collection must not be {@literal null}.
 	 * @param find must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws IllegalArgumentException if one of the required arguments is {@literal null}.
 	 * @since 2.2
 	 */

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

@@ -23,7 +23,7 @@ import com.mongodb.MongoException;
 /**
  * An interface used by {@link MongoTemplate} for processing documents returned from a MongoDB query on a per-document
  * basis. Implementations of this interface perform the actual work of prcoessing each document but don't need to worry
- * about exception handling. {@MongoException}s will be caught and translated by the calling MongoTemplate An
+ * about exception handling. {@link MongoException}s will be caught and translated by the calling MongoTemplate An
  * DocumentCallbackHandler is typically stateful: It keeps the result state within the object, to be available later for
  * later inspection.
  *

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

@@ -66,7 +66,7 @@ class EntityOperations {
 	 * Creates a new {@link Entity} for the given bean.
 	 *
 	 * @param entity must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Entity}.
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
 	public <T> Entity<T> forEntity(T entity) {
@@ -89,7 +89,7 @@ class EntityOperations {
 	 *
 	 * @param entity must not be {@literal null}.
 	 * @param conversionService must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link AdaptibleEntity}.
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
 	public <T> AdaptibleEntity<T> forEntity(T entity, ConversionService conversionService) {
@@ -108,6 +108,10 @@ class EntityOperations {
 		return AdaptibleMappedEntity.of(entity, context, conversionService);
 	}
 
+	/**
+	 * @param entityClass should not be null.
+	 * @return the {@link MongoPersistentEntity#getCollection() collection name}.
+	 */
 	public String determineCollectionName(@Nullable Class<?> entityClass) {
 
 		if (entityClass == null) {
@@ -138,7 +142,7 @@ class EntityOperations {
 	 * {@code _id} if no identifier property can be found.
 	 *
 	 * @param type must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public String getIdPropertyName(Class<?> type) {
 

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

@@ -88,10 +88,10 @@ public interface ExecutableAggregationOperation {
 
 		/**
 		 * Apply pipeline operations as specified and stream all matching elements. <br />
-		 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.Cursor}
+		 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.FindIterable}
 		 *
-		 * @return a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.Cursor} that needs to be closed.
-		 *         Never {@literal null}.
+		 * @return a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.FindIterable} that needs
+		 *         to be closed. Never {@literal null}.
 		 */
 		CloseableIterator<T> stream();
 	}

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

@@ -117,8 +117,8 @@ public interface ExecutableFindOperation {
 		/**
 		 * Stream all matching elements.
 		 *
-		 * @return a {@link Stream} that wraps the a Mongo DB {@link com.mongodb.Cursor} that needs to be closed. Never
-		 *         {@literal null}.
+		 * @return a {@link Stream} that wraps the a Mongo DB {@link com.mongodb.client.FindIterable} that needs to be
+		 *         closed. Never {@literal null}.
 		 */
 		Stream<T> stream();
 

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

@@ -15,7 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import com.mongodb.ReadPreference;
 import lombok.AccessLevel;
 import lombok.NonNull;
 import lombok.RequiredArgsConstructor;
@@ -26,6 +25,7 @@ import java.util.Optional;
 import java.util.stream.Stream;
 
 import org.bson.Document;
+
 import org.springframework.dao.IncorrectResultSizeDataAccessException;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
@@ -37,6 +37,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
+import com.mongodb.ReadPreference;
 import com.mongodb.client.FindIterable;
 
 /**
@@ -257,9 +258,9 @@ class ExecutableFindOperationSupport implements ExecutableFindOperation {
 		 * @see org.springframework.data.mongodb.core.CursorPreparer#prepare(com.mongodb.clientFindIterable)
 		 */
 		@Override
-		public FindIterable<Document> prepare(FindIterable<Document> cursor) {
+		public FindIterable<Document> prepare(FindIterable<Document> iterable) {
 
-			FindIterable<Document> target = delegate != null ? delegate.prepare(cursor) : cursor;
+			FindIterable<Document> target = delegate != null ? delegate.prepare(iterable) : iterable;
 			return limit.map(target::limit).orElse(target);
 		}
 

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

@@ -17,10 +17,8 @@ package org.springframework.data.mongodb.core;
 
 import java.util.Optional;
 
-import org.springframework.data.mongodb.core.aggregation.AggregationUpdate;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.Update;
-import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.lang.Nullable;
 
 import com.mongodb.client.result.UpdateResult;
@@ -153,16 +151,13 @@ public interface ExecutableUpdateOperation {
 	interface UpdateWithUpdate<T> {
 
 		/**
-		 * Set the {@link UpdateDefinition} to be applied.
+		 * Set the {@link Update} to be applied.
 		 *
 		 * @param update must not be {@literal null}.
 		 * @return new instance of {@link TerminatingUpdate}.
 		 * @throws IllegalArgumentException if update is {@literal null}.
-		 * @since 3.0
-		 * @see Update
-		 * @see AggregationUpdate
 		 */
-		TerminatingUpdate<T> apply(UpdateDefinition update);
+		TerminatingUpdate<T> apply(Update update);
 
 		/**
 		 * Specify {@code replacement} object.

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

@@ -21,7 +21,7 @@ import lombok.RequiredArgsConstructor;
 import lombok.experimental.FieldDefaults;
 
 import org.springframework.data.mongodb.core.query.Query;
-import org.springframework.data.mongodb.core.query.UpdateDefinition;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -67,7 +67,7 @@ class ExecutableUpdateOperationSupport implements ExecutableUpdateOperation {
 		@NonNull MongoTemplate template;
 		@NonNull Class domainType;
 		Query query;
-		@Nullable UpdateDefinition update;
+		@Nullable Update update;
 		@Nullable String collection;
 		@Nullable FindAndModifyOptions findAndModifyOptions;
 		@Nullable FindAndReplaceOptions findAndReplaceOptions;
@@ -76,10 +76,10 @@ class ExecutableUpdateOperationSupport implements ExecutableUpdateOperation {
 
 		/*
 		 * (non-Javadoc)
-		 * @see org.springframework.data.mongodb.core.ExecutableUpdateOperation.UpdateWithUpdate#apply(org.springframework.data.mongodb.core.query.UpdateDefinition)
+		 * @see org.springframework.data.mongodb.core.ExecutableUpdateOperation.UpdateWithUpdate#apply(Update)
 		 */
 		@Override
-		public TerminatingUpdate<T> apply(UpdateDefinition update) {
+		public TerminatingUpdate<T> apply(Update update) {
 
 			Assert.notNull(update, "Update must not be null!");
 

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

@@ -78,7 +78,7 @@ public class FindAndModifyOptions {
 	}
 
 	/**
-	 * Create new {@link FindAndModifyOptions} based on option of given {@litearl source}.
+	 * Create new {@link FindAndModifyOptions} based on option of given {@literal source}.
 	 *
 	 * @param source can be {@literal null}.
 	 * @return new instance of {@link FindAndModifyOptions}.
@@ -117,8 +117,8 @@ public class FindAndModifyOptions {
 	/**
 	 * Define the {@link Collation} specifying language-specific rules for string comparison.
 	 *
-	 * @param collation
-	 * @return
+	 * @param collation can be {@literal null}.
+	 * @return this.
 	 * @since 2.0
 	 */
 	public FindAndModifyOptions collation(@Nullable Collation collation) {
@@ -142,7 +142,7 @@ public class FindAndModifyOptions {
 	/**
 	 * Get the {@link Collation} specifying language-specific rules for string comparison.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.0
 	 */
 	public Optional<Collation> getCollation() {

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

@@ -116,7 +116,7 @@ public class FindAndReplaceOptions {
 	/**
 	 * Get the bit indicating to return the replacement document.
 	 *
-	 * @return
+	 * @return {@literal true} if set.
 	 */
 	public boolean isReturnNew() {
 		return returnNew;
@@ -125,7 +125,7 @@ public class FindAndReplaceOptions {
 	/**
 	 * Get the bit indicating if to create a new document if not exists.
 	 *
-	 * @return
+	 * @return {@literal true} if set.
 	 */
 	public boolean isUpsert() {
 		return upsert;

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

@@ -54,7 +54,7 @@ public interface FindPublisherPreparer extends ReadPreferenceAware {
 	 *
 	 * @param collection must not be {@literal null}.
 	 * @param find must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws IllegalArgumentException if one of the required arguments is {@literal null}.
 	 * @since 2.2
 	 */

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

@@ -47,7 +47,7 @@ class GeoCommandStatistics {
 	 * Creates a new {@link GeoCommandStatistics} from the given command result extracting the statistics.
 	 *
 	 * @param commandResult must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static GeoCommandStatistics from(Document commandResult) {
 
@@ -61,7 +61,7 @@ class GeoCommandStatistics {
 	 * Returns the average distance reported by the command result. Mitigating a removal of the field in case the command
 	 * didn't return any result introduced in MongoDB 3.2 RC1.
 	 *
-	 * @return
+	 * @return never {@literal null}, uses {@link Double#NaN} if {@literal avgDistance} does not exist.
 	 * @see <a href="https://jira.mongodb.org/browse/SERVER-21024">MongoDB Jira SERVER-21024</a>
 	 */
 	public double getAverageDistance() {

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

@@ -265,7 +265,7 @@ public abstract class MongoDbFactorySupport<C> implements MongoDbFactory {
 			factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, ClientSession.class, MongoDatabase.class,
 					this::proxyDatabase, MongoCollection.class, this::proxyCollection));
 
-			return targetType.cast(factory.getProxy());
+			return targetType.cast(factory.getProxy(target.getClass().getClassLoader()));
 		}
 	}
 

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

@@ -27,7 +27,6 @@ import org.springframework.data.mongodb.core.BulkOperations.BulkMode;
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
 import org.springframework.data.mongodb.core.aggregation.AggregationResults;
-import org.springframework.data.mongodb.core.aggregation.AggregationUpdate;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
 import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
@@ -41,7 +40,6 @@ import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.Update;
-import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.util.CloseableIterator;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -59,7 +57,7 @@ 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/>
+ * <p />
  * <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>.
@@ -80,7 +78,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * The collection name used for the specified class by this template.
 	 *
 	 * @param entityClass must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	String getCollectionName(Class<?> entityClass);
 
@@ -177,7 +175,7 @@ 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/>
+	 * <p />
 	 * <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}.
 	 *
@@ -213,7 +211,7 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Obtain a {@link ClientSession} bound instance of {@link MongoOperations}.
-	 * <p/>
+	 * <p />
 	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle.
 	 *
 	 * @param session must not be {@literal null}.
@@ -362,7 +360,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Returns the {@link ScriptOperations} that can be performed on {@link com.mongodb.DB} level.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 1.7
 	 * @deprecated since 2.2. The {@code eval} command has been removed without replacement in MongoDB Server 4.2.0.
 	 */
@@ -885,15 +883,12 @@ public interface MongoOperations extends FluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents. Must not be {@literal null}.
+	 * @param update the {@link Update} to apply on matching documents. Must not be {@literal null}.
 	 * @param entityClass the parametrized type. Must not be {@literal null}.
 	 * @return the converted object that was updated before it was updated or {@literal null}, if not found.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
 	@Nullable
-	<T> T findAndModify(Query query, UpdateDefinition update, Class<T> entityClass);
+	<T> T findAndModify(Query query, Update update, Class<T> entityClass);
 
 	/**
 	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
@@ -901,16 +896,13 @@ public interface MongoOperations extends FluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents. Must not be {@literal null}.
+	 * @param update the {@link Update} to apply on matching documents. Must not be {@literal null}.
 	 * @param entityClass the parametrized type. Must not be {@literal null}.
 	 * @param collectionName the collection to query. Must not be {@literal null}.
 	 * @return the converted object that was updated before it was updated or {@literal null}, if not found.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
 	@Nullable
-	<T> T findAndModify(Query query, UpdateDefinition update, Class<T> entityClass, String collectionName);
+	<T> T findAndModify(Query query, Update update, Class<T> entityClass, String collectionName);
 
 	/**
 	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
@@ -919,18 +911,15 @@ public interface MongoOperations extends FluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents.
+	 * @param update the {@link Update} to apply on matching documents.
 	 * @param options the {@link FindAndModifyOptions} holding additional information.
 	 * @param entityClass the parametrized type.
 	 * @return the converted object that was updated or {@literal null}, if not found. Depending on the value of
 	 *         {@link FindAndModifyOptions#isReturnNew()} this will either be the object as it was before the update or as
 	 *         it is after the update.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
 	@Nullable
-	<T> T findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass);
+	<T> T findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass);
 
 	/**
 	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
@@ -939,19 +928,16 @@ public interface MongoOperations extends FluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents. Must not be {@literal null}.
+	 * @param update the {@link Update} to apply on matching documents. Must not be {@literal null}.
 	 * @param options the {@link FindAndModifyOptions} holding additional information. Must not be {@literal null}.
 	 * @param entityClass the parametrized type. Must not be {@literal null}.
 	 * @param collectionName the collection to query. Must not be {@literal null}.
 	 * @return the converted object that was updated or {@literal null}, if not found. Depending on the value of
 	 *         {@link FindAndModifyOptions#isReturnNew()} this will either be the object as it was before the update or as
 	 *         it is after the update.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
 	@Nullable
-	<T> T findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass,
+	<T> T findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass,
 			String collectionName);
 
 	/**
@@ -1300,112 +1286,99 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * Performs an upsert. If no document is found that matches the query, a new document is created and inserted by
 	 * combining the query document and the update document. <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
-	 * Use {@link #findAndModify(Query, UpdateDefinition, FindAndModifyOptions, Class, String)} instead.
+	 * Use {@link #findAndModify(Query, Update, FindAndModifyOptions, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be upserted. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing object. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing
+	 *          object. Must not be {@literal null}.
 	 * @param entityClass class that determines the collection to use. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult upsert(Query query, UpdateDefinition update, Class<?> entityClass);
+	UpdateResult upsert(Query query, Update update, Class<?> entityClass);
 
 	/**
 	 * Performs an upsert. If no document is found that matches the query, a new document is created and inserted by
 	 * combining the query document and the update document. <br />
 	 * <strong>NOTE:</strong> Any additional support for field mapping, versions, etc. is not available due to the lack of
-	 * domain type information. Use {@link #upsert(Query, UpdateDefinition, Class, String)} to get full type specific
-	 * support. <br />
+	 * domain type information. Use {@link #upsert(Query, Update, Class, String)} to get full type specific support.
+	 * <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
-	 * Use {@link #findAndModify(Query, UpdateDefinition, FindAndModifyOptions, Class, String)} instead.
+	 * Use {@link #findAndModify(Query, Update, FindAndModifyOptions, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be upserted. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing object. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing
+	 *          object. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult upsert(Query query, UpdateDefinition update, String collectionName);
+	UpdateResult upsert(Query query, Update update, String collectionName);
 
 	/**
 	 * Performs an upsert. If no document is found that matches the query, a new document is created and inserted by
-	 * combining the query document and the update document.
-	 *
+	 * combining the query document and the update document. <br />
+	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
+	 * Use {@link #findAndModify(Query, Update, FindAndModifyOptions, Class, String)} instead.
+	 * 
 	 * @param query the query document that specifies the criteria used to select a record to be upserted. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing object. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing
+	 *          object. Must not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult upsert(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName);
+	UpdateResult upsert(Query query, Update update, Class<?> entityClass, String collectionName);
 
 	/**
 	 * Updates the first object that is found in the collection of the entity class that matches the query document with
-	 * the provided update document.
+	 * the provided update document. <br />
+	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
+	 * Use {@link #findAndModify(Query, Update, Class)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class that determines the collection to use.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult updateFirst(Query query, UpdateDefinition update, Class<?> entityClass);
+	UpdateResult updateFirst(Query query, Update update, Class<?> entityClass);
 
 	/**
 	 * Updates the first object that is found in the specified collection that matches the query document criteria with
 	 * the provided updated document. <br />
 	 * <strong>NOTE:</strong> Any additional support for field mapping, versions, etc. is not available due to the lack of
-	 * domain type information. Use {@link #updateFirst(Query, UpdateDefinition, Class, String)} to get full type specific
-	 * support.
+	 * domain type information. Use {@link #updateFirst(Query, Update, Class, String)} to get full type specific support.
 	 * <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
 	 * Use {@link #findAndModify(Query, Update, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult updateFirst(Query query, UpdateDefinition update, String collectionName);
+	UpdateResult updateFirst(Query query, Update update, String collectionName);
 
 	/**
 	 * Updates the first object that is found in the specified collection that matches the query document criteria with
 	 * the provided updated document. <br />
+	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
+	 * Use {@link #findAndModify(Query, Update, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult updateFirst(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName);
+	UpdateResult updateFirst(Query query, Update update, Class<?> entityClass, String collectionName);
 
 	/**
 	 * Updates all objects that are found in the collection for the entity class that matches the query document criteria
@@ -1413,34 +1386,27 @@ public interface MongoOperations extends FluentMongoOperations {
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult updateMulti(Query query, UpdateDefinition update, Class<?> entityClass);
+	UpdateResult updateMulti(Query query, Update update, Class<?> entityClass);
 
 	/**
 	 * Updates all objects that are found in the specified collection that matches the query document criteria with the
 	 * provided updated document. <br />
 	 * <strong>NOTE:</strong> Any additional support for field mapping, versions, etc. is not available due to the lack of
-	 * domain type information. Use {@link #updateMulti(Query, UpdateDefinition, Class, String)} to get full type specific
-	 * support.
+	 * domain type information. Use {@link #updateMulti(Query, Update, Class, String)} to get full type specific support.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult updateMulti(Query query, UpdateDefinition update, String collectionName);
+	UpdateResult updateMulti(Query query, Update update, String collectionName);
 
 	/**
 	 * Updates all objects that are found in the collection for the entity class that matches the query document criteria
@@ -1448,16 +1414,13 @@ public interface MongoOperations extends FluentMongoOperations {
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	UpdateResult updateMulti(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName);
+	UpdateResult updateMulti(Query query, Update update, Class<?> entityClass, String collectionName);
 
 	/**
 	 * Remove the given object from the collection by {@literal id} and (if applicable) its
@@ -1557,7 +1520,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Returns the underlying {@link MongoConverter}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	MongoConverter getConverter();
 }

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

@@ -35,7 +35,6 @@ import org.bson.codecs.Codec;
 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;
@@ -69,9 +68,7 @@ import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
 import org.springframework.data.mongodb.core.aggregation.AggregationResults;
-import org.springframework.data.mongodb.core.aggregation.AggregationUpdate;
 import org.springframework.data.mongodb.core.aggregation.Fields;
-import org.springframework.data.mongodb.core.aggregation.RelaxedTypeBasedAggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.TypeBasedAggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
 import org.springframework.data.mongodb.core.convert.DbRefResolver;
@@ -111,6 +108,7 @@ import org.springframework.data.mongodb.core.query.Meta;
 import org.springframework.data.mongodb.core.query.Meta.CursorOption;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.mongodb.core.query.UpdateDefinition.ArrayFilter;
 import org.springframework.data.mongodb.core.validation.Validator;
@@ -437,7 +435,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@SuppressWarnings("ConstantConditions")
-	protected <T> CloseableIterator<T> doStream(Query query, Class<?> entityType, String collectionName,
+	protected <T> CloseableIterator<T> doStream(Query query, final Class<?> entityType, String collectionName,
 			Class<T> returnType) {
 
 		Assert.notNull(query, "Query must not be null!");
@@ -471,7 +469,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 */
 	@Override
 	@SuppressWarnings("ConstantConditions")
-	public Document executeCommand(String jsonCommand) {
+	public Document executeCommand(final String jsonCommand) {
 
 		Assert.hasText(jsonCommand, "JsonCommand must not be null nor empty!");
 
@@ -484,7 +482,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 */
 	@Override
 	@SuppressWarnings("ConstantConditions")
-	public Document executeCommand(Document command) {
+	public Document executeCommand(final Document command) {
 
 		Assert.notNull(command, "Command must not be null!");
 
@@ -655,7 +653,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.MongoOperations#createCollection(java.lang.String)
 	 */
-	public MongoCollection<Document> createCollection(String collectionName) {
+	public MongoCollection<Document> createCollection(final String collectionName) {
 
 		Assert.notNull(collectionName, "CollectionName must not be null!");
 
@@ -666,8 +664,8 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.MongoOperations#createCollection(java.lang.String, org.springframework.data.mongodb.core.CollectionOptions)
 	 */
-	public MongoCollection<Document> createCollection(String collectionName,
-			@Nullable CollectionOptions collectionOptions) {
+	public MongoCollection<Document> createCollection(final String collectionName,
+			final @Nullable CollectionOptions collectionOptions) {
 
 		Assert.notNull(collectionName, "CollectionName must not be null!");
 		return doCreateCollection(collectionName, convertToDocument(collectionOptions));
@@ -678,7 +676,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @see org.springframework.data.mongodb.core.MongoOperations#getCollection(java.lang.String)
 	 */
 	@SuppressWarnings("ConstantConditions")
-	public MongoCollection<Document> getCollection(String collectionName) {
+	public MongoCollection<Document> getCollection(final String collectionName) {
 
 		Assert.notNull(collectionName, "CollectionName must not be null!");
 
@@ -698,7 +696,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @see org.springframework.data.mongodb.core.ExecutableInsertOperation#getCollection(java.lang.String)
 	 */
 	@SuppressWarnings("ConstantConditions")
-	public boolean collectionExists(String collectionName) {
+	public boolean collectionExists(final String collectionName) {
 
 		Assert.notNull(collectionName, "CollectionName must not be null!");
 
@@ -729,13 +727,15 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		Assert.notNull(collectionName, "CollectionName must not be null!");
 
-		execute(collectionName, (CollectionCallback<Void>) collection -> {
-			collection.drop();
-			if (LOGGER.isDebugEnabled()) {
-				LOGGER.debug("Dropped collection [{}]",
-						collection.getNamespace() != null ? collection.getNamespace().getCollectionName() : collectionName);
+		execute(collectionName, new CollectionCallback<Void>() {
+			public Void doInCollection(MongoCollection<Document> collection) throws MongoException, DataAccessException {
+				collection.drop();
+				if (LOGGER.isDebugEnabled()) {
+					LOGGER.debug("Dropped collection [{}]",
+							collection.getNamespace() != null ? collection.getNamespace().getCollectionName() : collectionName);
+				}
+				return null;
 			}
-			return null;
 		});
 	}
 
@@ -1045,25 +1045,25 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 	@Nullable
 	@Override
-	public <T> T findAndModify(Query query, UpdateDefinition update, Class<T> entityClass) {
+	public <T> T findAndModify(Query query, Update update, Class<T> entityClass) {
 		return findAndModify(query, update, new FindAndModifyOptions(), entityClass, getCollectionName(entityClass));
 	}
 
 	@Nullable
 	@Override
-	public <T> T findAndModify(Query query, UpdateDefinition update, Class<T> entityClass, String collectionName) {
+	public <T> T findAndModify(Query query, Update update, Class<T> entityClass, String collectionName) {
 		return findAndModify(query, update, new FindAndModifyOptions(), entityClass, collectionName);
 	}
 
 	@Nullable
 	@Override
-	public <T> T findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass) {
+	public <T> T findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass) {
 		return findAndModify(query, update, options, entityClass, getCollectionName(entityClass));
 	}
 
 	@Nullable
 	@Override
-	public <T> T findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass,
+	public <T> T findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass,
 			String collectionName) {
 
 		Assert.notNull(query, "Query must not be null!");
@@ -1152,7 +1152,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@Override
-	public long count(Query query, String collectionName) {
+	public long count(final Query query, String collectionName) {
 		return count(query, null, collectionName);
 	}
 
@@ -1191,8 +1191,11 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			LOGGER.debug("Executing count: {} in collection: {}", serializeToJsonSafely(filter), collectionName);
 		}
 
-		return execute(collectionName,
-				collection -> collection.countDocuments(CountQuery.of(filter).toQueryDocument(), options));
+		if (MongoDatabaseUtils.isTransactionActive(getMongoDbFactory())) {
+			return execute(collectionName, collection -> collection.countDocuments(filter, options));
+		}
+
+		return execute(collectionName, collection -> collection.count(filter, options));
 	}
 
 	/*
@@ -1233,7 +1236,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 	/**
 	 * Prepare the collection before any processing is done using it. This allows a convenient way to apply settings like
-	 * slaveOk() etc. Can be overridden in sub-classes.
+	 * withCodecRegistry() etc. Can be overridden in sub-classes.
 	 *
 	 * @param collection
 	 */
@@ -1482,7 +1485,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@SuppressWarnings("ConstantConditions")
-	protected Object insertDocument(String collectionName, Document document, Class<?> entityClass) {
+	protected Object insertDocument(final String collectionName, final Document document, final Class<?> entityClass) {
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("Inserting Document containing fields: {} in collection: {}", document.keySet(), collectionName);
@@ -1503,7 +1506,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		});
 	}
 
-	protected List<Object> insertDocumentList(String collectionName, List<Document> documents) {
+	protected List<Object> insertDocumentList(final String collectionName, final List<Document> documents) {
 
 		if (documents.isEmpty()) {
 			return Collections.emptyList();
@@ -1531,47 +1534,49 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		return MappedDocument.toIds(documents);
 	}
 
-	protected Object saveDocument(String collectionName, Document dbDoc, Class<?> entityClass) {
+	protected Object saveDocument(final String collectionName, final Document dbDoc, final Class<?> entityClass) {
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("Saving Document containing fields: {}", dbDoc.keySet());
 		}
 
-		return execute(collectionName, collection -> {
-			MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.SAVE, collectionName, entityClass,
-					dbDoc, null);
-			WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
+		return execute(collectionName, new CollectionCallback<Object>() {
+			public Object doInCollection(MongoCollection<Document> collection) throws MongoException, DataAccessException {
+				MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.SAVE, collectionName, entityClass,
+						dbDoc, null);
+				WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
 
-			MappedDocument mapped = MappedDocument.of(dbDoc);
+				MappedDocument mapped = MappedDocument.of(dbDoc);
 
-			if (!mapped.hasId()) {
-				if (writeConcernToUse == null) {
-					collection.insertOne(dbDoc);
+				if (!mapped.hasId()) {
+					if (writeConcernToUse == null) {
+						collection.insertOne(dbDoc);
+					} else {
+						collection.withWriteConcern(writeConcernToUse).insertOne(dbDoc);
+					}
+				} else if (writeConcernToUse == null) {
+					collection.replaceOne(mapped.getIdFilter(), dbDoc, new ReplaceOptions().upsert(true));
 				} else {
-					collection.withWriteConcern(writeConcernToUse).insertOne(dbDoc);
+					collection.withWriteConcern(writeConcernToUse).replaceOne(mapped.getIdFilter(), dbDoc,
+							new ReplaceOptions().upsert(true));
 				}
-			} else if (writeConcernToUse == null) {
-				collection.replaceOne(mapped.getIdFilter(), dbDoc, new ReplaceOptions().upsert(true));
-			} else {
-				collection.withWriteConcern(writeConcernToUse).replaceOne(mapped.getIdFilter(), dbDoc,
-						new ReplaceOptions().upsert(true));
+				return mapped.getId();
 			}
-			return mapped.getId();
 		});
 	}
 
 	@Override
-	public UpdateResult upsert(Query query, UpdateDefinition update, Class<?> entityClass) {
+	public UpdateResult upsert(Query query, Update update, Class<?> entityClass) {
 		return doUpdate(getCollectionName(entityClass), query, update, entityClass, true, false);
 	}
 
 	@Override
-	public UpdateResult upsert(Query query, UpdateDefinition update, String collectionName) {
+	public UpdateResult upsert(Query query, Update update, String collectionName) {
 		return doUpdate(collectionName, query, update, null, true, false);
 	}
 
 	@Override
-	public UpdateResult upsert(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName) {
+	public UpdateResult upsert(Query query, Update update, Class<?> entityClass, String collectionName) {
 
 		Assert.notNull(entityClass, "EntityClass must not be null!");
 
@@ -1579,17 +1584,17 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@Override
-	public UpdateResult updateFirst(Query query, UpdateDefinition update, Class<?> entityClass) {
+	public UpdateResult updateFirst(Query query, Update update, Class<?> entityClass) {
 		return doUpdate(getCollectionName(entityClass), query, update, entityClass, false, false);
 	}
 
 	@Override
-	public UpdateResult updateFirst(Query query, UpdateDefinition update, String collectionName) {
+	public UpdateResult updateFirst(final Query query, final Update update, final String collectionName) {
 		return doUpdate(collectionName, query, update, null, false, false);
 	}
 
 	@Override
-	public UpdateResult updateFirst(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName) {
+	public UpdateResult updateFirst(Query query, Update update, Class<?> entityClass, String collectionName) {
 
 		Assert.notNull(entityClass, "EntityClass must not be null!");
 
@@ -1597,17 +1602,17 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@Override
-	public UpdateResult updateMulti(Query query, UpdateDefinition update, Class<?> entityClass) {
+	public UpdateResult updateMulti(Query query, Update update, Class<?> entityClass) {
 		return doUpdate(getCollectionName(entityClass), query, update, entityClass, false, true);
 	}
 
 	@Override
-	public UpdateResult updateMulti(Query query, UpdateDefinition update, String collectionName) {
+	public UpdateResult updateMulti(final Query query, final Update update, String collectionName) {
 		return doUpdate(collectionName, query, update, null, false, true);
 	}
 
 	@Override
-	public UpdateResult updateMulti(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName) {
+	public UpdateResult updateMulti(final Query query, final Update update, Class<?> entityClass, String collectionName) {
 
 		Assert.notNull(entityClass, "EntityClass must not be null!");
 
@@ -1615,8 +1620,8 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@SuppressWarnings("ConstantConditions")
-	protected UpdateResult doUpdate(String collectionName, Query query, UpdateDefinition update,
-			@Nullable Class<?> entityClass, boolean upsert, boolean multi) {
+	protected UpdateResult doUpdate(final String collectionName, final Query query, final UpdateDefinition update,
+			@Nullable final Class<?> entityClass, final boolean upsert, final boolean multi) {
 
 		Assert.notNull(collectionName, "CollectionName must not be null!");
 		Assert.notNull(query, "Query must not be null!");
@@ -1628,54 +1633,25 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 					upsert ? "Upsert" : "UpdateFirst", serializeToJsonSafely(query.getSortObject()));
 		}
 
-		MongoPersistentEntity<?> entity = entityClass == null ? null : getPersistentEntity(entityClass);
-		increaseVersionForUpdateIfNecessary(entity, update);
-
-		UpdateOptions opts = new UpdateOptions();
-		opts.upsert(upsert);
-
-		if (update.hasArrayFilters()) {
-			opts.arrayFilters(update.getArrayFilters().stream().map(ArrayFilter::asDocument).collect(Collectors.toList()));
-		}
-
-		Document queryObj = new Document();
-
-		if (query != null) {
-			queryObj.putAll(queryMapper.getMappedObject(query.getQueryObject(), entity));
-		}
-
-		if (multi && update.isIsolated() && !queryObj.containsKey("$isolated")) {
-			queryObj.put("$isolated", 1);
-		}
-
-		if (update instanceof AggregationUpdate) {
-
-			AggregationOperationContext context = entityClass != null
-					? new RelaxedTypeBasedAggregationOperationContext(entityClass, mappingContext, queryMapper)
-					: Aggregation.DEFAULT_CONTEXT;
-
-			List<Document> pipeline = new AggregationUtil(queryMapper, mappingContext)
-					.createPipeline((AggregationUpdate) update, context);
-
-			return execute(collectionName, collection -> {
-
-				if (LOGGER.isDebugEnabled()) {
-					LOGGER.debug("Calling update using query: {} and update: {} in collection: {}",
-							serializeToJsonSafely(queryObj), serializeToJsonSafely(pipeline), collectionName);
-				}
-
-				MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.UPDATE, collectionName,
-						entityClass, update.getUpdateObject(), queryObj);
-				WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
-
-				collection = writeConcernToUse != null ? collection.withWriteConcern(writeConcernToUse) : collection;
-
-				return multi ? collection.updateMany(queryObj, pipeline, opts) : collection.updateOne(queryObj, pipeline, opts);
-			});
-		}
-
 		return execute(collectionName, collection -> {
 
+			MongoPersistentEntity<?> entity = entityClass == null ? null : getPersistentEntity(entityClass);
+
+			increaseVersionForUpdateIfNecessary(entity, update);
+
+			UpdateOptions opts = new UpdateOptions();
+			opts.upsert(upsert);
+
+			if (update.hasArrayFilters()) {
+				opts.arrayFilters(update.getArrayFilters().stream().map(ArrayFilter::asDocument).collect(Collectors.toList()));
+			}
+
+			Document queryObj = new Document();
+
+			if (query != null) {
+				queryObj.putAll(queryMapper.getMappedObject(query.getQueryObject(), entity));
+			}
+
 			operations.forType(entityClass) //
 					.getCollation(query) //
 					.map(Collation::toMongoCollation) //
@@ -1684,6 +1660,10 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			Document updateObj = update instanceof MappedUpdate ? update.getUpdateObject()
 					: updateMapper.getMappedObject(update.getUpdateObject(), entity);
 
+			if (multi && update.isIsolated() && !queryObj.containsKey("$isolated")) {
+				queryObj.put("$isolated", 1);
+			}
+
 			if (LOGGER.isDebugEnabled()) {
 				LOGGER.debug("Calling update using query: {} and update: {} in collection: {}", serializeToJsonSafely(queryObj),
 						serializeToJsonSafely(updateObj), collectionName);
@@ -1703,8 +1683,11 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 				return collection.replaceOne(queryObj, updateObj, replaceOptions);
 			} else {
-				return multi ? collection.updateMany(queryObj, updateObj, opts)
-						: collection.updateOne(queryObj, updateObj, opts);
+				if (multi) {
+					return collection.updateMany(queryObj, updateObj, opts);
+				} else {
+					return collection.updateOne(queryObj, updateObj, opts);
+				}
 			}
 		});
 	}
@@ -1757,14 +1740,14 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	@SuppressWarnings("ConstantConditions")
-	protected <T> DeleteResult doRemove(String collectionName, Query query, @Nullable Class<T> entityClass,
-			boolean multi) {
+	protected <T> DeleteResult doRemove(final String collectionName, final Query query,
+			@Nullable final Class<T> entityClass, boolean multi) {
 
 		Assert.notNull(query, "Query must not be null!");
 		Assert.hasText(collectionName, "Collection name must not be null or empty!");
 
-		MongoPersistentEntity<?> entity = getPersistentEntity(entityClass);
-		Document queryObject = queryMapper.getMappedObject(query.getQueryObject(), entity);
+		final MongoPersistentEntity<?> entity = getPersistentEntity(entityClass);
+		final Document queryObject = queryMapper.getMappedObject(query.getQueryObject(), entity);
 
 		return execute(collectionName, collection -> {
 
@@ -2162,7 +2145,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	protected <O> AggregationResults<O> doAggregate(Aggregation aggregation, String collectionName, Class<O> outputType,
 			AggregationOperationContext context) {
 
-		ReadDocumentCallback<O> callback = new ReadDocumentCallback<>(mongoConverter, outputType, collectionName);
+		DocumentCallback<O> callback = new UnwrapAndReadDocumentCallback<>(mongoConverter, outputType, collectionName);
 
 		AggregationOptions options = aggregation.getOptions();
 		AggregationUtil aggregationUtil = new AggregationUtil(queryMapper, mappingContext);
@@ -2207,10 +2190,6 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			options.getComment().ifPresent(aggregateIterable::comment);
 
-			if (options.hasExecutionTimeLimit()) {
-				aggregateIterable = aggregateIterable.maxTime(options.getMaxTime().toMillis(), TimeUnit.MILLISECONDS);
-			}
-
 			MongoIterable<O> iterable = aggregateIterable.map(val -> {
 
 				rawResult.add(val);
@@ -2672,7 +2651,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 	@SuppressWarnings("ConstantConditions")
 	protected <T> T doFindAndModify(String collectionName, Document query, Document fields, Document sort,
-			Class<T> entityClass, UpdateDefinition update, @Nullable FindAndModifyOptions options) {
+			Class<T> entityClass, Update update, @Nullable FindAndModifyOptions options) {
 
 		EntityReader<? super T, Bson> readerToUse = this.mongoConverter;
 
@@ -2685,18 +2664,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		increaseVersionForUpdateIfNecessary(entity, update);
 
 		Document mappedQuery = queryMapper.getMappedObject(query, entity);
-
-		Object mappedUpdate;
-		if (update instanceof AggregationUpdate) {
-
-			AggregationOperationContext context = entityClass != null
-					? new RelaxedTypeBasedAggregationOperationContext(entityClass, mappingContext, queryMapper)
-					: Aggregation.DEFAULT_CONTEXT;
-
-			mappedUpdate = new AggregationUtil(queryMapper, mappingContext).createPipeline((Aggregation) update, context);
-		} else {
-			mappedUpdate = updateMapper.getMappedObject(update.getUpdateObject(), entity);
-		}
+		Document mappedUpdate = updateMapper.getMappedObject(update.getUpdateObject(), entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug(
@@ -3070,11 +3038,11 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		private final Document query;
 		private final Document fields;
 		private final Document sort;
-		private final Object update;
+		private final Document update;
 		private final List<Document> arrayFilters;
 		private final FindAndModifyOptions options;
 
-		public FindAndModifyCallback(Document query, Document fields, Document sort, Object update,
+		public FindAndModifyCallback(Document query, Document fields, Document sort, Document update,
 				List<Document> arrayFilters, FindAndModifyOptions options) {
 			this.query = query;
 			this.fields = fields;
@@ -3102,13 +3070,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				opts.arrayFilters(arrayFilters);
 			}
 
-			if (update instanceof Document) {
-				return collection.findOneAndUpdate(query, (Document) update, opts);
-			} else if (update instanceof List) {
-				return collection.findOneAndUpdate(query, (List<Document>) update, opts);
-			}
-
-			throw new IllegalArgumentException(String.format("Using %s is not supported in findOneAndUpdate", update));
+			return collection.findOneAndUpdate(query, update, opts);
 		}
 	}
 
@@ -3193,12 +3155,13 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		@Nullable
 		public T doWith(@Nullable Document object) {
 
+			T source = null;
+
 			if (null != object) {
 				maybeEmitEvent(new AfterLoadEvent<>(object, type, collectionName));
+				source = reader.read(type, object);
 			}
 
-			T source = reader.read(type, object);
-
 			if (null != source) {
 				maybeEmitEvent(new AfterConvertEvent<>(object, source, collectionName));
 			}
@@ -3238,9 +3201,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			Class<?> typeToRead = targetType.isInterface() || targetType.isAssignableFrom(entityType) ? entityType
 					: targetType;
 
-			if (null != object) {
-				maybeEmitEvent(new AfterLoadEvent<>(object, targetType, collectionName));
-			}
+			maybeEmitEvent(new AfterLoadEvent<>(object, targetType, collectionName));
 
 			Object source = reader.read(typeToRead, object);
 			Object result = targetType.isInterface() ? projectionFactory.createProjection(targetType, source) : source;
@@ -3253,6 +3214,39 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		}
 	}
 
+	class UnwrapAndReadDocumentCallback<T> extends ReadDocumentCallback<T> {
+
+		public UnwrapAndReadDocumentCallback(EntityReader<? super T, Bson> reader, Class<T> type, String collectionName) {
+			super(reader, type, collectionName);
+		}
+
+		@Override
+		public T doWith(@Nullable Document object) {
+
+			if (object == null) {
+				return null;
+			}
+
+			Object idField = object.get(Fields.UNDERSCORE_ID);
+
+			if (!(idField instanceof Document)) {
+				return super.doWith(object);
+			}
+
+			Document toMap = new Document();
+			Document nested = (Document) idField;
+			toMap.putAll(nested);
+
+			for (String key : object.keySet()) {
+				if (!Fields.UNDERSCORE_ID.equals(key)) {
+					toMap.put(key, object.get(key));
+				}
+			}
+
+			return super.doWith(toMap);
+		}
+	}
+
 	class QueryCursorPreparer implements CursorPreparer {
 
 		private final Query query;
@@ -3268,9 +3262,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.CursorPreparer#prepare(com.mongodb.DBCursor)
 		 */
-		public FindIterable<Document> prepare(FindIterable<Document> cursor) {
+		public FindIterable<Document> prepare(FindIterable<Document> iterable) {
 
-			FindIterable<Document> cursorToUse = cursor;
+			FindIterable<Document> cursorToUse = iterable;
 
 			operations.forType(type).getCollation(query) //
 					.map(Collation::toMongoCollation) //
@@ -3330,6 +3324,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 							case PARTIAL:
 								cursorToUse = cursorToUse.partial(true);
 								break;
+							case SECONDARY_READS:
 							case SLAVE_OK:
 								break;
 							default:
@@ -3347,7 +3342,8 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		@Override
 		public ReadPreference getReadPreference() {
-			return query.getMeta().getFlags().contains(CursorOption.SLAVE_OK) ? ReadPreference.primaryPreferred() : null;
+			return (query.getMeta().getFlags().contains(CursorOption.SECONDARY_READS)
+					|| query.getMeta().getFlags().contains(CursorOption.SLAVE_OK)) ? ReadPreference.primaryPreferred() : null;
 		}
 	}
 
@@ -3528,5 +3524,19 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			// native MongoDB objects that offer methods with ClientSession must not be proxied.
 			return delegate.getDb();
 		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.MongoTemplate#doCount(java.lang.String, org.bson.Document, com.mongodb.client.model.CountOptions)
+		 */
+		@Override
+		protected long doCount(String collectionName, Document filter, CountOptions options) {
+
+			if (!session.hasActiveTransaction()) {
+				return super.doCount(collectionName, filter, options);
+			}
+
+			return execute(collectionName, collection -> collection.countDocuments(filter, options));
+		}
 	}
 }

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

@@ -29,7 +29,6 @@ import org.springframework.data.geo.GeoResult;
 import org.springframework.data.mongodb.ReactiveMongoDatabaseFactory;
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
-import org.springframework.data.mongodb.core.aggregation.AggregationUpdate;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
 import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
@@ -40,7 +39,6 @@ import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.Update;
-import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.lang.Nullable;
 import org.springframework.transaction.reactive.TransactionalOperator;
 import org.springframework.util.Assert;
@@ -680,14 +678,11 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents. Must not be {@literal null}.
+	 * @param update the {@link Update} to apply on matching documents. Must not be {@literal null}.
 	 * @param entityClass the parametrized type. Must not be {@literal null}.
 	 * @return the converted object that was updated before it was updated.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, Class<T> entityClass);
+	<T> Mono<T> findAndModify(Query query, Update update, Class<T> entityClass);
 
 	/**
 	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
@@ -695,15 +690,12 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents. Must not be {@literal null}.
+	 * @param update the {@link Update} to apply on matching documents. Must not be {@literal null}.
 	 * @param entityClass the parametrized type. Must not be {@literal null}.
 	 * @param collectionName the collection to query. Must not be {@literal null}.
 	 * @return the converted object that was updated before it was updated.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, Class<T> entityClass, String collectionName);
+	<T> Mono<T> findAndModify(Query query, Update update, Class<T> entityClass, String collectionName);
 
 	/**
 	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
@@ -712,16 +704,13 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents.
+	 * @param update the {@link Update} to apply on matching documents.
 	 * @param options the {@link FindAndModifyOptions} holding additional information.
 	 * @param entityClass the parametrized type.
 	 * @return the converted object that was updated. Depending on the value of {@link FindAndModifyOptions#isReturnNew()}
 	 *         this will either be the object as it was before the update or as it is after the update.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass);
+	<T> Mono<T> findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass);
 
 	/**
 	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify<a/>
@@ -730,17 +719,14 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
-	 * @param update the {@link UpdateDefinition} to apply on matching documents. Must not be {@literal null}.
+	 * @param update the {@link Update} to apply on matching documents. Must not be {@literal null}.
 	 * @param options the {@link FindAndModifyOptions} holding additional information. Must not be {@literal null}.
 	 * @param entityClass the parametrized type. Must not be {@literal null}.
 	 * @param collectionName the collection to query. Must not be {@literal null}.
 	 * @return the converted object that was updated. Depending on the value of {@link FindAndModifyOptions#isReturnNew()}
 	 *         this will either be the object as it was before the update or as it is after the update.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	<T> Mono<T> findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options, Class<T> entityClass,
+	<T> Mono<T> findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass,
 			String collectionName);
 
 	/**
@@ -1163,111 +1149,99 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * Performs an upsert. If no document is found that matches the query, a new document is created and inserted by
 	 * combining the query document and the update document. <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
-	 * Use {@link #findAndModify(Query, UpdateDefinition, Class)} instead.
+	 * Use {@link #findAndModify(Query, Update, Class)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be upserted. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing object. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing
+	 *          object. Must not be {@literal null}.
 	 * @param entityClass class that determines the collection to use. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> upsert(Query query, UpdateDefinition update, Class<?> entityClass);
+	Mono<UpdateResult> upsert(Query query, Update update, Class<?> entityClass);
 
 	/**
 	 * Performs an upsert. If no document is found that matches the query, a new document is created and inserted by
 	 * combining the query document and the update document. <br />
 	 * <strong>NOTE:</strong> Any additional support for field mapping, versions, etc. is not available due to the lack of
-	 * domain type information. Use {@link #upsert(Query, UpdateDefinition, Class, String)} to get full type specific
-	 * support.
+	 * domain type information. Use {@link #upsert(Query, Update, Class, String)} to get full type specific support.
+	 * <br />
+	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
+	 * Use {@link #findAndModify(Query, Update, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be upserted. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing object. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing
+	 *          object. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> upsert(Query query, UpdateDefinition update, String collectionName);
+	Mono<UpdateResult> upsert(Query query, Update update, String collectionName);
 
 	/**
 	 * Performs an upsert. If no document is found that matches the query, a new document is created and inserted by
-	 * combining the query document and the update document.
+	 * combining the query document and the update document. <br />
+	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
+	 * Use {@link #findAndModify(Query, Update, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be upserted. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing object. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing
+	 *          object. Must not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> upsert(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName);
+	Mono<UpdateResult> upsert(Query query, Update update, Class<?> entityClass, String collectionName);
 
 	/**
 	 * Updates the first object that is found in the collection of the entity class that matches the query document with
 	 * the provided update document. <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
-	 * Use {@link #findAndModify(Query, UpdateDefinition, Class)} instead.
+	 * Use {@link #findAndModify(Query, Update, Class)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class that determines the collection to use.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> updateFirst(Query query, UpdateDefinition update, Class<?> entityClass);
+	Mono<UpdateResult> updateFirst(Query query, Update update, Class<?> entityClass);
 
 	/**
 	 * Updates the first object that is found in the specified collection that matches the query document criteria with
 	 * the provided updated document. <br />
 	 * <strong>NOTE:</strong> Any additional support for field mapping, versions, etc. is not available due to the lack of
-	 * domain type information. Use {@link #updateFirst(Query, UpdateDefinition, Class, String)} to get full type specific
-	 * support. <br />
+	 * domain type information. Use {@link #updateFirst(Query, Update, Class, String)} to get full type specific support.
+	 * <br />
+	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
+	 * Use {@link #findAndModify(Query, UpdateDefinition, Class, String)} instead.
+	 *
+	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
+	 *          {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
+	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
+	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
+	 */
+	Mono<UpdateResult> updateFirst(Query query, Update update, String collectionName);
+
+	/**
+	 * Updates the first object that is found in the specified collection that matches the query document criteria with
+	 * the provided updated document. <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
 	 * Use {@link #findAndModify(Query, Update, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
-	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
-	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
-	 */
-	Mono<UpdateResult> updateFirst(Query query, UpdateDefinition update, String collectionName);
-
-	/**
-	 * Updates the first object that is found in the specified collection that matches the query document criteria with
-	 * the provided updated document. <br />
-	 *
-	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
-	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> updateFirst(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName);
+	Mono<UpdateResult> updateFirst(Query query, Update update, Class<?> entityClass, String collectionName);
 
 	/**
 	 * Updates all objects that are found in the collection for the entity class that matches the query document criteria
@@ -1275,34 +1249,27 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> updateMulti(Query query, UpdateDefinition update, Class<?> entityClass);
+	Mono<UpdateResult> updateMulti(Query query, Update update, Class<?> entityClass);
 
 	/**
 	 * Updates all objects that are found in the specified collection that matches the query document criteria with the
 	 * provided updated document. <br />
 	 * <strong>NOTE:</strong> Any additional support for field mapping, versions, etc. is not available due to the lack of
-	 * domain type information. Use {@link #updateMulti(Query, UpdateDefinition, Class, String)} to get full type specific
-	 * support.
+	 * domain type information. Use {@link #updateMulti(Query, Update, Class, String)} to get full type specific support.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> updateMulti(Query query, UpdateDefinition update, String collectionName);
+	Mono<UpdateResult> updateMulti(Query query, Update update, String collectionName);
 
 	/**
 	 * Updates all objects that are found in the collection for the entity class that matches the query document criteria
@@ -1310,16 +1277,13 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
-	 * @param update the {@link UpdateDefinition} that contains the updated object or {@code $} operators to manipulate
-	 *          the existing. Must not be {@literal null}.
+	 * @param update the update document that contains the updated object or $ operators to manipulate the existing. Must
+	 *          not be {@literal null}.
 	 * @param entityClass class of the pojo to be operated on. Must not be {@literal null}.
 	 * @param collectionName name of the collection to update the object in. Must not be {@literal null}.
 	 * @return the {@link UpdateResult} which lets you access the results of the previous write.
-	 * @since 3.0
-	 * @see Update
-	 * @see AggregationUpdate
 	 */
-	Mono<UpdateResult> updateMulti(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName);
+	Mono<UpdateResult> updateMulti(Query query, Update update, Class<?> entityClass, String collectionName);
 
 	/**
 	 * Remove the given object from the collection by id.
@@ -1571,7 +1535,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Returns the underlying {@link MongoConverter}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	MongoConverter getConverter();
 
@@ -1579,7 +1543,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * The collection name used for the specified class by this template.
 	 *
 	 * @param entityClass must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.1
 	 */
 	String getCollectionName(Class<?> entityClass);

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

@@ -40,6 +40,7 @@ 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;
@@ -69,9 +70,7 @@ import org.springframework.data.mongodb.core.EntityOperations.AdaptibleEntity;
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
-import org.springframework.data.mongodb.core.aggregation.AggregationUpdate;
 import org.springframework.data.mongodb.core.aggregation.PrefixingDelegatingAggregationOperationContext;
-import org.springframework.data.mongodb.core.aggregation.RelaxedTypeBasedAggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.TypeBasedAggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
 import org.springframework.data.mongodb.core.convert.DbRefResolver;
@@ -107,6 +106,7 @@ import org.springframework.data.mongodb.core.query.Meta;
 import org.springframework.data.mongodb.core.query.Meta.CursorOption;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.mongodb.core.query.UpdateDefinition.ArrayFilter;
 import org.springframework.data.mongodb.core.validation.Validator;
@@ -123,9 +123,6 @@ import org.springframework.util.StringUtils;
 
 import com.mongodb.ClientSessionOptions;
 import com.mongodb.CursorType;
-import com.mongodb.DBCollection;
-import com.mongodb.DBCursor;
-import com.mongodb.Mongo;
 import com.mongodb.MongoException;
 import com.mongodb.ReadPreference;
 import com.mongodb.WriteConcern;
@@ -323,8 +320,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/**
 	 * Configures the {@link WriteConcern} to be used with the template. If none is configured the {@link WriteConcern}
-	 * configured on the {@link MongoDbFactory} will apply. If you configured a {@link Mongo} instance no
-	 * {@link WriteConcern} will be used.
+	 * configured on the {@link MongoDbFactory} will apply.
 	 *
 	 * @param writeConcern can be {@literal null}.
 	 */
@@ -1074,10 +1070,6 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 				.map(Collation::toMongoCollation) //
 				.ifPresent(cursor::collation);
 
-		if (options.hasExecutionTimeLimit()) {
-			cursor = cursor.maxTime(options.getMaxTime().toMillis(), TimeUnit.MILLISECONDS);
-		}
-
 		return Flux.from(cursor).map(readCallback::doWith);
 	}
 
@@ -1126,38 +1118,36 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class)
 	 */
-	public <T> Mono<T> findAndModify(Query query, UpdateDefinition update, Class<T> entityClass) {
+	public <T> Mono<T> findAndModify(Query query, Update update, Class<T> entityClass) {
 		return findAndModify(query, update, new FindAndModifyOptions(), entityClass, getCollectionName(entityClass));
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class, java.lang.String)
 	 */
-	public <T> Mono<T> findAndModify(Query query, UpdateDefinition update, Class<T> entityClass, String collectionName) {
+	public <T> Mono<T> findAndModify(Query query, Update update, Class<T> entityClass, String collectionName) {
 		return findAndModify(query, update, new FindAndModifyOptions(), entityClass, collectionName);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, org.springframework.data.mongodb.core.FindAndModifyOptions, java.lang.Class)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, org.springframework.data.mongodb.core.FindAndModifyOptions, java.lang.Class)
 	 */
-	public <T> Mono<T> findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options,
-			Class<T> entityClass) {
+	public <T> Mono<T> findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass) {
 		return findAndModify(query, update, options, entityClass, getCollectionName(entityClass));
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, org.springframework.data.mongodb.core.FindAndModifyOptions, java.lang.Class, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#findAndModify(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, org.springframework.data.mongodb.core.FindAndModifyOptions, java.lang.Class, java.lang.String)
 	 */
-	public <T> Mono<T> findAndModify(Query query, UpdateDefinition update, FindAndModifyOptions options,
-			Class<T> entityClass, String collectionName) {
+	public <T> Mono<T> findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass,
+			String collectionName) {
 
 		Assert.notNull(options, "Options must not be null! ");
-		Assert.notNull(entityClass, "Entity class must not be null!");
 
 		FindAndModifyOptions optionsToUse = FindAndModifyOptions.of(options);
 
@@ -1168,6 +1158,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		if (!optionsToUse.getCollation().isPresent()) {
 			operations.forType(entityClass).getCollation(query).ifPresent(optionsToUse::collation);
+			;
 		}
 
 		return doFindAndModify(collectionName, query.getQueryObject(), query.getFieldsObject(),
@@ -1306,8 +1297,9 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 */
 	protected Mono<Long> doCount(String collectionName, Document filter, CountOptions options) {
 
-		return createMono(collectionName,
-				collection -> collection.countDocuments(CountQuery.of(filter).toQueryDocument(), options));
+		return ReactiveMongoDatabaseUtils.isTransactionActive(mongoDatabaseFactory) //
+				.flatMap(txActive -> createMono(collectionName,
+						collection -> txActive ? collection.countDocuments(filter, options) : collection.count(filter, options)));
 	}
 
 	/*
@@ -1689,75 +1681,73 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#upsert(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#upsert(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class)
 	 */
-	public Mono<UpdateResult> upsert(Query query, UpdateDefinition update, Class<?> entityClass) {
+	public Mono<UpdateResult> upsert(Query query, Update update, Class<?> entityClass) {
 		return doUpdate(getCollectionName(entityClass), query, update, entityClass, true, false);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#upsert(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#upsert(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.String)
 	 */
-	public Mono<UpdateResult> upsert(Query query, UpdateDefinition update, String collectionName) {
+	public Mono<UpdateResult> upsert(Query query, Update update, String collectionName) {
 		return doUpdate(collectionName, query, update, null, true, false);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#upsert(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#upsert(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class, java.lang.String)
 	 */
-	public Mono<UpdateResult> upsert(Query query, UpdateDefinition update, Class<?> entityClass, String collectionName) {
+	public Mono<UpdateResult> upsert(Query query, Update update, Class<?> entityClass, String collectionName) {
 		return doUpdate(collectionName, query, update, entityClass, true, false);
 	}
 
 	/*
 	 * (non-Javadoc))
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateFirst(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateFirst(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class)
 	 */
-	public Mono<UpdateResult> updateFirst(Query query, UpdateDefinition update, Class<?> entityClass) {
+	public Mono<UpdateResult> updateFirst(Query query, Update update, Class<?> entityClass) {
 		return doUpdate(getCollectionName(entityClass), query, update, entityClass, false, false);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateFirst(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateFirst(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.String)
 	 */
-	public Mono<UpdateResult> updateFirst(Query query, UpdateDefinition update, String collectionName) {
+	public Mono<UpdateResult> updateFirst(Query query, Update update, String collectionName) {
 		return doUpdate(collectionName, query, update, null, false, false);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateFirst(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateFirst(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class, java.lang.String)
 	 */
-	public Mono<UpdateResult> updateFirst(Query query, UpdateDefinition update, Class<?> entityClass,
-			String collectionName) {
+	public Mono<UpdateResult> updateFirst(Query query, Update update, Class<?> entityClass, String collectionName) {
 		return doUpdate(collectionName, query, update, entityClass, false, false);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateMulti(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateMulti(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class)
 	 */
-	public Mono<UpdateResult> updateMulti(Query query, UpdateDefinition update, Class<?> entityClass) {
+	public Mono<UpdateResult> updateMulti(Query query, Update update, Class<?> entityClass) {
 		return doUpdate(getCollectionName(entityClass), query, update, entityClass, false, true);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateMulti(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateMulti(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.String)
 	 */
-	public Mono<UpdateResult> updateMulti(Query query, UpdateDefinition update, String collectionName) {
+	public Mono<UpdateResult> updateMulti(Query query, Update update, String collectionName) {
 		return doUpdate(collectionName, query, update, null, false, true);
 	}
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateMulti(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.UpdateDefinition, java.lang.Class, java.lang.String)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#updateMulti(org.springframework.data.mongodb.core.query.Query, org.springframework.data.mongodb.core.query.Update, java.lang.Class, java.lang.String)
 	 */
-	public Mono<UpdateResult> updateMulti(Query query, UpdateDefinition update, Class<?> entityClass,
-			String collectionName) {
+	public Mono<UpdateResult> updateMulti(Query query, Update update, Class<?> entityClass, String collectionName) {
 		return doUpdate(collectionName, query, update, entityClass, false, true);
 	}
 
@@ -1771,87 +1761,56 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		}
 
 		MongoPersistentEntity<?> entity = entityClass == null ? null : getPersistentEntity(entityClass);
-		increaseVersionForUpdateIfNecessary(entity, update);
 
-		Document queryObj = queryMapper.getMappedObject(query.getQueryObject(), entity);
+		Flux<UpdateResult> result = execute(collectionName, collection -> {
 
-		UpdateOptions updateOptions = new UpdateOptions().upsert(upsert);
-		operations.forType(entityClass).getCollation(query) //
-				.map(Collation::toMongoCollation) //
-				.ifPresent(updateOptions::collation);
+			increaseVersionForUpdateIfNecessary(entity, update);
 
-		if (update.hasArrayFilters()) {
-			updateOptions.arrayFilters(update.getArrayFilters().stream().map(ArrayFilter::asDocument)
-					.map(it -> queryMapper.getMappedObject(it, entity)).collect(Collectors.toList()));
-		}
+			Document queryObj = queryMapper.getMappedObject(query.getQueryObject(), entity);
+			Document updateObj = update == null ? new Document()
+					: updateMapper.getMappedObject(update.getUpdateObject(), entity);
 
-		if (multi && update.isIsolated() && !queryObj.containsKey("$isolated")) {
-			queryObj.put("$isolated", 1);
-		}
+			if (LOGGER.isDebugEnabled()) {
+				LOGGER.debug(String.format("Calling update using query: %s and update: %s in collection: %s",
+						serializeToJsonSafely(queryObj), serializeToJsonSafely(updateObj), collectionName));
+			}
 
-		Flux<UpdateResult> result;
+			MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.UPDATE, collectionName, entityClass,
+					updateObj, queryObj);
+			WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
+			MongoCollection<Document> collectionToUse = prepareCollection(collection, writeConcernToUse);
 
-		if (update instanceof AggregationUpdate) {
+			UpdateOptions updateOptions = new UpdateOptions().upsert(upsert);
+			operations.forType(entityClass).getCollation(query) //
+					.map(Collation::toMongoCollation) //
+					.ifPresent(updateOptions::collation);
 
-			AggregationOperationContext context = entityClass != null
-					? new RelaxedTypeBasedAggregationOperationContext(entityClass, mappingContext, queryMapper)
-					: Aggregation.DEFAULT_CONTEXT;
+			if (update.hasArrayFilters()) {
+				updateOptions.arrayFilters(update.getArrayFilters().stream().map(ArrayFilter::asDocument)
+						.map(it -> queryMapper.getMappedObject(it, entity)).collect(Collectors.toList()));
+			}
 
-			List<Document> pipeline = new AggregationUtil(queryMapper, mappingContext)
-					.createPipeline((AggregationUpdate) update, context);
+			if (!UpdateMapper.isUpdateObject(updateObj)) {
 
-			result = execute(collectionName, collection -> {
+				ReplaceOptions replaceOptions = new ReplaceOptions();
+				replaceOptions.upsert(updateOptions.isUpsert());
+				replaceOptions.collation(updateOptions.getCollation());
 
-				if (LOGGER.isDebugEnabled()) {
-					LOGGER.debug(String.format("Calling update using query: %s and update: %s in collection: %s",
-							serializeToJsonSafely(queryObj), serializeToJsonSafely(pipeline), collectionName));
-				}
-
-				MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.UPDATE, collectionName,
-						entityClass, update.getUpdateObject(), queryObj);
-				WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
-
-				collection = writeConcernToUse != null ? collection.withWriteConcern(writeConcernToUse) : collection;
-
-				return multi ? collection.updateMany(queryObj, pipeline, updateOptions)
-						: collection.updateOne(queryObj, pipeline, updateOptions);
-			});
-		} else {
-
-			result = execute(collectionName, collection -> {
-
-				Document updateObj = updateMapper.getMappedObject(update.getUpdateObject(), entity);
-
-				if (LOGGER.isDebugEnabled()) {
-					LOGGER.debug(String.format("Calling update using query: %s and update: %s in collection: %s",
-							serializeToJsonSafely(queryObj), serializeToJsonSafely(updateObj), collectionName));
-				}
-
-				MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.UPDATE, collectionName,
-						entityClass, updateObj, queryObj);
-				WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
-				MongoCollection<Document> collectionToUse = prepareCollection(collection, writeConcernToUse);
-
-				if (!UpdateMapper.isUpdateObject(updateObj)) {
-
-					ReplaceOptions replaceOptions = new ReplaceOptions();
-					replaceOptions.upsert(updateOptions.isUpsert());
-					replaceOptions.collation(updateOptions.getCollation());
-
-					return collectionToUse.replaceOne(queryObj, updateObj, replaceOptions);
-				}
-
-				return multi ? collectionToUse.updateMany(queryObj, updateObj, updateOptions)
-						: collectionToUse.updateOne(queryObj, updateObj, updateOptions);
-			});
-		}
-
-		result = result.doOnNext(updateResult -> {
+				return collectionToUse.replaceOne(queryObj, updateObj, replaceOptions);
+			}
+			if (multi) {
+				return collectionToUse.updateMany(queryObj, updateObj, updateOptions);
+			}
+			return collectionToUse.updateOne(queryObj, updateObj, updateOptions);
+		}).doOnNext(updateResult -> {
 
 			if (entity != null && entity.hasVersionProperty() && !multi) {
 				if (updateResult.wasAcknowledged() && updateResult.getMatchedCount() == 0) {
 
-					Document updateObj = updateMapper.getMappedObject(update.getUpdateObject(), entity);
+					Document queryObj = query == null ? new Document()
+							: queryMapper.getMappedObject(query.getQueryObject(), entity);
+					Document updateObj = update == null ? new Document()
+							: updateMapper.getMappedObject(update.getUpdateObject(), entity);
 					if (containsVersionProperty(queryObj, entity))
 						throw new OptimisticLockingFailureException("Optimistic lock exception on saving entity: "
 								+ updateObj.toString() + " to collection " + collectionName);
@@ -2085,7 +2044,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		if (query == null) {
 
-			LOGGER.debug(String.format("Tail for class: %s in collection: %s", entityClass, collectionName));
+			// TODO: clean up
+			LOGGER.debug(String.format("find for class: %s in collection: %s", entityClass, collectionName));
 
 			return executeFindMultiInternal(
 					collection -> new FindCallback(null).doInCollection(collection).cursorType(CursorType.TailableAwait),
@@ -2437,8 +2397,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @param query the query document that specifies the criteria used to find a record.
 	 * @param fields the document that specifies the fields to be returned.
 	 * @param entityClass the parameterized type of the returned list.
-	 * @param preparer allows for customization of the {@link DBCursor} used when iterating over the result set, (apply
-	 *          limits, skips and so on).
+	 * @param preparer allows for customization of the {@link com.mongodb.client.FindIterable} used when iterating over
+	 *          the result set, (apply limits, skips and so on).
 	 * @return the {@link List} of converted objects.
 	 */
 	protected <T> Flux<T> doFind(String collectionName, Document query, Document fields, Class<T> entityClass,
@@ -2576,25 +2536,16 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	}
 
 	protected <T> Mono<T> doFindAndModify(String collectionName, Document query, Document fields, Document sort,
-			Class<T> entityClass, UpdateDefinition update, FindAndModifyOptions options) {
+			Class<T> entityClass, Update update, FindAndModifyOptions options) {
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(entityClass);
-		increaseVersionForUpdateIfNecessary(entity, update);
 
 		return Mono.defer(() -> {
 
+			increaseVersionForUpdateIfNecessary(entity, update);
+
 			Document mappedQuery = queryMapper.getMappedObject(query, entity);
-
-			Object mappedUpdate;
-			if (update instanceof AggregationUpdate) {
-
-				AggregationOperationContext context = new RelaxedTypeBasedAggregationOperationContext(entityClass,
-						mappingContext, queryMapper);
-
-				mappedUpdate = new AggregationUtil(queryMapper, mappingContext).createPipeline((Aggregation) update, context);
-			} else {
-				mappedUpdate = updateMapper.getMappedObject(update.getUpdateObject(), entity);
-			}
+			Document mappedUpdate = updateMapper.getMappedObject(update.getUpdateObject(), entity);
 
 			if (LOGGER.isDebugEnabled()) {
 				LOGGER.debug(String.format(
@@ -2699,7 +2650,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/**
 	 * Prepare the collection before any processing is done using it. This allows a convenient way to apply settings like
-	 * slaveOk() etc. Can be overridden in sub-classes.
+	 * withCodecRegistry() etc. Can be overridden in sub-classes.
 	 *
 	 * @param collection
 	 */
@@ -2858,7 +2809,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/**
 	 * Simple {@link ReactiveCollectionCallback} that takes a query {@link Document} plus an optional fields specification
-	 * {@link Document} and executes that against the {@link DBCollection}.
+	 * {@link Document} and executes that against the {@link MongoCollection}.
 	 *
 	 * @author Oliver Gierke
 	 * @author Thomas Risberg
@@ -2971,7 +2922,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		private final Document query;
 		private final Document fields;
 		private final Document sort;
-		private final Object update;
+		private final Document update;
 		private final List<Document> arrayFilters;
 		private final FindAndModifyOptions options;
 
@@ -2990,14 +2941,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 			FindOneAndUpdateOptions findOneAndUpdateOptions = convertToFindOneAndUpdateOptions(options, fields, sort,
 					arrayFilters);
-			if (update instanceof Document) {
-				return collection.findOneAndUpdate(query, (Document) update, findOneAndUpdateOptions);
-			} else if (update instanceof List) {
-				return collection.findOneAndUpdate(query, (List<Document>) update, findOneAndUpdateOptions);
-			}
-
-			return Flux
-					.error(new IllegalArgumentException(String.format("Using %s is not supported in findOneAndUpdate", update)));
+			return collection.findOneAndUpdate(query, update, findOneAndUpdateOptions);
 		}
 
 		private static FindOneAndUpdateOptions convertToFindOneAndUpdateOptions(FindAndModifyOptions options,
@@ -3308,7 +3252,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		@Override
 		public ReadPreference getReadPreference() {
-			return query.getMeta().getFlags().contains(CursorOption.SLAVE_OK) ? ReadPreference.primaryPreferred() : null;
+			return (query.getMeta().getFlags().contains(CursorOption.SECONDARY_READS)
+					|| query.getMeta().getFlags().contains(CursorOption.SLAVE_OK)) ? ReadPreference.primaryPreferred() : null;
 		}
 	}
 
@@ -3376,6 +3321,20 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			// native MongoDB objects that offer methods with ClientSession must not be proxied.
 			return delegate.getMongoDatabase();
 		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.ReactiveMongoTemplate#count(java.lang.String, org.bson.Document, com.mongodb.client.model.CountOptions)
+		 */
+		@Override
+		public Mono<Long> doCount(String collectionName, Document filter, CountOptions options) {
+
+			if (!session.hasActiveTransaction()) {
+				return super.doCount(collectionName, filter, options);
+			}
+
+			return createMono(collectionName, collection -> collection.countDocuments(filter, options));
+		}
 	}
 
 	@RequiredArgsConstructor

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import org.springframework.data.mongodb.core.aggregation.AggregationUpdate;
-import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import reactor.core.publisher.Mono;
 
 import org.springframework.data.mongodb.core.query.Query;
@@ -120,16 +118,13 @@ public interface ReactiveUpdateOperation {
 	interface UpdateWithUpdate<T> {
 
 		/**
-		 * Set the {@link UpdateDefinition} to be applied.
+		 * Set the {@link org.springframework.data.mongodb.core.query.Update} to be applied.
 		 *
 		 * @param update must not be {@literal null}.
 		 * @return new instance of {@link TerminatingUpdate}. Never {@literal null}.
 		 * @throws IllegalArgumentException if update is {@literal null}.
-		 * @since 3.0
-		 * @see Update
-		 * @see AggregationUpdate
 		 */
-		TerminatingUpdate<T> apply(UpdateDefinition update);
+		TerminatingUpdate<T> apply(org.springframework.data.mongodb.core.query.Update update);
 
 		/**
 		 * Specify {@code replacement} object.

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

@@ -63,7 +63,7 @@ class ReactiveUpdateOperationSupport implements ReactiveUpdateOperation {
 		@NonNull ReactiveMongoTemplate template;
 		@NonNull Class<?> domainType;
 		Query query;
-		org.springframework.data.mongodb.core.query.UpdateDefinition update;
+		org.springframework.data.mongodb.core.query.Update update;
 		@Nullable String collection;
 		@Nullable FindAndModifyOptions findAndModifyOptions;
 		@Nullable FindAndReplaceOptions findAndReplaceOptions;
@@ -72,10 +72,10 @@ class ReactiveUpdateOperationSupport implements ReactiveUpdateOperation {
 
 		/*
 		 * (non-Javadoc)
-		 * @see org.springframework.data.mongodb.core.ReactiveUpdateOperation.UpdateWithUpdate#apply(org.springframework.data.mongodb.core.query.UpdateDefinition)
+		 * @see org.springframework.data.mongodb.core.ReactiveUpdateOperation.UpdateWithUpdate#apply(org.springframework.data.mongodb.core.query.Update)
 		 */
 		@Override
-		public TerminatingUpdate<T> apply(org.springframework.data.mongodb.core.query.UpdateDefinition update) {
+		public TerminatingUpdate<T> apply(org.springframework.data.mongodb.core.query.Update update) {
 
 			Assert.notNull(update, "Update must not be null!");
 

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

@@ -242,7 +242,7 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 			factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, ClientSession.class, MongoDatabase.class,
 					this::proxyDatabase, MongoCollection.class, this::proxyCollection));
 
-			return targetType.cast(factory.getProxy());
+			return targetType.cast(factory.getProxy(target.getClass().getClassLoader()));
 		}
 	}
 }

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

@@ -16,8 +16,8 @@
 package org.springframework.data.mongodb.core;
 
 /**
- * Enum to represent how strict the check of {@link com.mongodb.WriteResult} shall be. It can either be skipped entirely
- * (use {@link #NONE}) or cause an exception to be thrown {@link #EXCEPTION}.
+ * Enum to represent how strict the check of {@link com.mongodb.WriteConcernResult} shall be. It can either be skipped
+ * entirely (use {@link #NONE}) or cause an exception to be thrown {@link #EXCEPTION}.
  *
  * @author Thomas Risberg
  * @author Oliver Gierke

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.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.LinkedHashMap;
 import java.util.List;
@@ -100,14 +101,14 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return value;
 	}
 
-	protected List<Object> append(Object value) {
+	protected List<Object> append(Object value, Expand expandList) {
 
 		if (this.value instanceof List) {
 
 			List<Object> clone = new ArrayList<Object>((List) this.value);
 
-			if (value instanceof List) {
-				clone.addAll((List) value);
+			if (value instanceof Collection && Expand.EXPAND_VALUES.equals(expandList)) {
+				clone.addAll((Collection<?>) value);
 			} else {
 				clone.add(value);
 			}
@@ -117,6 +118,17 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return Arrays.asList(this.value, value);
 	}
 
+	/**
+	 * Expand a nested list of values to single entries or keep the list.
+	 */
+	protected enum Expand {
+		EXPAND_VALUES, KEEP_SOURCE
+	}
+
+	protected List<Object> append(Object value) {
+		return append(value, Expand.EXPAND_VALUES);
+	}
+
 	@SuppressWarnings("unchecked")
 	protected java.util.Map<String, Object> append(String key, Object value) {
 

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

@@ -34,7 +34,7 @@ public class AccumulatorOperators {
 	 * Take the numeric value referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link AccumulatorOperatorFactory}.
 	 */
 	public static AccumulatorOperatorFactory valueOf(String fieldReference) {
 		return new AccumulatorOperatorFactory(fieldReference);
@@ -44,7 +44,7 @@ public class AccumulatorOperators {
 	 * Take the numeric value referenced resulting from given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link AccumulatorOperatorFactory}.
 	 */
 	public static AccumulatorOperatorFactory valueOf(AggregationExpression expression) {
 		return new AccumulatorOperatorFactory(expression);
@@ -86,7 +86,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates and
 		 * returns the sum.
 		 *
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum sum() {
 			return usesFieldRef() ? Sum.sumOf(fieldReference) : Sum.sumOf(expression);
@@ -96,7 +96,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * average value.
 		 *
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg avg() {
 			return usesFieldRef() ? Avg.avgOf(fieldReference) : Avg.avgOf(expression);
@@ -106,7 +106,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * maximum value.
 		 *
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max max() {
 			return usesFieldRef() ? Max.maxOf(fieldReference) : Max.maxOf(expression);
@@ -116,7 +116,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * minimum value.
 		 *
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min min() {
 			return usesFieldRef() ? Min.minOf(fieldReference) : Min.minOf(expression);
@@ -126,7 +126,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates the
 		 * population standard deviation of the input values.
 		 *
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop stdDevPop() {
 			return usesFieldRef() ? StdDevPop.stdDevPopOf(fieldReference) : StdDevPop.stdDevPopOf(expression);
@@ -136,7 +136,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates the
 		 * sample standard deviation of the input values.
 		 *
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp stdDevSamp() {
 			return usesFieldRef() ? StdDevSamp.stdDevSampOf(fieldReference) : StdDevSamp.stdDevSampOf(expression);
@@ -167,7 +167,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Sum}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public static Sum sumOf(String fieldReference) {
 
@@ -179,7 +179,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Sum}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public static Sum sumOf(AggregationExpression expression) {
 
@@ -192,7 +192,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum and(String fieldReference) {
 
@@ -205,7 +205,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum and(AggregationExpression expression) {
 
@@ -264,7 +264,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Avg}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public static Avg avgOf(String fieldReference) {
 
@@ -276,7 +276,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Avg}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public static Avg avgOf(AggregationExpression expression) {
 
@@ -289,7 +289,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg and(String fieldReference) {
 
@@ -302,7 +302,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg and(AggregationExpression expression) {
 
@@ -347,7 +347,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Max}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public static Max maxOf(String fieldReference) {
 
@@ -359,7 +359,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Max}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public static Max maxOf(AggregationExpression expression) {
 
@@ -372,7 +372,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max and(String fieldReference) {
 
@@ -385,7 +385,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max and(AggregationExpression expression) {
 
@@ -430,7 +430,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Min}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public static Min minOf(String fieldReference) {
 
@@ -442,7 +442,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Min}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public static Min minOf(AggregationExpression expression) {
 
@@ -455,7 +455,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min and(String fieldReference) {
 
@@ -468,7 +468,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min and(AggregationExpression expression) {
 
@@ -513,7 +513,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevPop}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public static StdDevPop stdDevPopOf(String fieldReference) {
 
@@ -525,7 +525,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevPop} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public static StdDevPop stdDevPopOf(AggregationExpression expression) {
 
@@ -538,7 +538,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop and(String fieldReference) {
 
@@ -551,7 +551,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop and(AggregationExpression expression) {
 
@@ -596,7 +596,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevSamp}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public static StdDevSamp stdDevSampOf(String fieldReference) {
 
@@ -608,7 +608,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevSamp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public static StdDevSamp stdDevSampOf(AggregationExpression expression) {
 
@@ -621,7 +621,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp and(String fieldReference) {
 
@@ -634,7 +634,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp and(AggregationExpression expression) {
 

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

@@ -115,23 +115,12 @@ public class Aggregation {
 		return new Aggregation(operations);
 	}
 
-	/**
-	 * Creates a new {@link AggregationUpdate} from the given {@link AggregationOperation}s.
-	 * 
-	 * @param operations can be {@literal empty} but must not be {@literal null}.
-	 * @return new instance of {@link AggregationUpdate}.
-	 * @since 3.0
-	 */
-	public static AggregationUpdate newUpdate(AggregationOperation... operations) {
-		return AggregationUpdate.from(Arrays.asList(operations));
-	}
-
 	/**
 	 * Returns a copy of this {@link Aggregation} with the given {@link AggregationOptions} set. Note that options are
 	 * supported in MongoDB version 2.6+.
 	 *
 	 * @param options must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Aggregation}.
 	 * @since 1.6
 	 */
 	public Aggregation withOptions(AggregationOptions options) {
@@ -192,12 +181,13 @@ public class Aggregation {
 	/**
 	 * Creates a new {@link Aggregation} from the given {@link AggregationOperation}s.
 	 *
-	 * @param aggregationOperations must not be {@literal null}.
+	 * @param aggregationOperations must not be {@literal null} or empty.
 	 * @param options must not be {@literal null} or empty.
 	 */
 	protected Aggregation(List<AggregationOperation> aggregationOperations, AggregationOptions options) {
 
 		Assert.notNull(aggregationOperations, "AggregationOperations must not be null!");
+		Assert.isTrue(!aggregationOperations.isEmpty(), "At least one AggregationOperation has to be provided");
 		Assert.notNull(options, "AggregationOptions must not be null!");
 
 		// check $out is the last operation if it exists
@@ -238,7 +228,7 @@ public class Aggregation {
 	 * Creates a new {@link ProjectionOperation} including the given fields.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ProjectionOperation}.
 	 */
 	public static ProjectionOperation project(String... fields) {
 		return project(fields(fields));
@@ -248,7 +238,7 @@ public class Aggregation {
 	 * Creates a new {@link ProjectionOperation} including the given {@link Fields}.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ProjectionOperation}.
 	 */
 	public static ProjectionOperation project(Fields fields) {
 		return new ProjectionOperation(fields);
@@ -271,7 +261,7 @@ public class Aggregation {
 	 * Factory method to create a new {@link UnwindOperation} for the field with the given name.
 	 *
 	 * @param field must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link UnwindOperation}.
 	 */
 	public static UnwindOperation unwind(String field) {
 		return new UnwindOperation(field(field));
@@ -281,7 +271,7 @@ public class Aggregation {
 	 * Factory method to create a new {@link ReplaceRootOperation} for the field with the given name.
 	 *
 	 * @param fieldName must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link ReplaceRootOperation}.
 	 * @since 1.10
 	 */
 	public static ReplaceRootOperation replaceRoot(String fieldName) {
@@ -293,7 +283,7 @@ public class Aggregation {
 	 * {@link AggregationExpression}.
 	 *
 	 * @param aggregationExpression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ReplaceRootOperation}.
 	 * @since 1.10
 	 */
 	public static ReplaceRootOperation replaceRoot(AggregationExpression aggregationExpression) {
@@ -359,7 +349,7 @@ public class Aggregation {
 	 * Creates a new {@link GroupOperation} for the given fields.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link GroupOperation}.
 	 */
 	public static GroupOperation group(String... fields) {
 		return group(fields(fields));
@@ -380,7 +370,7 @@ public class Aggregation {
 	 * {@link GraphLookupOperation} given {@literal fromCollection}.
 	 *
 	 * @param fromCollection must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link StartWithBuilder} for creating a {@link GraphLookupOperation}.
 	 * @since 1.10
 	 */
 	public static StartWithBuilder graphLookup(String fromCollection) {
@@ -391,7 +381,7 @@ public class Aggregation {
 	 * Factory method to create a new {@link SortOperation} for the given {@link Sort}.
 	 *
 	 * @param sort must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SortOperation}.
 	 */
 	public static SortOperation sort(Sort sort) {
 		return new SortOperation(sort);
@@ -402,7 +392,7 @@ public class Aggregation {
 	 *
 	 * @param direction must not be {@literal null}.
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SortOperation}.
 	 */
 	public static SortOperation sort(Direction direction, String... fields) {
 		return new SortOperation(Sort.by(direction, fields));
@@ -412,7 +402,7 @@ public class Aggregation {
 	 * Creates a new {@link SortByCountOperation} given {@literal groupByField}.
 	 *
 	 * @param field must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link SortByCountOperation}.
 	 * @since 2.1
 	 */
 	public static SortByCountOperation sortByCount(String field) {
@@ -423,7 +413,7 @@ public class Aggregation {
 	 * Creates a new {@link SortByCountOperation} given {@link AggregationExpression group and sort expression}.
 	 *
 	 * @param groupAndSortExpression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SortByCountOperation}.
 	 * @since 2.1
 	 */
 	public static SortByCountOperation sortByCount(AggregationExpression groupAndSortExpression) {
@@ -434,9 +424,10 @@ public class Aggregation {
 	 * Creates a new {@link SkipOperation} skipping the given number of elements.
 	 *
 	 * @param elementsToSkip must not be less than zero.
-	 * @return
+	 * @return new instance of {@link SkipOperation}.
 	 * @deprecated prepare to get this one removed in favor of {@link #skip(long)}.
 	 */
+	@Deprecated
 	public static SkipOperation skip(int elementsToSkip) {
 		return new SkipOperation(elementsToSkip);
 	}
@@ -445,7 +436,7 @@ public class Aggregation {
 	 * Creates a new {@link SkipOperation} skipping the given number of elements.
 	 *
 	 * @param elementsToSkip must not be less than zero.
-	 * @return
+	 * @return new instance of {@link SkipOperation}.
 	 */
 	public static SkipOperation skip(long elementsToSkip) {
 		return new SkipOperation(elementsToSkip);
@@ -455,7 +446,7 @@ public class Aggregation {
 	 * Creates a new {@link LimitOperation} limiting the result to the given number of elements.
 	 *
 	 * @param maxElements must not be less than zero.
-	 * @return
+	 * @return new instance of {@link LimitOperation}.
 	 */
 	public static LimitOperation limit(long maxElements) {
 		return new LimitOperation(maxElements);
@@ -465,7 +456,7 @@ public class Aggregation {
 	 * Creates a new {@link SampleOperation} to select the specified number of documents from its input randomly.
 	 *
 	 * @param sampleSize must not be less than zero.
-	 * @return
+	 * @return new instance of {@link SampleOperation}.
 	 * @since 2.0
 	 */
 	public static SampleOperation sample(long sampleSize) {
@@ -476,7 +467,7 @@ public class Aggregation {
 	 * Creates a new {@link MatchOperation} using the given {@link Criteria}.
 	 *
 	 * @param criteria must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link MatchOperation}.
 	 */
 	public static MatchOperation match(Criteria criteria) {
 		return new MatchOperation(criteria);
@@ -486,7 +477,7 @@ public class Aggregation {
 	 * Creates a new {@link MatchOperation} using the given {@link CriteriaDefinition}.
 	 *
 	 * @param criteria must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link MatchOperation}.
 	 * @since 1.10
 	 */
 	public static MatchOperation match(CriteriaDefinition criteria) {
@@ -501,7 +492,7 @@ public class Aggregation {
 	 *          collection in the current database if one does not already exist. The collection is not visible until the
 	 *          aggregation completes. If the aggregation fails, MongoDB does not create the collection. Must not be
 	 *          {@literal null}.
-	 * @return
+	 * @return new instance of {@link OutOperation}.
 	 */
 	public static OutOperation out(String outCollectionName) {
 		return new OutOperation(outCollectionName);
@@ -511,7 +502,7 @@ public class Aggregation {
 	 * Creates a new {@link BucketOperation} given {@literal groupByField}.
 	 *
 	 * @param groupByField must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 * @since 1.10
 	 */
 	public static BucketOperation bucket(String groupByField) {
@@ -522,7 +513,7 @@ public class Aggregation {
 	 * Creates a new {@link BucketOperation} given {@link AggregationExpression group-by expression}.
 	 *
 	 * @param groupByExpression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 * @since 1.10
 	 */
 	public static BucketOperation bucket(AggregationExpression groupByExpression) {
@@ -534,7 +525,7 @@ public class Aggregation {
 	 *
 	 * @param groupByField must not be {@literal null} or empty.
 	 * @param buckets number of buckets, must be a positive integer.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 * @since 1.10
 	 */
 	public static BucketAutoOperation bucketAuto(String groupByField, int buckets) {
@@ -546,7 +537,7 @@ public class Aggregation {
 	 *
 	 * @param groupByExpression must not be {@literal null}.
 	 * @param buckets number of buckets, must be a positive integer.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 * @since 1.10
 	 */
 	public static BucketAutoOperation bucketAuto(AggregationExpression groupByExpression, int buckets) {
@@ -556,7 +547,7 @@ public class Aggregation {
 	/**
 	 * Creates a new {@link FacetOperation}.
 	 *
-	 * @return
+	 * @return new instance of {@link FacetOperation}.
 	 * @since 1.10
 	 */
 	public static FacetOperation facet() {
@@ -567,7 +558,7 @@ public class Aggregation {
 	 * Creates a new {@link FacetOperationBuilder} given {@link Aggregation}.
 	 *
 	 * @param aggregationOperations the sub-pipeline, must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link FacetOperation}.
 	 * @since 1.10
 	 */
 	public static FacetOperationBuilder facet(AggregationOperation... aggregationOperations) {
@@ -616,7 +607,7 @@ public class Aggregation {
 	 * Creates a new {@link Fields} instance for the given field names.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Fields}.
 	 * @see Fields#fields(String...)
 	 */
 	public static Fields fields(String... fields) {
@@ -628,7 +619,7 @@ public class Aggregation {
 	 *
 	 * @param name must not be {@literal null} or empty.
 	 * @param target must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link Fields}.
 	 */
 	public static Fields bind(String name, String target) {
 		return Fields.from(field(name, target));
@@ -650,7 +641,7 @@ public class Aggregation {
 	/**
 	 * Returns a new {@link AggregationOptions.Builder}.
 	 *
-	 * @return
+	 * @return new instance of {@link AggregationOptions.Builder}.
 	 * @since 1.6
 	 */
 	public static AggregationOptions.Builder newAggregationOptions() {
@@ -712,7 +703,7 @@ public class Aggregation {
 		 * otherwise.
 		 *
 		 * @param fieldRef may be {@literal null}.
-		 * @return
+		 * @return {@literal true} if the given field refers to a {@link SystemVariable}.
 		 */
 		public static boolean isReferingToSystemVariable(@Nullable String fieldRef) {
 

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

@@ -31,8 +31,8 @@ public interface AggregationExpression {
 	 * Turns the {@link AggregationExpression} into a {@link Document} within the given
 	 * {@link AggregationOperationContext}.
 	 *
-	 * @param context
-	 * @return
+	 * @param context must not be {@literal null}.
+	 * @return the MongoDB native ({@link Document}) form of the expression.
 	 */
 	Document toDocument(AggregationOperationContext context);
 }

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

@@ -53,7 +53,7 @@ interface AggregationExpressionTransformer
 		 * @param currentNode must not be {@literal null}.
 		 * @param parentNode may be {@literal null}.
 		 * @param previousOperationObject may be {@literal null}.
-		 * @param aggregationContext must not be {@literal null}.
+		 * @param context must not be {@literal null}.
 		 */
 		public AggregationExpressionTransformationContext(T currentNode, @Nullable ExpressionNode parentNode,
 				@Nullable Document previousOperationObject, AggregationOperationContext context) {
@@ -67,7 +67,7 @@ interface AggregationExpressionTransformer
 		/**
 		 * Returns the underlying {@link AggregationOperationContext}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public AggregationOperationContext getAggregationContext() {
 			return aggregationContext;
@@ -76,7 +76,7 @@ interface AggregationExpressionTransformer
 		/**
 		 * Returns the {@link FieldReference} for the current {@link ExpressionNode}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public FieldReference getFieldReference() {
 			return aggregationContext.getReference(getCurrentNode().getName());

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

@@ -41,7 +41,7 @@ public enum AggregationFunctionExpressions {
 	 * Returns an {@link AggregationExpression} build from the current {@link Enum} name and the given parameters.
 	 *
 	 * @param parameters must not be {@literal null}
-	 * @return
+	 * @return new instance of {@link AggregationExpression}.
 	 */
 	public AggregationExpression of(Object... parameters) {
 

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

@@ -58,20 +58,20 @@ public interface AggregationOperationContext {
 	Document getMappedObject(Document document, @Nullable Class<?> type);
 
 	/**
-	 * Returns a {@link FieldReference} for the given field or {@literal null} if the context does not expose the given
-	 * field.
+	 * Returns a {@link FieldReference} for the given field.
 	 *
 	 * @param field must not be {@literal null}.
-	 * @return
+	 * @return the {@link FieldReference} for the given {@link Field}.
+	 * @throws IllegalArgumentException if the context does not expose a field with the given name
 	 */
 	FieldReference getReference(Field field);
 
 	/**
-	 * Returns the {@link FieldReference} for the field with the given name or {@literal null} if the context does not
-	 * expose a field with the given name.
+	 * Returns the {@link FieldReference} for the field with the given name.
 	 *
 	 * @param name must not be {@literal null} or empty.
-	 * @return
+	 * @return the {@link FieldReference} for the field with given {@literal name}.
+	 * @throws IllegalArgumentException if the context does not expose a field with the given name
 	 */
 	FieldReference getReference(String name);
 

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

@@ -15,7 +15,6 @@
  */
 package org.springframework.data.mongodb.core.aggregation;
 
-import java.time.Duration;
 import java.util.Optional;
 
 import org.bson.Document;
@@ -46,14 +45,12 @@ public class AggregationOptions {
 	private static final String ALLOW_DISK_USE = "allowDiskUse";
 	private static final String COLLATION = "collation";
 	private static final String COMMENT = "comment";
-	private static final String MAX_TIME = "maxTimeMS";
 
 	private final boolean allowDiskUse;
 	private final boolean explain;
 	private final Optional<Document> cursor;
 	private final Optional<Collation> collation;
 	private final Optional<String> comment;
-	private Duration maxTime = Duration.ZERO;
 
 	/**
 	 * Creates a new {@link AggregationOptions}.
@@ -132,11 +129,7 @@ public class AggregationOptions {
 				: null;
 		String comment = document.getString(COMMENT);
 
-		AggregationOptions options = new AggregationOptions(allowDiskUse, explain, cursor, collation, comment);
-		if (document.containsKey(MAX_TIME)) {
-			options.maxTime = Duration.ofMillis(document.getLong(MAX_TIME));
-		}
-		return options;
+		return new AggregationOptions(allowDiskUse, explain, cursor, collation, comment);
 	}
 
 	/**
@@ -153,7 +146,7 @@ public class AggregationOptions {
 	 * Enables writing to temporary files. When set to true, aggregation stages can write data to the _tmp subdirectory in
 	 * the dbPath directory.
 	 *
-	 * @return
+	 * @return {@literal true} if enabled.
 	 */
 	public boolean isAllowDiskUse() {
 		return allowDiskUse;
@@ -162,7 +155,7 @@ public class AggregationOptions {
 	/**
 	 * Specifies to return the information on the processing of the pipeline.
 	 *
-	 * @return
+	 * @return {@literal true} if enabled.
 	 */
 	public boolean isExplain() {
 		return explain;
@@ -187,7 +180,7 @@ public class AggregationOptions {
 	/**
 	 * Specify a document that contains options that control the creation of the cursor object.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public Optional<Document> getCursor() {
 		return cursor;
@@ -196,7 +189,7 @@ public class AggregationOptions {
 	/**
 	 * Get collation settings for string comparison.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.0
 	 */
 	public Optional<Collation> getCollation() {
@@ -206,21 +199,13 @@ public class AggregationOptions {
 	/**
 	 * Get the comment for the aggregation.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.2
 	 */
 	public Optional<String> getComment() {
 		return comment;
 	}
 
-	/**
-	 * @return the time limit for processing. {@link Duration#ZERO} is used for the default unbounded behavior.
-	 * @since 3.0
-	 */
-	public Duration getMaxTime() {
-		return maxTime;
-	}
-
 	/**
 	 * Returns a new potentially adjusted copy for the given {@code aggregationCommandObject} with the configuration
 	 * applied.
@@ -248,17 +233,13 @@ public class AggregationOptions {
 			collation.map(Collation::toDocument).ifPresent(val -> result.append(COLLATION, val));
 		}
 
-		if (hasExecutionTimeLimit() && !result.containsKey(MAX_TIME)) {
-			result.append(MAX_TIME, maxTime.toMillis());
-		}
-
 		return result;
 	}
 
 	/**
 	 * Returns a {@link Document} representation of this {@link AggregationOptions}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public Document toDocument() {
 
@@ -270,21 +251,9 @@ public class AggregationOptions {
 		collation.ifPresent(val -> document.append(COLLATION, val.toDocument()));
 		comment.ifPresent(val -> document.append(COMMENT, val));
 
-		if (hasExecutionTimeLimit()) {
-			document.append(MAX_TIME, maxTime.toMillis());
-		}
-
 		return document;
 	}
 
-	/**
-	 * @return
-	 * @since 3.0
-	 */
-	public boolean hasExecutionTimeLimit() {
-		return !maxTime.isZero() && !maxTime.isNegative();
-	}
-
 	/* (non-Javadoc)
 	 * @see java.lang.Object#toString()
 	 */
@@ -310,13 +279,12 @@ public class AggregationOptions {
 		private @Nullable Document cursor;
 		private @Nullable Collation collation;
 		private @Nullable String comment;
-		private @Nullable Duration maxTime;
 
 		/**
 		 * Defines whether to off-load intensive sort-operations to disk.
 		 *
-		 * @param allowDiskUse
-		 * @return
+		 * @param allowDiskUse use {@literal true} to allow disk use during the aggregation.
+		 * @return this.
 		 */
 		public Builder allowDiskUse(boolean allowDiskUse) {
 
@@ -327,8 +295,8 @@ public class AggregationOptions {
 		/**
 		 * Defines whether to get the execution plan for the aggregation instead of the actual results.
 		 *
-		 * @param explain
-		 * @return
+		 * @param explain use {@literal true} to enable explain feature.
+		 * @return this.
 		 */
 		public Builder explain(boolean explain) {
 
@@ -339,8 +307,8 @@ public class AggregationOptions {
 		/**
 		 * Additional options to the aggregation.
 		 *
-		 * @param cursor
-		 * @return
+		 * @param cursor must not be {@literal null}.
+		 * @return this.
 		 */
 		public Builder cursor(Document cursor) {
 
@@ -351,8 +319,8 @@ public class AggregationOptions {
 		/**
 		 * Define the initial cursor batch size.
 		 *
-		 * @param batchSize
-		 * @return
+		 * @param batchSize use a positive int.
+		 * @return this.
 		 * @since 2.0
 		 */
 		public Builder cursorBatchSize(int batchSize) {
@@ -365,7 +333,7 @@ public class AggregationOptions {
 		 * Define collation settings for string comparison.
 		 *
 		 * @param collation can be {@literal null}.
-		 * @return
+		 * @return this.
 		 * @since 2.0
 		 */
 		public Builder collation(@Nullable Collation collation) {
@@ -378,7 +346,7 @@ public class AggregationOptions {
 		 * Define a comment to describe the execution.
 		 *
 		 * @param comment can be {@literal null}.
-		 * @return
+		 * @return this.
 		 * @since 2.2
 		 */
 		public Builder comment(@Nullable String comment) {
@@ -387,33 +355,13 @@ public class AggregationOptions {
 			return this;
 		}
 
-		/**
-		 * Set the time limit for processing.
-		 *
-		 * @param maxTime {@link Duration#ZERO} is used for the default unbounded behavior. {@link Duration#isNegative()
-		 *          Negative} values will be ignored.
-		 * @return this.
-		 * @since 3.0
-		 */
-		public Builder maxTime(@Nullable Duration maxTime) {
-
-			this.maxTime = maxTime;
-			return this;
-		}
-
 		/**
 		 * Returns a new {@link AggregationOptions} instance with the given configuration.
 		 *
-		 * @return
+		 * @return new instance of {@link AggregationOptions}.
 		 */
 		public AggregationOptions build() {
-
-			AggregationOptions options = new AggregationOptions(allowDiskUse, explain, cursor, collation, comment);
-			if (maxTime != null) {
-				options.maxTime = maxTime;
-			}
-
-			return options;
+			return new AggregationOptions(allowDiskUse, explain, cursor, collation, comment);
 		}
 	}
 }

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

@@ -59,7 +59,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the aggregation results.
 	 *
-	 * @return
+	 * @return the list of already mapped results or an empty one if none found.
 	 */
 	public List<T> getMappedResults() {
 		return mappedResults;
@@ -68,7 +68,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the unique mapped result. Assumes no result or exactly one.
 	 *
-	 * @return
+	 * @return the single already mapped result object or raise an error if more than one found.
 	 * @throws IllegalArgumentException in case more than one result is available.
 	 */
 	@Nullable
@@ -88,7 +88,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the server that has been used to perform the aggregation.
 	 *
-	 * @return
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	public String getServerUsed() {
@@ -98,7 +98,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the raw result that was returned by the server.
 	 *
-	 * @return
+	 * @return the raw response.
 	 * @since 1.6
 	 */
 	public Document getRawResults() {

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

@@ -56,7 +56,7 @@ public class AggregationSpELExpression implements AggregationExpression {
 	 *
 	 * @param expressionString must not be {@literal null}.
 	 * @param parameters can be empty.
-	 * @return
+	 * @return new instance of {@link AggregationSpELExpression}.
 	 */
 	public static AggregationSpELExpression expressionOf(String expressionString, Object... parameters) {
 

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

@@ -1,325 +0,0 @@
-/*
- * Copyright 2019-2020 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.ArrayList;
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Set;
-import java.util.StringJoiner;
-import java.util.stream.Collectors;
-
-import org.bson.Document;
-
-import org.springframework.data.mongodb.core.query.Query;
-import org.springframework.data.mongodb.core.query.SerializationUtils;
-import org.springframework.data.mongodb.core.query.UpdateDefinition;
-import org.springframework.lang.Nullable;
-import org.springframework.util.Assert;
-
-/**
- * Abstraction for {@code db.collection.update()} using an aggregation pipeline. Aggregation pipeline updates use a more
- * expressive update statement expressing conditional updates based on current field values or updating one field using
- * the value of another field(s).
- *
- * <pre class="code">
- * AggregationUpdate update = AggregationUpdate.update().set("average")
- * 		.toValue(ArithmeticOperators.valueOf("tests").avg()).set("grade")
- * 		.toValue(ConditionalOperators
- * 				.switchCases(CaseOperator.when(Gte.valueOf("average").greaterThanEqualToValue(90)).then("A"),
- * 						CaseOperator.when(Gte.valueOf("average").greaterThanEqualToValue(80)).then("B"),
- * 						CaseOperator.when(Gte.valueOf("average").greaterThanEqualToValue(70)).then("C"),
- * 						CaseOperator.when(Gte.valueOf("average").greaterThanEqualToValue(60)).then("D"))
- * 				.defaultTo("F"));
- * </pre>
- *
- * The above sample is equivalent to the JSON update statement:
- *
- * <pre class="code">
- * db.collection.update(
- *    { },
- *    [
- *      { $set: { average : { $avg: "$tests" } } },
- *      { $set: { grade: { $switch: {
- *                            branches: [
- *                                { case: { $gte: [ "$average", 90 ] }, then: "A" },
- *                                { case: { $gte: [ "$average", 80 ] }, then: "B" },
- *                                { case: { $gte: [ "$average", 70 ] }, then: "C" },
- *                                { case: { $gte: [ "$average", 60 ] }, then: "D" }
- *                            ],
- *                            default: "F"
- *      } } } }
- *    ],
- *    { multi: true }
- * )
- * </pre>
- *
- * @author Christoph Strobl
- * @author Mark Paluch
- * @see <a href=
- *      "https://docs.mongodb.com/manual/reference/method/db.collection.update/#update-with-aggregation-pipeline">MongoDB
- *      Reference Documentation</a>
- * @since 3.0
- */
-public class AggregationUpdate extends Aggregation implements UpdateDefinition {
-
-	private boolean isolated = false;
-	private Set<String> keysTouched = new HashSet<>();
-
-	/**
-	 * Create new {@link AggregationUpdate}.
-	 */
-	protected AggregationUpdate() {
-		this(new ArrayList<>());
-	}
-
-	/**
-	 * Create new {@link AggregationUpdate} with the given aggregation pipeline to apply.
-	 *
-	 * @param pipeline must not be {@literal null}.
-	 */
-	protected AggregationUpdate(List<AggregationOperation> pipeline) {
-
-		super(pipeline);
-
-		for (AggregationOperation operation : pipeline) {
-			if (operation instanceof FieldsExposingAggregationOperation) {
-				((FieldsExposingAggregationOperation) operation).getFields().forEach(it -> {
-					keysTouched.add(it.getName());
-				});
-			}
-		}
-	}
-
-	/**
-	 * Start defining the update pipeline to execute.
-	 *
-	 * @return new instance of {@link AggregationUpdate}.
-	 */
-	public static AggregationUpdate update() {
-		return new AggregationUpdate();
-	}
-
-	/**
-	 * Create a new AggregationUpdate from the given {@link AggregationOperation}s.
-	 *
-	 * @return new instance of {@link AggregationUpdate}.
-	 */
-	public static AggregationUpdate from(List<AggregationOperation> pipeline) {
-		return new AggregationUpdate(pipeline);
-	}
-
-	/**
-	 * Adds new fields to documents. {@code $set} outputs documents that contain all existing fields from the input
-	 * documents and newly added fields.
-	 *
-	 * @param setOperation must not be {@literal null}.
-	 * @return this.
-	 * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/set/">$set Aggregation Reference</a>
-	 */
-	public AggregationUpdate set(SetOperation setOperation) {
-
-		Assert.notNull(setOperation, "SetOperation must not be null!");
-
-		setOperation.getFields().forEach(it -> {
-			keysTouched.add(it.getName());
-		});
-		operations.add(setOperation);
-		return this;
-	}
-
-	/**
-	 * {@code $unset} removes/excludes fields from documents.
-	 *
-	 * @param unsetOperation must not be {@literal null}.
-	 * @return this.
-	 * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/unset/">$unset Aggregation
-	 *      Reference</a>
-	 */
-	public AggregationUpdate unset(UnsetOperation unsetOperation) {
-
-		Assert.notNull(unsetOperation, "UnsetOperation must not be null!");
-
-		operations.add(unsetOperation);
-		keysTouched.addAll(unsetOperation.removedFieldNames());
-		return this;
-	}
-
-	/**
-	 * {@code $replaceWith} replaces the input document with the specified document. The operation replaces all existing
-	 * fields in the input document, including the <strong>_id</strong> field.
-	 *
-	 * @param replaceWithOperation
-	 * @return this.
-	 * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceWith/">$replaceWith Aggregation
-	 *      Reference</a>
-	 */
-	public AggregationUpdate replaceWith(ReplaceWithOperation replaceWithOperation) {
-
-		Assert.notNull(replaceWithOperation, "ReplaceWithOperation must not be null!");
-		operations.add(replaceWithOperation);
-		return this;
-	}
-
-	/**
-	 * {@code $replaceWith} replaces the input document with the value.
-	 *
-	 * @param value must not be {@literal null}.
-	 * @return this.
-	 */
-	public AggregationUpdate replaceWith(Object value) {
-
-		Assert.notNull(value, "Value must not be null!");
-		return replaceWith(ReplaceWithOperation.replaceWithValue(value));
-	}
-
-	/**
-	 * Fluent API variant for {@code $set} adding a single {@link SetOperation pipeline operation} every time. To update
-	 * multiple fields within one {@link SetOperation} use {@link #set(SetOperation)}.
-	 *
-	 * @param key must not be {@literal null}.
-	 * @return new instance of {@link SetValueAppender}.
-	 * @see #set(SetOperation)
-	 */
-	public SetValueAppender set(String key) {
-
-		Assert.notNull(key, "Key must not be null!");
-
-		return new SetValueAppender() {
-
-			@Override
-			public AggregationUpdate toValue(@Nullable Object value) {
-				return set(SetOperation.builder().set(key).toValue(value));
-			}
-
-			@Override
-			public AggregationUpdate toValueOf(Object value) {
-
-				Assert.notNull(value, "Value must not be null!");
-				return set(SetOperation.builder().set(key).toValueOf(value));
-			}
-		};
-	}
-
-	/**
-	 * Short for {@link #unset(UnsetOperation)}.
-	 *
-	 * @param keys
-	 * @return
-	 */
-	public AggregationUpdate unset(String... keys) {
-
-		Assert.notNull(keys, "Keys must not be null!");
-		Assert.noNullElements(keys, "Keys must not contain null elements.");
-
-		return unset(new UnsetOperation(Arrays.stream(keys).map(Fields::field).collect(Collectors.toList())));
-	}
-
-	/**
-	 * Prevents a write operation that affects <strong>multiple</strong> documents from yielding to other reads or writes
-	 * once the first document is written. <br />
-	 * Use with {@link org.springframework.data.mongodb.core.MongoOperations#updateMulti(Query, UpdateDefinition, Class)}.
-	 *
-	 * @return never {@literal null}.
-	 */
-	public AggregationUpdate isolated() {
-
-		isolated = true;
-		return this;
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.query.UpdateDefinition#isIsolated()
-	 */
-	@Override
-	public Boolean isIsolated() {
-		return isolated;
-	}
-
-	/*
-	 * Returns a update document containing the update pipeline.
-	 * The resulting document needs to be unwrapped to be used with update operations.
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.query.UpdateDefinition#getUpdateObject()
-	 */
-	@Override
-	public Document getUpdateObject() {
-		return new Document("", toPipeline(Aggregation.DEFAULT_CONTEXT));
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.query.UpdateDefinition#modifies(java.lang.String)
-	 */
-	@Override
-	public boolean modifies(String key) {
-		return keysTouched.contains(key);
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.query.UpdateDefinition#inc(java.lang.String)
-	 */
-	@Override
-	public void inc(String key) {
-		set(new SetOperation(key, ArithmeticOperators.valueOf(key).add(1)));
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.query.UpdateDefinition#getArrayFilters()
-	 */
-	@Override
-	public List<ArrayFilter> getArrayFilters() {
-		return Collections.emptyList();
-	}
-
-	@Override
-	public String toString() {
-
-		StringJoiner joiner = new StringJoiner(",\n", "[\n", "\n]");
-		toPipeline(Aggregation.DEFAULT_CONTEXT).stream().map(SerializationUtils::serializeToJsonSafely)
-				.forEach(joiner::add);
-		return joiner.toString();
-	}
-
-	/**
-	 * Fluent API AggregationUpdate builder.
-	 *
-	 * @author Christoph Strobl
-	 */
-	public interface SetValueAppender {
-
-		/**
-		 * Define the target value as is.
-		 *
-		 * @param value can be {@literal null}.
-		 * @return never {@literal null}.
-		 */
-		AggregationUpdate toValue(@Nullable Object value);
-
-		/**
-		 * Define the target value as value, an {@link AggregationExpression} or a {@link Field} reference.
-		 *
-		 * @param value can be {@literal null}.
-		 * @return never {@literal null}.
-		 */
-		AggregationUpdate toValueOf(Object value);
-	}
-}

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

@@ -32,9 +32,9 @@ interface AggregationUtils {
 	 * Converts the given {@link Range} into an array of values.
 	 *
 	 * @param range must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
-	public static List<Long> toRangeValues(Range<Long> range) {
+	static List<Long> toRangeValues(Range<Long> range) {
 
 		Assert.notNull(range, "Range must not be null!");
 

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

@@ -43,7 +43,7 @@ public class ArrayOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ArrayOperatorFactory}.
 	 */
 	public static ArrayOperatorFactory arrayOf(String fieldReference) {
 		return new ArrayOperatorFactory(fieldReference);
@@ -53,7 +53,7 @@ public class ArrayOperators {
 	 * Take the array referenced resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ArrayOperatorFactory}.
 	 */
 	public static ArrayOperatorFactory arrayOf(AggregationExpression expression) {
 		return new ArrayOperatorFactory(expression);
@@ -123,8 +123,8 @@ public class ArrayOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated array and returns the element at the
 		 * specified array {@literal position}.
 		 *
-		 * @param position
-		 * @return
+		 * @param position the element index.
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public ArrayElemAt elementAt(int position) {
 			return createArrayElemAt().elementAt(position);
@@ -135,7 +135,7 @@ public class ArrayOperators {
 		 * resulting form the given {@literal expression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public ArrayElemAt elementAt(AggregationExpression expression) {
 
@@ -148,7 +148,7 @@ public class ArrayOperators {
 		 * defined by the referenced {@literal field}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public ArrayElemAt elementAt(String fieldReference) {
 
@@ -170,7 +170,7 @@ public class ArrayOperators {
 		 * {@literal arrayFieldReference} to it.
 		 *
 		 * @param arrayFieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public ConcatArrays concat(String arrayFieldReference) {
 
@@ -183,7 +183,7 @@ public class ArrayOperators {
 		 * the given {@literal expression} to it.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public ConcatArrays concat(AggregationExpression expression) {
 
@@ -204,7 +204,7 @@ public class ArrayOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated array and selects a subset of the array to
 		 * return based on the specified condition.
 		 *
-		 * @return
+		 * @return new instance of {@link AsBuilder} to create a {@link Filter}.
 		 */
 		public AsBuilder filter() {
 
@@ -219,7 +219,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated array and an check if its an array.
 		 *
-		 * @return
+		 * @return new instance of {@link IsArray}.
 		 */
 		public IsArray isArray() {
 
@@ -231,7 +231,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated array and retrieves its length.
 		 *
-		 * @return
+		 * @return new instance of {@link Size}.
 		 */
 		public Size length() {
 
@@ -245,7 +245,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated array and selects a subset from it.
 		 *
-		 * @return
+		 * @return new instance of {@link Slice}.
 		 */
 		public Slice slice() {
 
@@ -261,7 +261,7 @@ public class ArrayOperators {
 		 * value and returns the array index (zero-based) of the first occurrence.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfArray}.
 		 */
 		public IndexOfArray indexOf(Object value) {
 
@@ -276,7 +276,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns an array with the elements in reverse order.
 		 *
-		 * @return
+		 * @return new instance of {@link ReverseArray}.
 		 */
 		public ReverseArray reverse() {
 
@@ -293,7 +293,7 @@ public class ArrayOperators {
 		 * an array and combines them into a single value.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReduceInitialValueBuilder} to create {@link Reduce}.
 		 */
 		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(AggregationExpression expression) {
 
@@ -305,8 +305,8 @@ public class ArrayOperators {
 		 * Start creating new {@link AggregationExpression} that applies an {@link AggregationExpression} to each element in
 		 * an array and combines them into a single value.
 		 *
-		 * @param expressions
-		 * @return
+		 * @param expressions must not be {@literal null}.
+		 * @return new instance of {@link ReduceInitialValueBuilder} to create {@link Reduce}.
 		 */
 		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(PropertyExpression... expressions) {
 
@@ -320,7 +320,7 @@ public class ArrayOperators {
 		 * the second input array, etc.
 		 *
 		 * @param arrays must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip zipWith(Object... arrays) {
 
@@ -336,7 +336,7 @@ public class ArrayOperators {
 		 * associated array.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link In}.
 		 */
 		public In containsValue(Object value) {
 
@@ -413,7 +413,7 @@ public class ArrayOperators {
 		 * Creates new {@link ArrayElemAt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public static ArrayElemAt arrayOf(String fieldReference) {
 
@@ -425,7 +425,7 @@ public class ArrayOperators {
 		 * Creates new {@link ArrayElemAt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public static ArrayElemAt arrayOf(AggregationExpression expression) {
 
@@ -446,16 +446,34 @@ public class ArrayOperators {
 			return new ArrayElemAt(Collections.singletonList(values));
 		}
 
+		/**
+		 * Use the element with given index number.
+		 *
+		 * @param index the index number
+		 * @return new instance of {@link ArrayElemAt}.
+		 */
 		public ArrayElemAt elementAt(int index) {
 			return new ArrayElemAt(append(index));
 		}
 
+		/**
+		 * Use the element at the index number evaluated from the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ArrayElemAt}.
+		 */
 		public ArrayElemAt elementAt(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new ArrayElemAt(append(expression));
 		}
 
+		/**
+		 * Use the element at the index number traken from the given field.
+		 *
+		 * @param arrayFieldReference the field name.
+		 * @return new instance of {@link ArrayElemAt}.
+		 */
 		public ArrayElemAt elementAt(String arrayFieldReference) {
 
 			Assert.notNull(arrayFieldReference, "ArrayReference must not be null!");
@@ -483,7 +501,7 @@ public class ArrayOperators {
 		 * Creates new {@link ConcatArrays}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public static ConcatArrays arrayOf(String fieldReference) {
 
@@ -495,7 +513,7 @@ public class ArrayOperators {
 		 * Creates new {@link ConcatArrays}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public static ConcatArrays arrayOf(AggregationExpression expression) {
 
@@ -516,12 +534,24 @@ public class ArrayOperators {
 			return new ConcatArrays(Collections.singletonList(values));
 		}
 
+		/**
+		 * Concat with the array stored at the given field.
+		 *
+		 * @param arrayFieldReference must not be {@literal null}.
+		 * @return new instance of {@link ConcatArrays}.
+		 */
 		public ConcatArrays concat(String arrayFieldReference) {
 
 			Assert.notNull(arrayFieldReference, "ArrayFieldReference must not be null!");
 			return new ConcatArrays(append(Fields.field(arrayFieldReference)));
 		}
 
+		/**
+		 * Concat with the array resulting from the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ConcatArrays}.
+		 */
 		public ConcatArrays concat(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
@@ -574,7 +604,7 @@ public class ArrayOperators {
 		 * Set the {@literal values} to apply the {@code $filter} to.
 		 *
 		 * @param values must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AsBuilder} to create the {@link Filter}.
 		 */
 		public static AsBuilder filter(List<?> values) {
 
@@ -651,7 +681,7 @@ public class ArrayOperators {
 			 * Set the {@literal variableName} for the elements in the input array.
 			 *
 			 * @param variableName must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			ConditionBuilder as(String variableName);
 		}
@@ -665,7 +695,7 @@ public class ArrayOperators {
 			 * Set the {@link AggregationExpression} that determines whether to include the element in the resulting array.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			Filter by(AggregationExpression expression);
 
@@ -673,7 +703,7 @@ public class ArrayOperators {
 			 * Set the {@literal expression} that determines whether to include the element in the resulting array.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			Filter by(String expression);
 
@@ -681,7 +711,7 @@ public class ArrayOperators {
 			 * Set the {@literal expression} that determines whether to include the element in the resulting array.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			Filter by(Document expression);
 		}
@@ -700,7 +730,7 @@ public class ArrayOperators {
 			/**
 			 * Creates new {@link InputBuilder}.
 			 *
-			 * @return
+			 * @return new instance of {@link FilterExpressionBuilder}.
 			 */
 			public static InputBuilder newBuilder() {
 				return new FilterExpressionBuilder();
@@ -800,7 +830,7 @@ public class ArrayOperators {
 		 * Creates new {@link IsArray}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsArray}.
 		 */
 		public static IsArray isArray(String fieldReference) {
 
@@ -812,7 +842,7 @@ public class ArrayOperators {
 		 * Creates new {@link IsArray}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsArray}.
 		 */
 		public static IsArray isArray(AggregationExpression expression) {
 
@@ -841,7 +871,7 @@ public class ArrayOperators {
 		 * Creates new {@link Size}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Size}.
 		 */
 		public static Size lengthOfArray(String fieldReference) {
 
@@ -853,7 +883,7 @@ public class ArrayOperators {
 		 * Creates new {@link Size}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Size}.
 		 */
 		public static Size lengthOfArray(AggregationExpression expression) {
 
@@ -895,7 +925,7 @@ public class ArrayOperators {
 		 * Creates new {@link Slice}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Slice}.
 		 */
 		public static Slice sliceArrayOf(String fieldReference) {
 
@@ -907,7 +937,7 @@ public class ArrayOperators {
 		 * Creates new {@link Slice}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Slice}.
 		 */
 		public static Slice sliceArrayOf(AggregationExpression expression) {
 
@@ -928,10 +958,22 @@ public class ArrayOperators {
 			return new Slice(Collections.singletonList(values));
 		}
 
+		/**
+		 * Slice the number of elements.
+		 *
+		 * @param nrElements elements to slice.
+		 * @return new instance of {@link Slice}.
+		 */
 		public Slice itemCount(int nrElements) {
 			return new Slice(append(nrElements));
 		}
 
+		/**
+		 * Slice using offset and count.
+		 *
+		 * @param position the start position
+		 * @return new instance of {@link SliceElementsBuilder} to create {@link Slice}.
+		 */
 		public SliceElementsBuilder offset(final int position) {
 
 			return new SliceElementsBuilder() {
@@ -978,7 +1020,7 @@ public class ArrayOperators {
 		 * Start creating new {@link IndexOfArray}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfArray}.
 		 */
 		public static IndexOfArrayBuilder arrayOf(String fieldReference) {
 
@@ -990,7 +1032,7 @@ public class ArrayOperators {
 		 * Start creating new {@link IndexOfArray}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfArray}.
 		 */
 		public static IndexOfArrayBuilder arrayOf(AggregationExpression expression) {
 
@@ -1002,7 +1044,7 @@ public class ArrayOperators {
 		 * Start creating new {@link IndexOfArray}.
 		 *
 		 * @param values must not be {@literal null}.
-		 * @return new instance of {@link IndexOfArray}.
+		 * @return new instance of {@link IndexOfArrayBuilder} to create {@link IndexOfArray}.
 		 * @since 2.2
 		 */
 		public static IndexOfArrayBuilder arrayOf(Collection<?> values) {
@@ -1011,6 +1053,12 @@ public class ArrayOperators {
 			return new IndexOfArrayBuilder(values);
 		}
 
+		/**
+		 * Lookup within a given range.
+		 *
+		 * @param range the lookup range.
+		 * @return new instance of {@link IndexOfArray}.
+		 */
 		public IndexOfArray within(Range<Long> range) {
 			return new IndexOfArray(append(AggregationUtils.toRangeValues(range)));
 		}
@@ -1030,7 +1078,7 @@ public class ArrayOperators {
 			 * Set the {@literal value} to check for its index in the array.
 			 *
 			 * @param value must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfArray}.
 			 */
 			public IndexOfArray indexOf(Object value) {
 
@@ -1060,7 +1108,7 @@ public class ArrayOperators {
 		 * Start creating new {@link RangeOperator}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link RangeOperatorBuilder} to create {@link RangeOperator}.
 		 */
 		public static RangeOperatorBuilder rangeStartingAt(String fieldReference) {
 			return new RangeOperatorBuilder(Fields.field(fieldReference));
@@ -1070,7 +1118,7 @@ public class ArrayOperators {
 		 * Start creating new {@link RangeOperator}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link RangeOperatorBuilder} to create {@link RangeOperator}.
 		 */
 		public static RangeOperatorBuilder rangeStartingAt(AggregationExpression expression) {
 			return new RangeOperatorBuilder(expression);
@@ -1080,7 +1128,7 @@ public class ArrayOperators {
 		 * Start creating new {@link RangeOperator}.
 		 *
 		 * @param value
-		 * @return
+		 * @return new instance of {@link RangeOperator}.
 		 */
 		public static RangeOperatorBuilder rangeStartingAt(long value) {
 			return new RangeOperatorBuilder(value);
@@ -1102,7 +1150,7 @@ public class ArrayOperators {
 			 * Creates new {@link RangeOperator}.
 			 *
 			 * @param index
-			 * @return
+			 * @return new instance of {@link RangeOperator}.
 			 */
 			public RangeOperator to(long index) {
 				return new RangeOperator(Arrays.asList(startPoint, index));
@@ -1112,7 +1160,7 @@ public class ArrayOperators {
 			 * Creates new {@link RangeOperator}.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link RangeOperator}.
 			 */
 			public RangeOperator to(AggregationExpression expression) {
 				return new RangeOperator(Arrays.asList(startPoint, expression));
@@ -1122,7 +1170,7 @@ public class ArrayOperators {
 			 * Creates new {@link RangeOperator}.
 			 *
 			 * @param fieldReference must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link RangeOperator}.
 			 */
 			public RangeOperator to(String fieldReference) {
 				return new RangeOperator(Arrays.asList(startPoint, Fields.field(fieldReference)));
@@ -1150,7 +1198,7 @@ public class ArrayOperators {
 		 * Creates new {@link ReverseArray} given {@literal fieldReference}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReverseArray}.
 		 */
 		public static ReverseArray reverseArrayOf(String fieldReference) {
 			return new ReverseArray(Fields.field(fieldReference));
@@ -1160,7 +1208,7 @@ public class ArrayOperators {
 		 * Creates new {@link ReverseArray} given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReverseArray}.
 		 */
 		public static ReverseArray reverseArrayOf(AggregationExpression expression) {
 			return new ReverseArray(expression);
@@ -1239,7 +1287,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Reduce}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InitialValueBuilder} to create {@link Reduce}.
 		 */
 		public static InitialValueBuilder arrayOf(final String fieldReference) {
 
@@ -1278,7 +1326,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Reduce}.
 		 *
 		 * @param arrayValueExpression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InitialValueBuilder} to create {@link Reduce}.
 		 */
 		public static InitialValueBuilder arrayOf(final AggregationExpression arrayValueExpression) {
 
@@ -1318,7 +1366,7 @@ public class ArrayOperators {
 			 * Define the initial cumulative value set before in is applied to the first element of the input array.
 			 *
 			 * @param initialValue must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			ReduceBuilder withInitialValue(Object initialValue);
 		}
@@ -1335,7 +1383,7 @@ public class ArrayOperators {
 			 * {@link Variable#VALUE} are available.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link Reduce}.
 			 */
 			Reduce reduce(AggregationExpression expression);
 
@@ -1346,7 +1394,7 @@ public class ArrayOperators {
 			 * {@link Variable#VALUE} are available.
 			 *
 			 * @param expressions must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link Reduce}.
 			 */
 			Reduce reduce(PropertyExpression... expressions);
 		}
@@ -1372,7 +1420,7 @@ public class ArrayOperators {
 			 * Define a result property for an {@link AggregationExpression} used in {@link Reduce}.
 			 *
 			 * @param name must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link AsBuilder} to create {@link Reduce}.
 			 */
 			public static AsBuilder property(final String name) {
 
@@ -1402,7 +1450,7 @@ public class ArrayOperators {
 				 * Set the {@link AggregationExpression} resulting in the properties value.
 				 *
 				 * @param expression must not be {@literal null}.
-				 * @return
+				 * @return never {@literal null}.
 				 */
 				PropertyExpression definedAs(AggregationExpression expression);
 			}
@@ -1459,7 +1507,7 @@ public class ArrayOperators {
 			 * eg. {@code $$value.product}
 			 *
 			 * @param property must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			public Field referringTo(final String property) {
 
@@ -1508,7 +1556,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Zip}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ZipBuilder} to create {@link Zip}.
 		 */
 		public static ZipBuilder arrayOf(String fieldReference) {
 
@@ -1520,7 +1568,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Zip}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ZipBuilder} to create {@link Zip}.
 		 */
 		public static ZipBuilder arrayOf(AggregationExpression expression) {
 
@@ -1544,7 +1592,7 @@ public class ArrayOperators {
 		/**
 		 * Create new {@link Zip} and set the {@code useLongestLength} property to {@literal true}.
 		 *
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip useLongestLength() {
 			return new Zip(append("useLongestLength", true));
@@ -1554,7 +1602,7 @@ public class ArrayOperators {
 		 * Optionally provide a default value.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip defaultTo(String fieldReference) {
 
@@ -1566,7 +1614,7 @@ public class ArrayOperators {
 		 * Optionally provide a default value.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip defaultTo(AggregationExpression expression) {
 
@@ -1578,7 +1626,7 @@ public class ArrayOperators {
 		 * Optionally provide a default value.
 		 *
 		 * @param array must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip defaultTo(Object[] array) {
 
@@ -1602,7 +1650,7 @@ public class ArrayOperators {
 			 * array, etc.
 			 *
 			 * @param arrays arrays to zip the referenced one with. must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link Zip}.
 			 */
 			public Zip zip(Object... arrays) {
 
@@ -1645,7 +1693,7 @@ public class ArrayOperators {
 		 * Start creating {@link In}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InBuilder} to create {@link In}.
 		 */
 		public static InBuilder arrayOf(String fieldReference) {
 
@@ -1662,7 +1710,7 @@ public class ArrayOperators {
 		 * Start creating {@link In}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InBuilder} to create {@link In}.
 		 */
 		public static InBuilder arrayOf(AggregationExpression expression) {
 
@@ -1704,7 +1752,7 @@ public class ArrayOperators {
 			 * Set the {@literal value} to check for existence in the array.
 			 *
 			 * @param value must not be {@literal value}.
-			 * @return
+			 * @return new instance of {@link In}.
 			 */
 			In containsValue(Object value);
 		}

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

@@ -34,7 +34,7 @@ public class BooleanOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BooleanOperatorFactory}.
 	 */
 	public static BooleanOperatorFactory valueOf(String fieldReference) {
 		return new BooleanOperatorFactory(fieldReference);
@@ -44,7 +44,7 @@ public class BooleanOperators {
 	 * Take the value resulting of the given {@link AggregationExpression}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BooleanOperatorFactory}.
 	 */
 	public static BooleanOperatorFactory valueOf(AggregationExpression fieldReference) {
 		return new BooleanOperatorFactory(fieldReference);
@@ -55,7 +55,7 @@ public class BooleanOperators {
 	 * opposite boolean value.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Not}.
 	 */
 	public static Not not(String fieldReference) {
 		return Not.not(fieldReference);
@@ -66,7 +66,7 @@ public class BooleanOperators {
 	 * and returns the opposite boolean value.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Not}.
 	 */
 	public static Not not(AggregationExpression expression) {
 		return Not.not(expression);
@@ -109,7 +109,7 @@ public class BooleanOperators {
 		 * all of the expressions are {@literal true}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And and(AggregationExpression expression) {
 
@@ -122,7 +122,7 @@ public class BooleanOperators {
 		 * all of the expressions are {@literal true}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And and(String fieldReference) {
 
@@ -139,7 +139,7 @@ public class BooleanOperators {
 		 * any of the expressions are {@literal true}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or or(AggregationExpression expression) {
 
@@ -152,7 +152,7 @@ public class BooleanOperators {
 		 * any of the expressions are {@literal true}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or or(String fieldReference) {
 
@@ -167,7 +167,7 @@ public class BooleanOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that evaluates a boolean and returns the opposite boolean value.
 		 *
-		 * @return
+		 * @return new instance of {@link Not}.
 		 */
 		public Not not() {
 			return usesFieldRef() ? Not.not(fieldReference) : Not.not(expression);
@@ -198,8 +198,8 @@ public class BooleanOperators {
 		 * Creates new {@link And} that evaluates one or more expressions and returns {@literal true} if all of the
 		 * expressions are {@literal true}.
 		 *
-		 * @param expressions
-		 * @return
+		 * @param expressions must not be {@literal null}.
+		 * @return new instance of {@link And}.
 		 */
 		public static And and(Object... expressions) {
 			return new And(Arrays.asList(expressions));
@@ -209,7 +209,7 @@ public class BooleanOperators {
 		 * Creates new {@link And} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And andExpression(AggregationExpression expression) {
 
@@ -221,7 +221,7 @@ public class BooleanOperators {
 		 * Creates new {@link And} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And andField(String fieldReference) {
 
@@ -233,7 +233,7 @@ public class BooleanOperators {
 		 * Creates new {@link And} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And andValue(Object value) {
 
@@ -263,7 +263,7 @@ public class BooleanOperators {
 		 * expressions are {@literal true}.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public static Or or(Object... expressions) {
 
@@ -275,7 +275,7 @@ public class BooleanOperators {
 		 * Creates new {@link Or} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or orExpression(AggregationExpression expression) {
 
@@ -287,7 +287,7 @@ public class BooleanOperators {
 		 * Creates new {@link Or} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or orField(String fieldReference) {
 
@@ -299,7 +299,7 @@ public class BooleanOperators {
 		 * Creates new {@link Or} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or orValue(Object value) {
 
@@ -329,7 +329,7 @@ public class BooleanOperators {
 		 * value.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Not}.
 		 */
 		public static Not not(String fieldReference) {
 
@@ -342,7 +342,7 @@ public class BooleanOperators {
 		 * returns the opposite boolean value.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Not}.
 		 */
 		public static Not not(AggregationExpression expression) {
 

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

@@ -15,12 +15,12 @@
  */
 package org.springframework.data.mongodb.core.aggregation;
 
+import org.bson.Document;
+
 import org.springframework.data.mongodb.core.aggregation.BucketAutoOperation.BucketAutoOperationOutputBuilder;
 import org.springframework.data.mongodb.core.aggregation.BucketOperationSupport.OutputBuilder;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Encapsulates the aggregation framework {@code $bucketAuto}-operation. <br />
  * Bucket stage is typically used with {@link Aggregation} and {@code $facet}. Categorizes incoming documents into a
@@ -113,7 +113,7 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 	 * Configures a number of bucket {@literal buckets} and return a new {@link BucketAutoOperation}.
 	 *
 	 * @param buckets must be a positive number.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 */
 	public BucketAutoOperation withBuckets(int buckets) {
 
@@ -128,7 +128,7 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 	 * Use either predefined {@link Granularities} or provide a own one.
 	 *
 	 * @param granularity must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 */
 	public BucketAutoOperation withGranularity(Granularity granularity) {
 
@@ -209,7 +209,7 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 		 *
 		 * @param expression must not be {@literal null}.
 		 * @param operation must not be {@literal null}.
-		 * @param parameters
+		 * @param parameters must not be {@literal null}.
 		 */
 		protected ExpressionBucketAutoOperationBuilder(String expression, BucketAutoOperation operation,
 				Object[] parameters) {

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

@@ -20,11 +20,11 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
+import org.bson.Document;
+
 import org.springframework.data.mongodb.core.aggregation.BucketOperation.BucketOperationOutputBuilder;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Encapsulates the aggregation framework {@code $bucket}-operation. <br />
  *
@@ -110,7 +110,7 @@ public class BucketOperation extends BucketOperationSupport<BucketOperation, Buc
 	 * Configures a default bucket {@literal literal} and return a new {@link BucketOperation}.
 	 *
 	 * @param literal must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 */
 	public BucketOperation withDefaultBucket(Object literal) {
 
@@ -123,7 +123,7 @@ public class BucketOperation extends BucketOperationSupport<BucketOperation, Buc
 	 * preserved and the new {@literal boundaries} are appended.
 	 *
 	 * @param boundaries must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 */
 	public BucketOperation withBoundaries(Object... boundaries) {
 
@@ -204,12 +204,12 @@ public class BucketOperation extends BucketOperationSupport<BucketOperation, Buc
 			extends ExpressionBucketOperationBuilderSupport<BucketOperationOutputBuilder, BucketOperation> {
 
 		/**
-		 * Creates a new {@link ExpressionBucketOperationBuilderSupport} for the given value, {@link BucketOperation}
-		 * and parameters.
+		 * Creates a new {@link ExpressionBucketOperationBuilderSupport} for the given value, {@link BucketOperation} and
+		 * parameters.
 		 *
 		 * @param expression must not be {@literal null}.
 		 * @param operation must not be {@literal null}.
-		 * @param parameters
+		 * @param parameters must not be {@literal null}.
 		 */
 		protected ExpressionBucketOperationBuilder(String expression, BucketOperation operation, Object[] parameters) {
 			super(expression, operation, parameters);

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

@@ -21,14 +21,13 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.BucketOperationSupport.OutputBuilder;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder;
 import org.springframework.expression.spel.ast.Projection;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Base class for bucket operations that support output expressions the aggregation framework. <br />
  * Bucket stages collect documents into buckets and can contribute output fields. <br />
@@ -104,7 +103,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 	 *
 	 * @param expression the SpEL expression, must not be {@literal null} or empty.
 	 * @param params must not be {@literal null}
-	 * @return
+	 * @return new instance of {@link ExpressionBucketOperationBuilderSupport} to create {@link BucketOperation}.
 	 */
 	public abstract ExpressionBucketOperationBuilderSupport<B, T> andOutputExpression(String expression,
 			Object... params);
@@ -114,7 +113,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 	 * resulting bucket documents.
 	 *
 	 * @param expression the SpEL expression, must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public abstract B andOutput(AggregationExpression expression);
 
@@ -124,14 +123,14 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 	 * {@literal fieldName}.
 	 *
 	 * @param fieldName must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public abstract B andOutput(String fieldName);
 
 	/**
 	 * Creates a new {@link BucketOperationSupport} given to add a count field to the resulting bucket documents.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public B andOutputCount() {
 		return andOutput(new AggregationExpression() {
@@ -231,7 +230,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Generates a builder for a {@code $sum}-expression. <br />
 		 * Count expressions are emulated via {@code $sum: 1}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B count() {
 			return sum(1);
@@ -240,7 +239,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for a {@code $sum}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B sum() {
 			return apply(Accumulators.SUM);
@@ -249,8 +248,8 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for a {@code $sum}-expression for the given {@literal value}.
 		 *
-		 * @param value
-		 * @return
+		 * @param value must not be {@literal null}.
+		 * @return never {@literal null}.
 		 */
 		public B sum(Number value) {
 			return apply(new OperationOutput(Accumulators.SUM.getMongoOperator(), Collections.singleton(value)));
@@ -259,7 +258,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $last}-expression for the current value..
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B last() {
 			return apply(Accumulators.LAST);
@@ -268,7 +267,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for a {@code $first}-expression the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B first() {
 			return apply(Accumulators.FIRST);
@@ -277,8 +276,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $avg}-expression for the current value.
 		 *
-		 * @param reference
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B avg() {
 			return apply(Accumulators.AVG);
@@ -287,7 +285,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $min}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B min() {
 			return apply(Accumulators.MIN);
@@ -296,7 +294,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $max}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B max() {
 			return apply(Accumulators.MAX);
@@ -305,7 +303,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $push}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B push() {
 			return apply(Accumulators.PUSH);
@@ -314,7 +312,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $addToSet}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B addToSet() {
 			return apply(Accumulators.ADDTOSET);
@@ -325,7 +323,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 *
 		 * @param operation the operation name, must not be {@literal null} or empty.
 		 * @param values must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B apply(String operation, Object... values) {
 
@@ -342,7 +340,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Apply an {@link OperationOutput} to this output.
 		 *
 		 * @param operationOutput must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		protected abstract B apply(OperationOutput operationOutput);
 
@@ -354,7 +352,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Returns the finally to be applied {@link BucketOperation} with the given alias.
 		 *
 		 * @param alias will never be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public T as(String alias) {
 
@@ -550,8 +548,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		public Document toDocument(AggregationOperationContext context) {
 
 			List<Object> operationArguments = getOperationArguments(context);
-			return new Document(operation,
-					operationArguments.size() == 1 ? operationArguments.get(0) : operationArguments);
+			return new Document(operation, operationArguments.size() == 1 ? operationArguments.get(0) : operationArguments);
 		}
 
 		protected List<Object> getOperationArguments(AggregationOperationContext context) {
@@ -579,7 +576,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Returns the field that holds the {@link ProjectionOperationBuilder.OperationProjection}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		protected Field getField() {
 			return getExposedField();
@@ -589,7 +586,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Creates a new instance of this {@link OperationOutput} with the given alias.
 		 *
 		 * @param alias the alias to set
-		 * @return
+		 * @return new instance of {@link OperationOutput}.
 		 */
 		public OperationOutput withAlias(String alias) {
 
@@ -658,8 +655,8 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Creates a new {@link AggregationExpressionOutput}.
 		 *
-		 * @param field
-		 * @param expression
+		 * @param field must not be {@literal null}.
+		 * @param expression must not be {@literal null}.
 		 */
 		protected AggregationExpressionOutput(Field field, AggregationExpression expression) {
 

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

@@ -32,7 +32,7 @@ public class ComparisonOperators {
 	 * Take the field referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ComparisonOperatorFactory}.
 	 */
 	public static ComparisonOperatorFactory valueOf(String fieldReference) {
 		return new ComparisonOperatorFactory(fieldReference);
@@ -42,7 +42,7 @@ public class ComparisonOperators {
 	 * Take the value resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ComparisonOperatorFactory}.
 	 */
 	public static ComparisonOperatorFactory valueOf(AggregationExpression expression) {
 		return new ComparisonOperatorFactory(expression);
@@ -81,7 +81,7 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(String fieldReference) {
 			return createCmp().compareTo(fieldReference);
@@ -91,7 +91,7 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(AggregationExpression expression) {
 			return createCmp().compareTo(expression);
@@ -101,7 +101,7 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareToValue(Object value) {
 			return createCmp().compareToValue(value);
@@ -116,7 +116,7 @@ public class ComparisonOperators {
 		 * value is equal to the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(String fieldReference) {
 			return createEq().equalTo(fieldReference);
@@ -127,7 +127,7 @@ public class ComparisonOperators {
 		 * value is equal to the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(AggregationExpression expression) {
 			return createEq().equalTo(expression);
@@ -138,7 +138,7 @@ public class ComparisonOperators {
 		 * value is equal to the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalToValue(Object value) {
 			return createEq().equalToValue(value);
@@ -153,7 +153,7 @@ public class ComparisonOperators {
 		 * value is greater than the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(String fieldReference) {
 			return createGt().greaterThan(fieldReference);
@@ -164,7 +164,7 @@ public class ComparisonOperators {
 		 * value is greater than the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(AggregationExpression expression) {
 			return createGt().greaterThan(expression);
@@ -175,7 +175,7 @@ public class ComparisonOperators {
 		 * value is greater than the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThanValue(Object value) {
 			return createGt().greaterThanValue(value);
@@ -190,7 +190,7 @@ public class ComparisonOperators {
 		 * value is greater than or equivalent to the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(String fieldReference) {
 			return createGte().greaterThanEqualTo(fieldReference);
@@ -201,7 +201,7 @@ public class ComparisonOperators {
 		 * value is greater than or equivalent to the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(AggregationExpression expression) {
 			return createGte().greaterThanEqualTo(expression);
@@ -212,7 +212,7 @@ public class ComparisonOperators {
 		 * value is greater than or equivalent to the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualToValue(Object value) {
 			return createGte().greaterThanEqualToValue(value);
@@ -227,7 +227,7 @@ public class ComparisonOperators {
 		 * value is less than the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(String fieldReference) {
 			return createLt().lessThan(fieldReference);
@@ -238,7 +238,7 @@ public class ComparisonOperators {
 		 * value is less than the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(AggregationExpression expression) {
 			return createLt().lessThan(expression);
@@ -249,7 +249,7 @@ public class ComparisonOperators {
 		 * value is less than to the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThanValue(Object value) {
 			return createLt().lessThanValue(value);
@@ -264,7 +264,7 @@ public class ComparisonOperators {
 		 * value is less than or equivalent to the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(String fieldReference) {
 			return createLte().lessThanEqualTo(fieldReference);
@@ -275,7 +275,7 @@ public class ComparisonOperators {
 		 * value is less than or equivalent to the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(AggregationExpression expression) {
 			return createLte().lessThanEqualTo(expression);
@@ -285,8 +285,8 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values and returns {@literal true} when the first
 		 * value is less than or equivalent to the given value.
 		 *
-		 * @param value
-		 * @return
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualToValue(Object value) {
 			return createLte().lessThanEqualToValue(value);
@@ -301,7 +301,7 @@ public class ComparisonOperators {
 		 * are not equivalent.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(String fieldReference) {
 			return createNe().notEqualTo(fieldReference);
@@ -312,7 +312,7 @@ public class ComparisonOperators {
 		 * are not equivalent.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(AggregationExpression expression) {
 			return createNe().notEqualTo(expression);
@@ -323,7 +323,7 @@ public class ComparisonOperators {
 		 * are not equivalent.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualToValue(Object value) {
 			return createNe().notEqualToValue(value);
@@ -358,7 +358,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public static Cmp valueOf(String fieldReference) {
 
@@ -370,7 +370,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public static Cmp valueOf(AggregationExpression expression) {
 
@@ -382,7 +382,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(String fieldReference) {
 
@@ -394,7 +394,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(AggregationExpression expression) {
 
@@ -406,12 +406,12 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareToValue(Object value) {
 
 			Assert.notNull(value, "Value must not be null!");
-			return new Cmp(append(value));
+			return new Cmp(append(value, Expand.KEEP_SOURCE));
 		}
 	}
 
@@ -435,7 +435,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public static Eq valueOf(String fieldReference) {
 
@@ -447,7 +447,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public static Eq valueOf(AggregationExpression expression) {
 
@@ -459,7 +459,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(String fieldReference) {
 
@@ -471,7 +471,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(AggregationExpression expression) {
 
@@ -483,12 +483,12 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalToValue(Object value) {
 
 			Assert.notNull(value, "Value must not be null!");
-			return new Eq(append(value));
+			return new Eq(append(value, Expand.KEEP_SOURCE));
 		}
 	}
 
@@ -512,7 +512,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public static Gt valueOf(String fieldReference) {
 
@@ -524,7 +524,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public static Gt valueOf(AggregationExpression expression) {
 
@@ -536,7 +536,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(String fieldReference) {
 
@@ -548,7 +548,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(AggregationExpression expression) {
 
@@ -560,7 +560,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThanValue(Object value) {
 
@@ -589,7 +589,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public static Lt valueOf(String fieldReference) {
 
@@ -601,7 +601,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public static Lt valueOf(AggregationExpression expression) {
 
@@ -613,7 +613,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(String fieldReference) {
 
@@ -625,7 +625,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(AggregationExpression expression) {
 
@@ -637,7 +637,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThanValue(Object value) {
 
@@ -666,7 +666,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public static Gte valueOf(String fieldReference) {
 
@@ -678,7 +678,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public static Gte valueOf(AggregationExpression expression) {
 
@@ -690,7 +690,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(String fieldReference) {
 
@@ -702,7 +702,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(AggregationExpression expression) {
 
@@ -714,7 +714,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualToValue(Object value) {
 
@@ -743,7 +743,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public static Lte valueOf(String fieldReference) {
 
@@ -755,7 +755,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public static Lte valueOf(AggregationExpression expression) {
 
@@ -767,7 +767,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(String fieldReference) {
 
@@ -779,7 +779,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(AggregationExpression expression) {
 
@@ -791,7 +791,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualToValue(Object value) {
 
@@ -820,7 +820,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public static Ne valueOf(String fieldReference) {
 
@@ -832,7 +832,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public static Ne valueOf(AggregationExpression expression) {
 
@@ -844,7 +844,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(String fieldReference) {
 
@@ -856,7 +856,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(AggregationExpression expression) {
 
@@ -868,12 +868,12 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualToValue(Object value) {
 
 			Assert.notNull(value, "Value must not be null!");
-			return new Ne(append(value));
+			return new Ne(append(value, Expand.KEEP_SOURCE));
 		}
 	}
 }

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

@@ -43,7 +43,7 @@ public class ConditionalOperators {
 	 * Take the field referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ConditionalOperatorFactory}.
 	 */
 	public static ConditionalOperatorFactory when(String fieldReference) {
 		return new ConditionalOperatorFactory(fieldReference);
@@ -53,7 +53,7 @@ public class ConditionalOperators {
 	 * Take the value resulting from the given {@literal expression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ConditionalOperatorFactory}.
 	 */
 	public static ConditionalOperatorFactory when(AggregationExpression expression) {
 		return new ConditionalOperatorFactory(expression);
@@ -63,7 +63,7 @@ public class ConditionalOperators {
 	 * Take the value resulting from the given {@literal criteriaDefinition}.
 	 *
 	 * @param criteriaDefinition must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ConditionalOperatorFactory}.
 	 */
 	public static ConditionalOperatorFactory when(CriteriaDefinition criteriaDefinition) {
 		return new ConditionalOperatorFactory(criteriaDefinition);
@@ -75,7 +75,7 @@ public class ConditionalOperators {
 	 * instances of undefined values or missing fields, returns the value of the replacement expression.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link IfNull.ThenBuilder} to create {@link IfNull}.
 	 */
 	public static IfNull.ThenBuilder ifNull(String fieldReference) {
 
@@ -89,7 +89,7 @@ public class ConditionalOperators {
 	 * instances of undefined values or missing fields, returns the value of the replacement expression.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link IfNull.ThenBuilder} to create {@link IfNull}.
 	 */
 	public static IfNull.ThenBuilder ifNull(AggregationExpression expression) {
 
@@ -103,7 +103,7 @@ public class ConditionalOperators {
 	 * out of the control flow.
 	 *
 	 * @param conditions must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Switch}.
 	 */
 	public static Switch switchCases(CaseOperator... conditions) {
 		return Switch.switchCases(conditions);
@@ -115,7 +115,7 @@ public class ConditionalOperators {
 	 * out of the control flow.
 	 *
 	 * @param conditions must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Switch}.
 	 */
 	public static Switch switchCases(List<CaseOperator> conditions) {
 		return Switch.switchCases(conditions);
@@ -176,7 +176,7 @@ public class ConditionalOperators {
 		 * return expressions.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link OtherwiseBuilder} to create {@link Cond}.
 		 */
 		public OtherwiseBuilder then(Object value) {
 
@@ -189,7 +189,7 @@ public class ConditionalOperators {
 		 * return expressions.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link OtherwiseBuilder} to create {@link Cond}.
 		 */
 		public OtherwiseBuilder thenValueOf(AggregationExpression expression) {
 
@@ -202,7 +202,7 @@ public class ConditionalOperators {
 		 * return expressions.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link OtherwiseBuilder} to create {@link Cond}.
 		 */
 		public OtherwiseBuilder thenValueOf(String fieldReference) {
 
@@ -233,9 +233,9 @@ public class ConditionalOperators {
 	 * field references}, {@link AggregationExpression expressions}, values of simple MongoDB types or values that can be
 	 * converted to a simple MongoDB type.
 	 *
+	 * @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>
-	 * @author Mark Paluch
 	 */
 	public static class IfNull implements AggregationExpression {
 
@@ -251,9 +251,9 @@ 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}
-		 *          .
-		 * @return
+		 * @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) {
 
@@ -266,7 +266,7 @@ public class ConditionalOperators {
 		 *
 		 * @param expression the expression to check for a {@literal null} value, field reference must not be
 		 *          {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public static ThenBuilder ifNull(AggregationExpression expression) {
 
@@ -338,19 +338,19 @@ public class ConditionalOperators {
 			 * @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}.
-			 * @return
+			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull then(Object value);
 
 			/**
 			 * @param fieldReference the field holding the replacement value, must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull thenValueOf(String fieldReference);
 
 			/**
 			 * @param expression the expression yielding to the replacement value, must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull thenValueOf(AggregationExpression expression);
 		}
@@ -461,6 +461,12 @@ public class ConditionalOperators {
 			return new Switch(Collections.<String, Object> singletonMap("branches", new ArrayList<CaseOperator>(conditions)));
 		}
 
+		/**
+		 * Set the default value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Switch}.
+		 */
 		public Switch defaultTo(Object value) {
 			return new Switch(append("default", value));
 		}
@@ -522,7 +528,7 @@ public class ConditionalOperators {
 				 * Set the then {@literal value}.
 				 *
 				 * @param value must not be {@literal null}.
-				 * @return
+				 * @return new instance of {@link CaseOperator}.
 				 */
 				CaseOperator then(Object value);
 			}
@@ -536,10 +542,10 @@ public class ConditionalOperators {
 	 * {@link AggregationExpression expressions}, values of simple MongoDB types or values that can be converted to a
 	 * simple MongoDB type.
 	 *
-	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/cond/">https://docs.mongodb.com/manual/reference/operator/aggregation/cond/</a>
 	 * @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>
 	 */
 	public static class Cond implements AggregationExpression {
 
@@ -664,7 +670,6 @@ public class ConditionalOperators {
 				}
 
 				clauses.add(new Document(key, args));
-
 			} else if (predicate instanceof Document) {
 
 				Document nested = (Document) predicate;
@@ -680,7 +685,6 @@ public class ConditionalOperators {
 					args.add(nested.get(s));
 					clauses.add(new Document(s, args));
 				}
-
 			} else if (!isKeyword(key)) {
 
 				List<Object> args = new ArrayList<Object>();

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

@@ -34,9 +34,9 @@ public class CountOperation implements FieldsExposingAggregationOperation {
 	private final String fieldName;
 
 	/**
-	 * Creates a new {@link CountOperation} given the {@link fieldName} field name.
+	 * Creates a new {@link CountOperation} given the {@literal fieldName} field name.
 	 *
-	 * @param asFieldName must not be {@literal null} or empty.
+	 * @param fieldName must not be {@literal null} or empty.
 	 */
 	public CountOperation(String fieldName) {
 
@@ -71,7 +71,7 @@ public class CountOperation implements FieldsExposingAggregationOperation {
 		 * Returns the finally to be applied {@link CountOperation} with the given alias.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return new instance of {@link CountOperation}.
 		 */
 		public CountOperation as(String fieldName) {
 			return new CountOperation(fieldName);

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

@@ -30,7 +30,7 @@ public class DataTypeOperators {
 	 * Return the BSON data type of the given {@literal field}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Type}.
 	 */
 	public static Type typeOf(String fieldReference) {
 		return Type.typeOf(fieldReference);
@@ -56,7 +56,7 @@ public class DataTypeOperators {
 		 * Creates new {@link Type}.
 		 *
 		 * @param field must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Type}.
 		 */
 		public static Type typeOf(String field) {
 

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

@@ -37,7 +37,7 @@ public class DateOperators {
 	 * Take the date referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link DateOperatorFactory}.
 	 */
 	public static DateOperatorFactory dateOf(String fieldReference) {
 
@@ -49,7 +49,7 @@ public class DateOperators {
 	 * Take the date resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link DateOperatorFactory}.
 	 */
 	public static DateOperatorFactory dateOf(AggregationExpression expression) {
 
@@ -278,7 +278,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the day of the year for a date as a number between 1 and
 		 * 366.
 		 *
-		 * @return
+		 * @return new instance of {@link DayOfYear}.
 		 */
 		public DayOfYear dayOfYear() {
 			return applyTimezone(DayOfYear.dayOfYear(dateReference()), timezone);
@@ -288,7 +288,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the day of the month for a date as a number between 1 and
 		 * 31.
 		 *
-		 * @return
+		 * @return new instance of {@link DayOfMonth}.
 		 */
 		public DayOfMonth dayOfMonth() {
 			return applyTimezone(DayOfMonth.dayOfMonth(dateReference()), timezone);
@@ -298,7 +298,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the day of the week for a date as a number between 1
 		 * (Sunday) and 7 (Saturday).
 		 *
-		 * @return
+		 * @return new instance of {@link DayOfWeek}.
 		 */
 		public DayOfWeek dayOfWeek() {
 			return applyTimezone(DayOfWeek.dayOfWeek(dateReference()), timezone);
@@ -307,7 +307,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the year portion of a date.
 		 *
-		 * @return
+		 * @return new instance of {@link Year}.
 		 */
 		public Year year() {
 			return applyTimezone(Year.year(dateReference()), timezone);
@@ -316,7 +316,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the month of a date as a number between 1 and 12.
 		 *
-		 * @return
+		 * @return new instance of {@link Month}.
 		 */
 		public Month month() {
 			return applyTimezone(Month.month(dateReference()), timezone);
@@ -326,7 +326,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the week of the year for a date as a number between 0 and
 		 * 53.
 		 *
-		 * @return
+		 * @return new instance of {@link Week}.
 		 */
 		public Week week() {
 			return applyTimezone(Week.week(dateReference()), timezone);
@@ -335,7 +335,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the hour portion of a date as a number between 0 and 23.
 		 *
-		 * @return
+		 * @return new instance of {@link Hour}.
 		 */
 		public Hour hour() {
 			return applyTimezone(Hour.hour(dateReference()), timezone);
@@ -344,7 +344,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the minute portion of a date as a number between 0 and 59.
 		 *
-		 * @return
+		 * @return new instance of {@link Minute}.
 		 */
 		public Minute minute() {
 			return applyTimezone(Minute.minute(dateReference()), timezone);
@@ -354,7 +354,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the second portion of a date as a number between 0 and 59,
 		 * but can be 60 to account for leap seconds.
 		 *
-		 * @return
+		 * @return new instance of {@link Second}.
 		 */
 		public Second second() {
 			return applyTimezone(Second.second(dateReference()), timezone);
@@ -364,7 +364,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the millisecond portion of a date as an integer between 0
 		 * and 999.
 		 *
-		 * @return
+		 * @return new instance of {@link Millisecond}.
 		 */
 		public Millisecond millisecond() {
 			return applyTimezone(Millisecond.millisecond(dateReference()), timezone);
@@ -375,7 +375,7 @@ public class DateOperators {
 		 * {@literal format}.
 		 *
 		 * @param format must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DateToString}.
 		 */
 		public DateToString toString(String format) {
 			return applyTimezone(DateToString.dateToString(dateReference()).toString(format), timezone);
@@ -396,7 +396,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the weekday number in ISO 8601-2018 format, ranging from 1
 		 * (for Monday) to 7 (for Sunday).
 		 *
-		 * @return
+		 * @return new instance of {@link IsoDayOfWeek}.
 		 */
 		public IsoDayOfWeek isoDayOfWeek() {
 			return applyTimezone(IsoDayOfWeek.isoDayWeek(dateReference()), timezone);
@@ -406,7 +406,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the week number in ISO 8601-2018 format, ranging from 1 to
 		 * 53.
 		 *
-		 * @return
+		 * @return new instance of {@link IsoWeek}.
 		 */
 		public IsoWeek isoWeek() {
 			return applyTimezone(IsoWeek.isoWeek(dateReference()), timezone);
@@ -415,7 +415,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the year number in ISO 8601-2018 format.
 		 *
-		 * @return
+		 * @return new instance of {@link IsoWeekYear}.
 		 */
 		public IsoWeekYear isoWeekYear() {
 			return applyTimezone(IsoWeekYear.isoWeekYear(dateReference()), timezone);
@@ -642,7 +642,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfYear}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfYear}.
 		 */
 		public static DayOfYear dayOfYear(String fieldReference) {
 
@@ -654,7 +654,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfYear}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfYear}.
 		 */
 		public static DayOfYear dayOfYear(AggregationExpression expression) {
 
@@ -714,7 +714,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfMonth}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfMonth}.
 		 */
 		public static DayOfMonth dayOfMonth(String fieldReference) {
 
@@ -726,7 +726,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfMonth}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfMonth}.
 		 */
 		public static DayOfMonth dayOfMonth(AggregationExpression expression) {
 
@@ -786,7 +786,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfWeek}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfWeek}.
 		 */
 		public static DayOfWeek dayOfWeek(String fieldReference) {
 
@@ -798,7 +798,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfWeek}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfWeek}.
 		 */
 		public static DayOfWeek dayOfWeek(AggregationExpression expression) {
 
@@ -858,7 +858,7 @@ public class DateOperators {
 		 * Creates new {@link Year}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Year}.
 		 */
 		public static Year yearOf(String fieldReference) {
 
@@ -870,7 +870,7 @@ public class DateOperators {
 		 * Creates new {@link Year}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Year}.
 		 */
 		public static Year yearOf(AggregationExpression expression) {
 
@@ -930,7 +930,7 @@ public class DateOperators {
 		 * Creates new {@link Month}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Month}.
 		 */
 		public static Month monthOf(String fieldReference) {
 
@@ -942,7 +942,7 @@ public class DateOperators {
 		 * Creates new {@link Month}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Month}.
 		 */
 		public static Month monthOf(AggregationExpression expression) {
 
@@ -1002,7 +1002,7 @@ public class DateOperators {
 		 * Creates new {@link Week}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Week}.
 		 */
 		public static Week weekOf(String fieldReference) {
 
@@ -1014,7 +1014,7 @@ public class DateOperators {
 		 * Creates new {@link Week}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Week}.
 		 */
 		public static Week weekOf(AggregationExpression expression) {
 
@@ -1074,7 +1074,7 @@ public class DateOperators {
 		 * Creates new {@link Hour}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Hour}.
 		 */
 		public static Hour hourOf(String fieldReference) {
 
@@ -1086,7 +1086,7 @@ public class DateOperators {
 		 * Creates new {@link Hour}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Hour}.
 		 */
 		public static Hour hourOf(AggregationExpression expression) {
 
@@ -1146,7 +1146,7 @@ public class DateOperators {
 		 * Creates new {@link Minute}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Minute}.
 		 */
 		public static Minute minuteOf(String fieldReference) {
 
@@ -1158,7 +1158,7 @@ public class DateOperators {
 		 * Creates new {@link Minute}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Minute}.
 		 */
 		public static Minute minuteOf(AggregationExpression expression) {
 
@@ -1218,7 +1218,7 @@ public class DateOperators {
 		 * Creates new {@link Second}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Second}.
 		 */
 		public static Second secondOf(String fieldReference) {
 
@@ -1230,7 +1230,7 @@ public class DateOperators {
 		 * Creates new {@link Second}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Second}.
 		 */
 		public static Second secondOf(AggregationExpression expression) {
 
@@ -1290,7 +1290,7 @@ public class DateOperators {
 		 * Creates new {@link Millisecond}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Millisecond}.
 		 */
 		public static Millisecond millisecondOf(String fieldReference) {
 
@@ -1302,7 +1302,7 @@ public class DateOperators {
 		 * Creates new {@link Millisecond}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Millisecond}.
 		 */
 		public static Millisecond millisecondOf(AggregationExpression expression) {
 
@@ -1376,7 +1376,7 @@ public class DateOperators {
 		 * Creates new {@link FormatBuilder} allowing to define the date format to apply.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link FormatBuilder} to crate {@link DateToString}.
 		 */
 		public static FormatBuilder dateOf(final String fieldReference) {
 
@@ -1388,7 +1388,7 @@ public class DateOperators {
 		 * Creates new {@link FormatBuilder} allowing to define the date format to apply.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link FormatBuilder} to crate {@link DateToString}.
 		 */
 		public static FormatBuilder dateOf(final AggregationExpression expression) {
 
@@ -1540,7 +1540,7 @@ public class DateOperators {
 		 * Creates new {@link IsoDayOfWeek}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoDayOfWeek}.
 		 */
 		public static IsoDayOfWeek isoDayOfWeek(String fieldReference) {
 
@@ -1552,7 +1552,7 @@ public class DateOperators {
 		 * Creates new {@link IsoDayOfWeek}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoDayOfWeek}.
 		 */
 		public static IsoDayOfWeek isoDayOfWeek(AggregationExpression expression) {
 
@@ -1612,7 +1612,7 @@ public class DateOperators {
 		 * Creates new {@link IsoWeek}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeek}.
 		 */
 		public static IsoWeek isoWeekOf(String fieldReference) {
 
@@ -1624,7 +1624,7 @@ public class DateOperators {
 		 * Creates new {@link IsoWeek}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeek}.
 		 */
 		public static IsoWeek isoWeekOf(AggregationExpression expression) {
 
@@ -1684,7 +1684,7 @@ public class DateOperators {
 		 * Creates new {@link IsoWeekYear}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeekYear}.
 		 */
 		public static IsoWeekYear isoWeekYearOf(String fieldReference) {
 
@@ -1696,7 +1696,7 @@ public class DateOperators {
 		 * Creates new {@link Millisecond}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeekYear}.
 		 */
 		public static IsoWeekYear isoWeekYearOf(AggregationExpression expression) {
 
@@ -2378,6 +2378,7 @@ public class DateOperators {
 	@SuppressWarnings("unchecked")
 	private static <T extends TimezonedDateAggregationExpression> T applyTimezone(T instance, Timezone timezone) {
 		return !ObjectUtils.nullSafeEquals(Timezone.none(), timezone) && !instance.hasTimezone()
-				? (T) instance.withTimezone(timezone) : instance;
+				? (T) instance.withTimezone(timezone)
+				: instance;
 	}
 }

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

@@ -46,7 +46,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	/**
 	 * Returns an empty {@link ExposedFields} instance.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.0
 	 */
 	public static ExposedFields empty() {
@@ -57,7 +57,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates a new {@link ExposedFields} instance from the given {@link ExposedField}s.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static ExposedFields from(ExposedField... fields) {
 		return from(Arrays.asList(fields));
@@ -67,7 +67,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates a new {@link ExposedFields} instance from the given {@link ExposedField}s.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	private static ExposedFields from(List<ExposedField> fields) {
 
@@ -84,7 +84,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates synthetic {@link ExposedFields} from the given {@link Fields}.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static ExposedFields synthetic(Fields fields) {
 		return createFields(fields, true);
@@ -94,7 +94,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates non-synthetic {@link ExposedFields} from the given {@link Fields}.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static ExposedFields nonSynthetic(Fields fields) {
 		return createFields(fields, false);
@@ -105,7 +105,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 *
 	 * @param fields must not be {@literal null}.
 	 * @param synthetic
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	private static ExposedFields createFields(Fields fields, boolean synthetic) {
 
@@ -135,7 +135,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates a new {@link ExposedFields} adding the given {@link ExposedField}.
 	 *
 	 * @param field must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ExposedFields}.
 	 */
 	public ExposedFields and(ExposedField field) {
 
@@ -151,8 +151,8 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	/**
 	 * Returns the field with the given name or {@literal null} if no field with the given name is available.
 	 *
-	 * @param name
-	 * @return
+	 * @param name must not be {@literal null}.
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	public ExposedField getField(String name) {

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

@@ -146,7 +146,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the {@literal collectionName} to apply the {@code $graphLookup} to.
 		 *
 		 * @param collectionName must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		StartWithBuilder from(String collectionName);
 	}
@@ -161,7 +161,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the startWith {@literal fieldReferences} to apply the {@code $graphLookup} to.
 		 *
 		 * @param fieldReferences must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ConnectFromBuilder startWith(String... fieldReferences);
 
@@ -169,7 +169,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the startWith {@literal expressions} to apply the {@code $graphLookup} to.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ConnectFromBuilder startWith(AggregationExpression... expressions);
 
@@ -178,7 +178,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * {@link AggregationExpression} to apply the {@code $graphLookup} to.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 * @throws IllegalArgumentException
 		 */
 		ConnectFromBuilder startWith(Object... expressions);
@@ -193,7 +193,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the connectFrom {@literal fieldName} to apply the {@code $graphLookup} to.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ConnectToBuilder connectFrom(String fieldName);
 	}
@@ -207,7 +207,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the connectTo {@literal fieldName} to apply the {@code $graphLookup} to.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		GraphLookupOperationBuilder connectTo(String fieldName);
 	}
@@ -360,7 +360,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Optionally limit the number of recursions.
 		 *
 		 * @param numberOfRecursions must be greater or equal to zero.
-		 * @return
+		 * @return this.
 		 */
 		public GraphLookupOperationBuilder maxDepth(long numberOfRecursions) {
 
@@ -374,7 +374,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Optionally add a depth field {@literal fieldName} to each traversed document in the search path.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return this.
 		 */
 		public GraphLookupOperationBuilder depthField(String fieldName) {
 

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

@@ -89,7 +89,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 	/**
 	 * Get a builder that allows creation of {@link LookupOperation}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static FromBuilder newLookup() {
 		return new LookupOperationBuilder();
@@ -99,7 +99,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 		/**
 		 * @param name the collection in the same database to perform the join with, must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		LocalFieldBuilder from(String name);
 	}
@@ -109,7 +109,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 		/**
 		 * @param name the field from the documents input to the {@code $lookup} stage, must not be {@literal null} or
 		 *          empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ForeignFieldBuilder localField(String name);
 	}
@@ -118,7 +118,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 		/**
 		 * @param name the field from the documents in the {@code from} collection, must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		AsBuilder foreignField(String name);
 	}
@@ -127,7 +127,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 		/**
 		 * @param name the name of the new array field to add to the input documents, must not be {@literal null} or empty.
-		 * @return
+		 * @return new instance of {@link LookupOperation}.
 		 */
 		LookupOperation as(String name);
 	}

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

@@ -31,7 +31,7 @@ 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 />
- * Fields to be excluded from prefixing my be added to a {@literal blacklist}.
+ * Fields to be excluded from prefixing my be added to a {@literal denylist}.
  *
  * @author Christoph Strobl
  * @author Mark Paluch
@@ -41,18 +41,18 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 
 	private final AggregationOperationContext delegate;
 	private final String prefix;
-	private final Set<String> blacklist;
+	private final Set<String> denylist;
 
 	public PrefixingDelegatingAggregationOperationContext(AggregationOperationContext delegate, String prefix) {
 		this(delegate, prefix, Collections.emptySet());
 	}
 
 	public PrefixingDelegatingAggregationOperationContext(AggregationOperationContext delegate, String prefix,
-			Collection<String> blacklist) {
+			Collection<String> denylist) {
 
 		this.delegate = delegate;
 		this.prefix = prefix;
-		this.blacklist = new HashSet<>(blacklist);
+		this.denylist = new HashSet<>(denylist);
 	}
 
 	/*
@@ -121,7 +121,7 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 	}
 
 	private String prefixKey(String key) {
-		return (key.startsWith("$") || isBlacklisted(key)) ? key : (prefix + "." + key);
+		return (key.startsWith("$") || isDenied(key)) ? key : (prefix + "." + key);
 	}
 
 	private Object prefixCollection(Collection<Object> sourceCollection) {
@@ -139,9 +139,9 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 		return prefixed;
 	}
 
-	private boolean isBlacklisted(String key) {
+	private boolean isDenied(String key) {
 
-		if (blacklist.contains(key)) {
+		if (denylist.contains(key)) {
 			return true;
 		}
 
@@ -149,8 +149,8 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 			return false;
 		}
 
-		for (String blacklisted : blacklist) {
-			if (key.startsWith(blacklisted + ".")) {
+		for (String denied : denylist) {
+			if (key.startsWith(denied + ".")) {
 				return true;
 			}
 		}

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

@@ -1,62 +0,0 @@
-/*
- * Copyright 2019-2020 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.springframework.data.mapping.context.InvalidPersistentPropertyPath;
-import org.springframework.data.mapping.context.MappingContext;
-import org.springframework.data.mongodb.core.aggregation.ExposedFields.DirectFieldReference;
-import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
-import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldReference;
-import org.springframework.data.mongodb.core.convert.QueryMapper;
-import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
-import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
-
-/**
- * A {@link TypeBasedAggregationOperationContext} with less restrictive field reference handling, suppressing
- * {@link InvalidPersistentPropertyPath} exceptions when resolving mapped field names.
- *
- * @author Christoph Strobl
- * @since 3.0
- */
-public class RelaxedTypeBasedAggregationOperationContext extends TypeBasedAggregationOperationContext {
-
-	/**
-	 * Creates a new {@link TypeBasedAggregationOperationContext} for the given type, {@link MappingContext} and
-	 * {@link QueryMapper}.
-	 *
-	 * @param type must not be {@literal null}.
-	 * @param mappingContext must not be {@literal null}.
-	 * @param mapper must not be {@literal null}.
-	 */
-	public RelaxedTypeBasedAggregationOperationContext(Class<?> type,
-			MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext, QueryMapper mapper) {
-		super(type, mappingContext, mapper);
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.TypeBasedAggregationOperationContext#getReferenceFor(rg.springframework.data.mongodb.core.aggregation.Field)
-	 */
-	@Override
-	protected FieldReference getReferenceFor(Field field) {
-
-		try {
-			return super.getReferenceFor(field);
-		} catch (InvalidPersistentPropertyPath e) {
-			return new DirectFieldReference(new ExposedField(field, true));
-		}
-	}
-}

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

@@ -33,8 +33,7 @@ import org.springframework.util.Assert;
  * @author Mark Paluch
  * @author Christoph Strobl
  * @since 1.10
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/">MongoDB Aggregation
- *      Framework: $replaceRoot</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceRoot/">MongoDB Aggregation Framework: $replaceRoot</a>
  */
 public class ReplaceRootOperation implements FieldsExposingAggregationOperation {
 
@@ -83,7 +82,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$replaceRoot", new Document("newRoot", getReplacement().toDocumentExpression(context)));
+		return new Document("$replaceRoot", new Document("newRoot", replacement.toDocumentExpression(context)));
 	}
 
 	/* (non-Javadoc)
@@ -94,16 +93,6 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		return ExposedFields.from();
 	}
 
-	/**
-	 * Obtain the {@link Replacement}.
-	 *
-	 * @return never {@literal null}.
-	 * @since 3.0
-	 */
-	protected Replacement getReplacement() {
-		return replacement;
-	}
-
 	/**
 	 * Builder for {@link ReplaceRootOperation}.
 	 *
@@ -166,7 +155,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 	 *
 	 * @author Mark Paluch
 	 */
-	static class ReplaceRootDocumentOperation extends ReplaceRootOperation {
+	public static class ReplaceRootDocumentOperation extends ReplaceRootOperation {
 
 		private final static ReplacementDocument EMPTY = new ReplacementDocument();
 		private final ReplacementDocument current;
@@ -346,7 +335,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		/**
 		 * Creates a {@link ReplacementDocument} given {@link ReplacementContributor}.
 		 *
-		 * @param contributor
+		 * @param contributor must not be {@literal null}.
 		 */
 		protected ReplacementDocument(ReplacementContributor contributor) {
 
@@ -362,7 +351,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * Creates a {@link ReplacementDocument} given a {@literal value}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReplacementDocument}.
 		 */
 		public static ReplacementDocument valueOf(Object value) {
 			return new ReplacementDocument(new DocumentContributor(value));
@@ -372,7 +361,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * Creates a {@link ReplacementDocument} given a single {@literal field} and {@link AggregationExpression}.
 		 *
 		 * @param aggregationExpression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReplacementDocument}.
 		 */
 		public static ReplacementDocument forExpression(String field, AggregationExpression aggregationExpression) {
 			return new ReplacementDocument(new ExpressionFieldContributor(Fields.field(field), aggregationExpression));
@@ -382,7 +371,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * Creates a {@link ReplacementDocument} given a single {@literal field} and {@literal value}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReplacementDocument}.
 		 */
 		public static ReplacementDocument forSingleValue(String field, Object value) {
 			return new ReplacementDocument(new ValueFieldContributor(Fields.field(field), value));
@@ -438,7 +427,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * {@link AggregationOperationContext}.
 		 *
 		 * @param context will never be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		Document toDocument(AggregationOperationContext context);
 	}

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

@@ -1,95 +0,0 @@
-/*
- * Copyright 2019-2020 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.Collection;
-import java.util.stream.Collectors;
-
-import org.bson.Document;
-import org.springframework.util.Assert;
-
-/**
- * Encapsulates the aggregation framework {@code $replaceRoot}-operation. <br />
- * The operation replaces all existing fields including the {@code id} field with @{code $replaceWith}. This way it is
- * possible to promote an embedded document to the top-level or specify a new document.
- *
- * @author Christoph Strobl
- * @since 3.0
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceWith/">MongoDB Aggregation
- *      Framework: $replaceWith</a>
- */
-public class ReplaceWithOperation extends ReplaceRootOperation {
-
-	/**
-	 * Creates new instance of {@link ReplaceWithOperation}.
-	 *
-	 * @param replacement must not be {@literal null}.
-	 */
-	public ReplaceWithOperation(Replacement replacement) {
-		super(replacement);
-	}
-
-	/**
-	 * Creates new instance of {@link ReplaceWithOperation}.
-	 *
-	 * @param value must not be {@literal null}.
-	 * @return new instance of {@link ReplaceWithOperation}.
-	 */
-	public static ReplaceWithOperation replaceWithValue(Object value) {
-		return new ReplaceWithOperation((ctx) -> value);
-	}
-
-	/**
-	 * Creates new instance of {@link ReplaceWithOperation} treating a given {@link String} {@literal value} as a
-	 * {@link Field field reference}.
-	 *
-	 * @param value must not be {@literal null}.
-	 * @return
-	 */
-	public static ReplaceWithOperation replaceWithValueOf(Object value) {
-
-		Assert.notNull(value, "Value must not be null!");
-		return new ReplaceWithOperation((ctx) -> {
-
-			Object target = value instanceof String ? Fields.field((String) value) : value;
-			return computeValue(target, ctx);
-		});
-	}
-
-	private static Object computeValue(Object value, AggregationOperationContext context) {
-
-		if (value instanceof Field) {
-			return context.getReference((Field) value).toString();
-		}
-		if (value instanceof AggregationExpression) {
-			return ((AggregationExpression) value).toDocument(context);
-		}
-		if (value instanceof Collection) {
-			return ((Collection) value).stream().map(it -> computeValue(it, context)).collect(Collectors.toList());
-		}
-
-		return value;
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
-	 */
-	@Override
-	public Document toDocument(AggregationOperationContext context) {
-		return context.getMappedObject(new Document("$replaceWith", getReplacement().toDocumentExpression(context)));
-	}
-}

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

@@ -1,250 +0,0 @@
-/*
- * Copyright 2019-2020 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.Collection;
-import java.util.Collections;
-import java.util.LinkedHashMap;
-import java.util.Map;
-import java.util.Map.Entry;
-import java.util.stream.Collectors;
-
-import org.bson.Document;
-import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
-import org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation;
-import org.springframework.data.mongodb.core.aggregation.SetOperation.FieldAppender.ValueAppender;
-import org.springframework.lang.Nullable;
-
-/**
- * Adds new fields to documents. {@code $set} outputs documents that contain all existing fields from the input
- * documents and newly added fields.
- * 
- * <pre class="code">
- * SetOperation.set("totalHomework").toValue("A+").and().set("totalQuiz").toValue("B-")
- * </pre>
- *
- * @author Christoph Strobl
- * @since 3.0
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/set/">MongoDB Aggregation Framework:
- *      $set</a>
- */
-public class SetOperation implements InheritsFieldsAggregationOperation {
-
-	private Map<Object, Object> valueMap;
-	private ExposedFields exposedFields = ExposedFields.empty();
-
-	/**
-	 * Create new instance of {@link SetOperation} adding map keys as exposed fields.
-	 *
-	 * @param source must not be {@literal null}.
-	 */
-	private SetOperation(Map<Object, Object> source) {
-
-		this.valueMap = new LinkedHashMap<>(source);
-		for (Object key : source.keySet()) {
-			this.exposedFields = add(key);
-		}
-	}
-
-	/**
-	 * Create new instance of {@link SetOperation}
-	 *
-	 * @param field must not be {@literal null}.
-	 * @param value can be {@literal null}.
-	 */
-	public SetOperation(Object field, @Nullable Object value) {
-		this(Collections.singletonMap(field, value));
-	}
-
-	/**
-	 * Define the {@link SetOperation} via {@link FieldAppender}.
-	 *
-	 * @return new instance of {@link FieldAppender}.
-	 */
-	public static FieldAppender builder() {
-		return new FieldAppender();
-	}
-
-	/**
-	 * Concatenate another field to set.
-	 *
-	 * @param field must not be {@literal null}.
-	 * @return new instance of {@link ValueAppender}.
-	 */
-	public static ValueAppender set(String field) {
-		return new FieldAppender().set(field);
-	}
-
-	/**
-	 * Append the value for a specific field to the operation.
-	 *
-	 * @param field the target field to set.
-	 * @param value the value to assign.
-	 * @return new instance of {@link SetOperation}.
-	 */
-	public SetOperation set(Object field, Object value) {
-
-		LinkedHashMap<Object, Object> target = new LinkedHashMap<>(this.valueMap);
-		target.put(field, value);
-
-		return new SetOperation(target);
-	}
-
-	/**
-	 * Concatenate additional fields to set.
-	 *
-	 * @return new instance of {@link FieldAppender}.
-	 */
-	public FieldAppender and() {
-		return new FieldAppender(this.valueMap);
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
-	 */
-	@Override
-	public Document toDocument(AggregationOperationContext context) {
-
-		InheritingExposedFieldsAggregationOperationContext operationContext = new InheritingExposedFieldsAggregationOperationContext(
-				exposedFields, context);
-
-		if (valueMap.size() == 1) {
-			return context
-					.getMappedObject(new Document("$set", toSetEntry(valueMap.entrySet().iterator().next(), operationContext)));
-		}
-
-		Document $set = new Document();
-		valueMap.entrySet().stream().map(it -> toSetEntry(it, operationContext)).forEach($set::putAll);
-		return context.getMappedObject(new Document("$set", $set));
-	}
-
-	/*
-	 * (non-Javadoc)
-	 * @see org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation#getFields()
-	 */
-	@Override
-	public ExposedFields getFields() {
-		return exposedFields;
-	}
-
-	private ExposedFields add(Object field) {
-
-		if (field instanceof Field) {
-			return exposedFields.and(new ExposedField((Field) field, true));
-		}
-		if (field instanceof String) {
-			return exposedFields.and(new ExposedField(Fields.field((String) field), true));
-		}
-
-		throw new IllegalArgumentException(String.format("Expected %s to be a field/property.", field));
-	}
-
-	private static Document toSetEntry(Entry<Object, Object> entry, AggregationOperationContext context) {
-
-		String field = entry.getKey() instanceof String ? context.getReference((String) entry.getKey()).getRaw()
-				: context.getReference((Field) entry.getKey()).getRaw();
-
-		Object value = computeValue(entry.getValue(), context);
-
-		return new Document(field, value);
-	}
-
-	private static Object computeValue(Object value, AggregationOperationContext context) {
-
-		if (value instanceof Field) {
-			return context.getReference((Field) value).toString();
-		}
-		if (value instanceof AggregationExpression) {
-			return ((AggregationExpression) value).toDocument(context);
-		}
-		if (value instanceof Collection) {
-			return ((Collection) value).stream().map(it -> computeValue(it, context)).collect(Collectors.toList());
-		}
-
-		return value;
-	}
-
-	/**
-	 * @author Christoph Strobl
-	 * @since 3.0
-	 */
-	public static class FieldAppender {
-
-		private final Map<Object, Object> valueMap;
-
-		private FieldAppender() {
-			this.valueMap = new LinkedHashMap<>();
-		}
-
-		private FieldAppender(Map<Object, Object> source) {
-			this.valueMap = new LinkedHashMap<>(source);
-		}
-
-		/**
-		 * Define the field to set.
-		 *
-		 * @param field must not be {@literal null}.
-		 * @return new instance of {@link ValueAppender}.
-		 */
-		public ValueAppender set(String field) {
-
-			return new ValueAppender() {
-
-				@Override
-				public SetOperation toValue(Object value) {
-
-					valueMap.put(field, value);
-					return FieldAppender.this.build();
-				}
-
-				@Override
-				public SetOperation toValueOf(Object value) {
-
-					valueMap.put(field, value instanceof String ? Fields.fields((String) value) : value);
-					return FieldAppender.this.build();
-				}
-			};
-		}
-
-		private SetOperation build() {
-			return new SetOperation(valueMap);
-		}
-
-		/**
-		 * @author Christoph Strobl
-		 * @since 3.0
-		 */
-		public interface ValueAppender {
-
-			/**
-			 * Define the value to assign as is.
-			 *
-			 * @param value can be {@literal null}.
-			 * @return new instance of {@link SetOperation}.
-			 */
-			SetOperation toValue(@Nullable Object value);
-
-			/**
-			 * Define the value to assign. Plain {@link String} values are treated as {@link Field field references}.
-			 *
-			 * @param value must not be {@literal null}.
-			 * @return new instance of {@link SetOperation}.
-			 */
-			SetOperation toValueOf(Object value);
-		}
-	}
-}

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

@@ -34,7 +34,7 @@ public class SetOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SetOperatorFactory}.
 	 */
 	public static SetOperatorFactory arrayAsSet(String fieldReference) {
 		return new SetOperatorFactory(fieldReference);
@@ -44,7 +44,7 @@ public class SetOperators {
 	 * Take the array resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SetOperatorFactory}.
 	 */
 	public static SetOperatorFactory arrayAsSet(AggregationExpression expression) {
 		return new SetOperatorFactory(expression);
@@ -87,7 +87,7 @@ public class SetOperators {
 		 * returns {@literal true} if they have the same distinct elements and {@literal false} otherwise.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(String... arrayReferences) {
 			return createSetEquals().isEqualTo(arrayReferences);
@@ -98,7 +98,7 @@ public class SetOperators {
 		 * returns {@literal true} if they have the same distinct elements and {@literal false} otherwise.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(AggregationExpression... expressions) {
 			return createSetEquals().isEqualTo(expressions);
@@ -113,7 +113,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in every of those.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(String... arrayReferences) {
 			return createSetIntersection().intersects(arrayReferences);
@@ -124,7 +124,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in every of those.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(AggregationExpression... expressions) {
 			return createSetIntersection().intersects(expressions);
@@ -139,7 +139,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in any of those.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(String... arrayReferences) {
 			return createSetUnion().union(arrayReferences);
@@ -150,7 +150,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in any of those.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(AggregationExpression... expressions) {
 			return createSetUnion().union(expressions);
@@ -165,7 +165,7 @@ public class SetOperators {
 		 * containing the elements that do not exist in the given {@literal arrayReference}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(String arrayReference) {
 			return createSetDifference().differenceTo(arrayReference);
@@ -176,7 +176,7 @@ public class SetOperators {
 		 * containing the elements that do not exist in the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(AggregationExpression expression) {
 			return createSetDifference().differenceTo(expression);
@@ -191,7 +191,7 @@ public class SetOperators {
 		 * {@literal true} if it is a subset of the given {@literal arrayReference}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(String arrayReference) {
 			return createSetIsSubset().isSubsetOf(arrayReference);
@@ -202,7 +202,7 @@ public class SetOperators {
 		 * {@literal true} if it is a subset of the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(AggregationExpression expression) {
 			return createSetIsSubset().isSubsetOf(expression);
@@ -216,7 +216,7 @@ public class SetOperators {
 		 * Creates new {@link AggregationExpression} that takes array of the previously mentioned field and returns
 		 * {@literal true} if any of the elements are {@literal true} and {@literal false} otherwise.
 		 *
-		 * @return
+		 * @return new instance of {@link AnyElementTrue}.
 		 */
 		public AnyElementTrue anyElementTrue() {
 			return usesFieldRef() ? AnyElementTrue.arrayAsSet(fieldReference) : AnyElementTrue.arrayAsSet(expression);
@@ -226,7 +226,7 @@ public class SetOperators {
 		 * Creates new {@link AggregationExpression} that tkes array of the previously mentioned field and returns
 		 * {@literal true} if no elements is {@literal false}.
 		 *
-		 * @return
+		 * @return new instance of {@link AllElementsTrue}.
 		 */
 		public AllElementsTrue allElementsTrue() {
 			return usesFieldRef() ? AllElementsTrue.arrayAsSet(fieldReference) : AllElementsTrue.arrayAsSet(expression);
@@ -257,7 +257,7 @@ public class SetOperators {
 		 * Create new {@link SetEquals}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public static SetEquals arrayAsSet(String arrayReference) {
 
@@ -269,7 +269,7 @@ public class SetOperators {
 		 * Create new {@link SetEquals}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public static SetEquals arrayAsSet(AggregationExpression expression) {
 
@@ -281,7 +281,7 @@ public class SetOperators {
 		 * Creates new {@link java.util.Set} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(String... arrayReferences) {
 
@@ -293,7 +293,7 @@ public class SetOperators {
 		 * Creates new {@link Sum} with all previously added arguments appending the given one.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(AggregationExpression... expressions) {
 
@@ -305,7 +305,7 @@ public class SetOperators {
 		 * Creates new {@link Sum} with all previously added arguments appending the given one.
 		 *
 		 * @param array must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(Object[] array) {
 
@@ -334,7 +334,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection}
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public static SetIntersection arrayAsSet(String arrayReference) {
 
@@ -346,7 +346,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public static SetIntersection arrayAsSet(AggregationExpression expression) {
 
@@ -358,7 +358,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(String... arrayReferences) {
 
@@ -370,7 +370,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection} with all previously added arguments appending the given one.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(AggregationExpression... expressions) {
 
@@ -399,7 +399,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public static SetUnion arrayAsSet(String arrayReference) {
 
@@ -411,7 +411,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public static SetUnion arrayAsSet(AggregationExpression expression) {
 
@@ -423,7 +423,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(String... arrayReferences) {
 
@@ -435,7 +435,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion} with all previously added arguments appending the given one.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(AggregationExpression... expressions) {
 
@@ -464,7 +464,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public static SetDifference arrayAsSet(String arrayReference) {
 
@@ -476,7 +476,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public static SetDifference arrayAsSet(AggregationExpression expression) {
 
@@ -488,7 +488,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(String arrayReference) {
 
@@ -500,7 +500,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(AggregationExpression expression) {
 
@@ -529,7 +529,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public static SetIsSubset arrayAsSet(String arrayReference) {
 
@@ -541,7 +541,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public static SetIsSubset arrayAsSet(AggregationExpression expression) {
 
@@ -553,7 +553,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(String arrayReference) {
 
@@ -565,7 +565,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(AggregationExpression expression) {
 
@@ -594,7 +594,7 @@ public class SetOperators {
 		 * Creates new {@link AnyElementTrue}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AnyElementTrue}.
 		 */
 		public static AnyElementTrue arrayAsSet(String arrayReference) {
 
@@ -606,7 +606,7 @@ public class SetOperators {
 		 * Creates new {@link AnyElementTrue}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AnyElementTrue}.
 		 */
 		public static AnyElementTrue arrayAsSet(AggregationExpression expression) {
 
@@ -639,7 +639,7 @@ public class SetOperators {
 		 * Creates new {@link AllElementsTrue}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AllElementsTrue}.
 		 */
 		public static AllElementsTrue arrayAsSet(String arrayReference) {
 
@@ -651,7 +651,7 @@ public class SetOperators {
 		 * Creates new {@link AllElementsTrue}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AllElementsTrue}.
 		 */
 		public static AllElementsTrue arrayAsSet(AggregationExpression expression) {
 

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

@@ -35,7 +35,7 @@ public class StringOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link StringOperatorFactory}.
 	 */
 	public static StringOperatorFactory valueOf(String fieldReference) {
 		return new StringOperatorFactory(fieldReference);
@@ -45,7 +45,7 @@ public class StringOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link StringOperatorFactory}.
 	 */
 	public static StringOperatorFactory valueOf(AggregationExpression fieldReference) {
 		return new StringOperatorFactory(fieldReference);
@@ -88,7 +88,7 @@ public class StringOperators {
 		 * of the referenced field to it.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public Concat concatValueOf(String fieldReference) {
 
@@ -101,7 +101,7 @@ public class StringOperators {
 		 * of the given {@link AggregationExpression} to it.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public Concat concatValueOf(AggregationExpression expression) {
 
@@ -114,7 +114,7 @@ public class StringOperators {
 		 * {@literal value} to it.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public Concat concat(String value) {
 
@@ -130,8 +130,8 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and returns a substring
 		 * starting at a specified index position.
 		 *
-		 * @param start
-		 * @return
+		 * @param start start index number (including)
+		 * @return new instance of {@link Substr}.
 		 */
 		public Substr substring(int start) {
 			return substring(start, -1);
@@ -141,9 +141,9 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and returns a substring
 		 * starting at a specified index position including the specified number of characters.
 		 *
-		 * @param start
-		 * @param nrOfChars
-		 * @return
+		 * @param start start index number (including)
+		 * @param nrOfChars number of characters.
+		 * @return new instance of {@link Substr}.
 		 */
 		public Substr substring(int start, int nrOfChars) {
 			return createSubstr().substring(start, nrOfChars);
@@ -156,7 +156,7 @@ public class StringOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and lowers it.
 		 *
-		 * @return
+		 * @return new instance of {@link ToLower}.
 		 */
 		public ToLower toLower() {
 			return usesFieldRef() ? ToLower.lowerValueOf(fieldReference) : ToLower.lowerValueOf(expression);
@@ -165,7 +165,7 @@ public class StringOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and uppers it.
 		 *
-		 * @return
+		 * @return new instance of {@link ToUpper}.
 		 */
 		public ToUpper toUpper() {
 			return usesFieldRef() ? ToUpper.upperValueOf(fieldReference) : ToUpper.upperValueOf(expression);
@@ -176,7 +176,7 @@ public class StringOperators {
 		 * case-insensitive comparison to the given {@literal value}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public StrCaseCmp strCaseCmp(String value) {
 
@@ -189,7 +189,7 @@ public class StringOperators {
 		 * case-insensitive comparison to the referenced {@literal fieldReference}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public StrCaseCmp strCaseCmpValueOf(String fieldReference) {
 
@@ -202,7 +202,7 @@ public class StringOperators {
 		 * case-insensitive comparison to the result of the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public StrCaseCmp strCaseCmpValueOf(AggregationExpression expression) {
 
@@ -220,7 +220,7 @@ public class StringOperators {
 		 * occurrence.
 		 *
 		 * @param substring must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes indexOf(String substring) {
 
@@ -234,7 +234,7 @@ public class StringOperators {
 		 * index (zero-based) of the first occurrence.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes indexOf(Field fieldReference) {
 
@@ -248,7 +248,7 @@ public class StringOperators {
 		 * byte index (zero-based) of the first occurrence.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes indexOf(AggregationExpression expression) {
 
@@ -266,7 +266,7 @@ public class StringOperators {
 		 * first occurrence.
 		 *
 		 * @param substring must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP indexOfCP(String substring) {
 
@@ -280,7 +280,7 @@ public class StringOperators {
 		 * point index (zero-based) of the first occurrence.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP indexOfCP(Field fieldReference) {
 
@@ -294,7 +294,7 @@ public class StringOperators {
 		 * code point index (zero-based) of the first occurrence.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP indexOfCP(AggregationExpression expression) {
 
@@ -311,7 +311,7 @@ public class StringOperators {
 		 * substrings based on the given delimiter.
 		 *
 		 * @param delimiter must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(String delimiter) {
 			return createSplit().split(delimiter);
@@ -322,7 +322,7 @@ public class StringOperators {
 		 * substrings based on the delimiter resulting from the referenced field..
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(Field fieldReference) {
 			return createSplit().split(fieldReference);
@@ -333,7 +333,7 @@ public class StringOperators {
 		 * substrings based on a delimiter resulting from the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(AggregationExpression expression) {
 			return createSplit().split(expression);
@@ -347,7 +347,7 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that returns the number of UTF-8 bytes in the associated string
 		 * representation.
 		 *
-		 * @return
+		 * @return new instance of {@link StrLenBytes}.
 		 */
 		public StrLenBytes length() {
 			return usesFieldRef() ? StrLenBytes.stringLengthOf(fieldReference) : StrLenBytes.stringLengthOf(expression);
@@ -357,7 +357,7 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that returns the number of UTF-8 code points in the associated string
 		 * representation.
 		 *
-		 * @return
+		 * @return new instance of {@link StrLenCP}.
 		 */
 		public StrLenCP lengthCP() {
 			return usesFieldRef() ? StrLenCP.stringLengthOfCP(fieldReference) : StrLenCP.stringLengthOfCP(expression);
@@ -368,7 +368,7 @@ public class StringOperators {
 		 * starting at a specified code point index position.
 		 *
 		 * @param codePointStart
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public SubstrCP substringCP(int codePointStart) {
 			return substringCP(codePointStart, -1);
@@ -378,9 +378,9 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and returns a substring
 		 * starting at a specified code point index position including the specified number of code points.
 		 *
-		 * @param codePointStart
+		 * @param codePointStart start point (including).
 		 * @param nrOfCodePoints
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public SubstrCP substringCP(int codePointStart, int nrOfCodePoints) {
 			return createSubstrCP().substringCP(codePointStart, nrOfCodePoints);
@@ -541,7 +541,7 @@ public class StringOperators {
 		 * Creates new {@link Concat}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public static Concat valueOf(String fieldReference) {
 
@@ -553,7 +553,7 @@ public class StringOperators {
 		 * Creates new {@link Concat}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public static Concat valueOf(AggregationExpression expression) {
 
@@ -565,7 +565,7 @@ public class StringOperators {
 		 * Creates new {@link Concat}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public static Concat stringValue(String value) {
 
@@ -573,18 +573,36 @@ public class StringOperators {
 			return new Concat(Collections.singletonList(value));
 		}
 
+		/**
+		 * Concat the value of the given field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Concat}.
+		 */
 		public Concat concatValueOf(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Concat(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Concat the value resulting from the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Concat}.
+		 */
 		public Concat concatValueOf(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Concat(append(expression));
 		}
 
+		/**
+		 * Concat the given value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Concat}.
+		 */
 		public Concat concat(String value) {
 			return new Concat(append(value));
 		}
@@ -610,7 +628,7 @@ public class StringOperators {
 		 * Creates new {@link Substr}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Substr}.
 		 */
 		public static Substr valueOf(String fieldReference) {
 
@@ -622,7 +640,7 @@ public class StringOperators {
 		 * Creates new {@link Substr}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Substr}.
 		 */
 		public static Substr valueOf(AggregationExpression expression) {
 
@@ -630,10 +648,19 @@ public class StringOperators {
 			return new Substr(Collections.singletonList(expression));
 		}
 
+		/**
+		 * @param start start index (including)
+		 * @return new instance of {@link Substr}.
+		 */
 		public Substr substring(int start) {
 			return substring(start, -1);
 		}
 
+		/**
+		 * @param start start index (including)
+		 * @param nrOfChars
+		 * @return new instance of {@link Substr}.
+		 */
 		public Substr substring(int start, int nrOfChars) {
 			return new Substr(append(Arrays.asList(start, nrOfChars)));
 		}
@@ -777,7 +804,7 @@ public class StringOperators {
 		 * Creates new {@link StrCaseCmp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public static StrCaseCmp valueOf(AggregationExpression expression) {
 
@@ -789,7 +816,7 @@ public class StringOperators {
 		 * Creates new {@link StrCaseCmp}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public static StrCaseCmp stringValue(String value) {
 
@@ -834,7 +861,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfBytes}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstringBuilder}.
 		 */
 		public static SubstringBuilder valueOf(String fieldReference) {
 
@@ -846,7 +873,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfBytes}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstringBuilder}.
 		 */
 		public static SubstringBuilder valueOf(AggregationExpression expression) {
 
@@ -858,7 +885,7 @@ public class StringOperators {
 		 * Optionally define the substring search start and end position.
 		 *
 		 * @param range must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes within(Range<Long> range) {
 			return new IndexOfBytes(append(AggregationUtils.toRangeValues(range)));
@@ -876,7 +903,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfBytes} given {@literal substring}.
 			 *
 			 * @param substring must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfBytes}.
 			 */
 			public IndexOfBytes indexOf(String substring) {
 				return new IndexOfBytes(Arrays.asList(stringExpression, substring));
@@ -886,7 +913,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfBytes} given {@link AggregationExpression} that resolves to the substring.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfBytes}.
 			 */
 			public IndexOfBytes indexOf(AggregationExpression expression) {
 				return new IndexOfBytes(Arrays.asList(stringExpression, expression));
@@ -896,7 +923,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfBytes} given {@link Field} that resolves to the substring.
 			 *
 			 * @param fieldReference must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfBytes}.
 			 */
 			public IndexOfBytes indexOf(Field fieldReference) {
 				return new IndexOfBytes(Arrays.asList(stringExpression, fieldReference));
@@ -924,7 +951,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfCP}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public static SubstringBuilder valueOf(String fieldReference) {
 
@@ -936,7 +963,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfCP}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public static SubstringBuilder valueOf(AggregationExpression expression) {
 
@@ -948,7 +975,7 @@ public class StringOperators {
 		 * Optionally define the substring search start and end position.
 		 *
 		 * @param range must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP within(Range<Long> range) {
 			return new IndexOfCP(append(AggregationUtils.toRangeValues(range)));
@@ -966,7 +993,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfCP} given {@literal substring}.
 			 *
 			 * @param substring must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfCP}.
 			 */
 			public IndexOfCP indexOf(String substring) {
 				return new IndexOfCP(Arrays.asList(stringExpression, substring));
@@ -976,7 +1003,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfCP} given {@link AggregationExpression} that resolves to the substring.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfCP}.
 			 */
 			public IndexOfCP indexOf(AggregationExpression expression) {
 				return new IndexOfCP(Arrays.asList(stringExpression, expression));
@@ -1014,7 +1041,7 @@ public class StringOperators {
 		 * Start creating a new {@link Split}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public static Split valueOf(String fieldReference) {
 
@@ -1026,7 +1053,7 @@ public class StringOperators {
 		 * Start creating a new {@link Split}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public static Split valueOf(AggregationExpression expression) {
 
@@ -1038,7 +1065,7 @@ public class StringOperators {
 		 * Use given {@link String} as delimiter.
 		 *
 		 * @param delimiter must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(String delimiter) {
 
@@ -1050,7 +1077,7 @@ public class StringOperators {
 		 * Use value of referenced field as delimiter.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(Field fieldReference) {
 
@@ -1062,7 +1089,7 @@ public class StringOperators {
 		 * Use value resulting from {@link AggregationExpression} as delimiter.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(AggregationExpression expression) {
 
@@ -1091,7 +1118,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenBytes}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenBytes}.
 		 */
 		public static StrLenBytes stringLengthOf(String fieldReference) {
 			return new StrLenBytes(Fields.field(fieldReference));
@@ -1101,7 +1128,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenBytes}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenBytes}.
 		 */
 		public static StrLenBytes stringLengthOf(AggregationExpression expression) {
 
@@ -1130,7 +1157,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenCP}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenCP}.
 		 */
 		public static StrLenCP stringLengthOfCP(String fieldReference) {
 			return new StrLenCP(Fields.field(fieldReference));
@@ -1140,7 +1167,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenCP}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenCP}.
 		 */
 		public static StrLenCP stringLengthOfCP(AggregationExpression expression) {
 
@@ -1169,7 +1196,7 @@ public class StringOperators {
 		 * Creates new {@link SubstrCP}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public static SubstrCP valueOf(String fieldReference) {
 
@@ -1181,7 +1208,7 @@ public class StringOperators {
 		 * Creates new {@link SubstrCP}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public static SubstrCP valueOf(AggregationExpression expression) {
 

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

@@ -21,6 +21,7 @@ import java.util.ArrayList;
 import java.util.List;
 
 import org.bson.Document;
+
 import org.springframework.data.mapping.PersistentPropertyPath;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.DirectFieldReference;
@@ -127,7 +128,7 @@ public class TypeBasedAggregationOperationContext implements AggregationOperatio
 		return Fields.fields(fields.toArray(new String[0]));
 	}
 
-	protected FieldReference getReferenceFor(Field field) {
+	private FieldReference getReferenceFor(Field field) {
 
 		PersistentPropertyPath<MongoPersistentProperty> propertyPath = mappingContext
 				.getPersistentPropertyPath(field.getTarget(), type);

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

@@ -1,144 +0,0 @@
-/*
- * Copyright 2019-2020 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.ArrayList;
-import java.util.Arrays;
-import java.util.Collection;
-import java.util.List;
-import java.util.stream.Collectors;
-
-import org.bson.Document;
-import org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation;
-import org.springframework.util.Assert;
-import org.springframework.util.CollectionUtils;
-
-/**
- * Removes fields from documents.
- *
- * @author Christoph Strobl
- * @since 3.0
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/unset/">MongoDB Aggregation Framework:
- *      $unset</a>
- */
-public class UnsetOperation implements InheritsFieldsAggregationOperation {
-
-	private final Collection<Object> fields;
-
-	/**
-	 * Create new instance of {@link UnsetOperation}.
-	 * 
-	 * @param fields must not be {@literal null}.
-	 */
-	public UnsetOperation(Collection<Object> fields) {
-
-		Assert.notNull(fields, "Fields must not be null!");
-		Assert.noNullElements(fields, "Fields must not contain null values.");
-
-		this.fields = fields;
-	}
-
-	/**
-	 * Create new instance of {@link UnsetOperation}.
-	 *
-	 * @param fields must not be {@literal null}.
-	 * @return new instance of {@link UnsetOperation}.
-	 */
-	public static UnsetOperation unset(String... fields) {
-		return new UnsetOperation(Arrays.asList(fields));
-	}
-
-	/**
-	 * Also unset the given fields.
-	 *
-	 * @param fields must not be {@literal null}.
-	 * @return new instance of {@link UnsetOperation}.
-	 */
-	public UnsetOperation and(String... fields) {
-
-		List<Object> target = new ArrayList<>(this.fields);
-		CollectionUtils.mergeArrayIntoCollection(fields, target);
-		return new UnsetOperation(target);
-	}
-
-	/**
-	 * Also unset the given fields.
-	 *
-	 * @param fields must not be {@literal null}.
-	 * @return new instance of {@link UnsetOperation}.
-	 */
-	public UnsetOperation and(Field... fields) {
-
-		List<Object> target = new ArrayList<>(this.fields);
-		CollectionUtils.mergeArrayIntoCollection(fields, target);
-		return new UnsetOperation(target);
-	}
-
-	/*
-	 * (non-Javadoc)
-	 *  @see org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation#getFields()
-	 */
-	@Override
-	public ExposedFields getFields() {
-		return ExposedFields.from();
-	}
-
-	Collection<String> removedFieldNames() {
-
-		List<String> fieldNames = new ArrayList<>(fields.size());
-		for (Object it : fields) {
-			if (it instanceof Field) {
-				fieldNames.add(((Field) it).getName());
-			} else {
-				fieldNames.add(it.toString());
-			}
-		}
-		return fieldNames;
-	}
-
-	/*
-	 * (non-Javadoc)
-	 *  @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
-	 */
-	@Override
-	public Document toDocument(AggregationOperationContext context) {
-
-		if (fields.size() == 1) {
-			return new Document("$unset", computeFieldName(fields.iterator().next(), context));
-		}
-
-		return new Document("$unset",
-				fields.stream().map(it -> computeFieldName(it, context)).collect(Collectors.toList()));
-	}
-
-	private Object computeFieldName(Object field, AggregationOperationContext context) {
-
-		if (field instanceof Field) {
-			return context.getReference((Field) field).getRaw();
-		}
-
-		if (field instanceof AggregationExpression) {
-			return ((AggregationExpression) field).toDocument(context);
-		}
-
-		if (field instanceof String) {
-			return context.getReference((String) field).getRaw();
-		}
-
-		return field;
-	}
-
-}

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

@@ -72,7 +72,7 @@ public abstract class AbstractMongoConverter implements MongoConverter, Initiali
 	/**
 	 * Registers {@link EntityInstantiators} to customize entity instantiation.
 	 *
-	 * @param instantiators
+	 * @param instantiators can be {@literal null}. Uses default {@link EntityInstantiators} if so.
 	 */
 	public void setInstantiators(@Nullable EntityInstantiators instantiators) {
 		this.instantiators = instantiators == null ? new EntityInstantiators() : instantiators;

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

@@ -46,7 +46,7 @@ public class CustomConversions extends MongoCustomConversions {
 	/**
 	 * Creates a new {@link CustomConversions} instance registering the given converters.
 	 *
-	 * @param converters
+	 * @param converters must not be {@literal null}.
 	 */
 	public CustomConversions(List<?> converters) {
 		super(converters);

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

@@ -45,7 +45,7 @@ public interface DbRefResolver {
 	 * @param property will never be {@literal null}.
 	 * @param dbref the {@link DBRef} to resolve.
 	 * @param callback will never be {@literal null}.
-	 * @return
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	Object resolveDbRef(MongoPersistentProperty property, @Nullable DBRef dbref, DbRefResolverCallback callback,
@@ -58,7 +58,7 @@ public interface DbRefResolver {
 	 * @param annotation will never be {@literal null}.
 	 * @param entity will never be {@literal null}.
 	 * @param id will never be {@literal null}.
-	 * @return
+	 * @return new instance of {@link DBRef}.
 	 */
 	default DBRef createDbRef(@Nullable org.springframework.data.mongodb.core.mapping.DBRef annotation,
 			MongoPersistentEntity<?> entity, Object id) {
@@ -74,7 +74,7 @@ public interface DbRefResolver {
 	 * Actually loads the {@link DBRef} from the datasource.
 	 *
 	 * @param dbRef must not be {@literal null}.
-	 * @return
+	 * @return can be {@literal null}.
 	 * @since 1.7
 	 */
 	@Nullable

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

@@ -34,6 +34,7 @@ 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;
@@ -44,6 +45,7 @@ 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.MongoDatabaseUtils;
 import org.springframework.data.mongodb.MongoDbFactory;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.lang.Nullable;
@@ -114,14 +116,16 @@ 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() : mongoDbFactory.getDb().getName(),
+					StringUtils.hasText(dbRef.getDatabaseName()) ? dbRef.getDatabaseName()
+							: mongoCollection.getNamespace().getDatabaseName(),
 					dbRef.getCollectionName());
 		}
 
-		StringUtils.hasText(dbRef.getDatabaseName());
-		return getCollection(dbRef).find(Filters.eq("_id", dbRef.getId())).first();
+		return mongoCollection.find(Filters.eq("_id", dbRef.getId())).first();
 	}
 
 	/*
@@ -152,15 +156,16 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		}
 
 		DBRef databaseSource = refs.iterator().next();
+		MongoCollection<Document> mongoCollection = getCollection(databaseSource);
 
 		if (LOGGER.isTraceEnabled()) {
 			LOGGER.trace("Bulk fetching DBRefs {} from {}.{}.", ids,
 					StringUtils.hasText(databaseSource.getDatabaseName()) ? databaseSource.getDatabaseName()
-							: mongoDbFactory.getDb().getName(),
+							: mongoCollection.getNamespace().getDatabaseName(),
 					databaseSource.getCollectionName());
 		}
 
-		List<Document> result = getCollection(databaseSource) //
+		List<Document> result = mongoCollection //
 				.find(new Document("_id", new Document("$in", ids))) //
 				.into(new ArrayList<>());
 
@@ -202,7 +207,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		proxyFactory.addInterface(propertyType);
 		proxyFactory.addAdvice(interceptor);
 
-		return handler.populateId(property, dbref, proxyFactory.getProxy());
+		return handler.populateId(property, dbref, proxyFactory.getProxy(LazyLoadingProxy.class.getClassLoader()));
 	}
 
 	/**
@@ -497,7 +502,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	 */
 	protected MongoCollection<Document> getCollection(DBRef dbref) {
 
-		return (StringUtils.hasText(dbref.getDatabaseName()) ? mongoDbFactory.getDb(dbref.getDatabaseName())
-				: mongoDbFactory.getDb()).getCollection(dbref.getCollectionName(), Document.class);
+		return MongoDatabaseUtils.getDatabase(dbref.getDatabaseName(), mongoDbFactory)
+				.getCollection(dbref.getCollectionName(), Document.class);
 	}
 }

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

@@ -23,7 +23,6 @@ import java.util.function.UnaryOperator;
 
 import org.bson.Document;
 import org.bson.conversions.Bson;
-
 import org.springframework.data.convert.CustomConversions;
 import org.springframework.data.convert.DefaultTypeMapper;
 import org.springframework.data.convert.SimpleTypeInformationMapper;
@@ -42,9 +41,7 @@ import com.mongodb.DBObject;
 
 /**
  * Default implementation of {@link MongoTypeMapper} allowing configuration of the key to lookup and store type
- * information in {@link Document}. The key defaults to {@link #DEFAULT_TYPE_KEY}. Actual type-to-{@link String}
- * conversion and back is done in {@link #getTypeString(TypeInformation)} or {@link #getTypeInformation(String)}
- * respectively.
+ * information in {@link Document}. The key defaults to {@link #DEFAULT_TYPE_KEY}.
  *
  * @author Oliver Gierke
  * @author Thomas Darimont
@@ -111,7 +108,7 @@ public class DefaultMongoTypeMapper extends DefaultTypeMapper<Bson> implements M
 	 * {@link TypeInformationMapper} to map type hints.
 	 *
 	 * @param typeKey name of the field to read and write type hints. Can be {@literal null} to disable type hints.
-	 * @param mappers
+	 * @param mappers must not be {@literal null}.
 	 */
 	public DefaultMongoTypeMapper(@Nullable String typeKey, List<? extends TypeInformationMapper> mappers) {
 		this(typeKey, new DocumentTypeAliasAccessor(typeKey), null, mappers);

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

@@ -70,10 +70,10 @@ class DocumentAccessor {
 	/**
 	 * Puts the given value into the backing {@link Document} based on the coordinates defined through the given
 	 * {@link MongoPersistentProperty}. By default this will be the plain field name. But field names might also consist
-	 * of path traversals so we might need to create intermediate {@link BasicDocument}s.
+	 * of path traversals so we might need to create intermediate {@link Document}s.
 	 *
 	 * @param prop must not be {@literal null}.
-	 * @param value
+	 * @param value can be {@literal null}.
 	 */
 	public void put(MongoPersistentProperty prop, @Nullable Object value) {
 
@@ -106,7 +106,7 @@ class DocumentAccessor {
 	 * a path expression in the field name metadata.
 	 *
 	 * @param property must not be {@literal null}.
-	 * @return
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	public Object get(MongoPersistentProperty property) {
@@ -150,7 +150,7 @@ class DocumentAccessor {
 	 * {@link MongoPersistentProperty}.
 	 *
 	 * @param property must not be {@literal null}.
-	 * @return
+	 * @return {@literal true} if no non {@literal null} value present.
 	 */
 	@SuppressWarnings("unchecked")
 	public boolean hasValue(MongoPersistentProperty property) {
@@ -203,7 +203,7 @@ class DocumentAccessor {
 	 * Returns the given source object as map, i.e. {@link Document}s and maps as is or {@literal null} otherwise.
 	 *
 	 * @param source can be {@literal null}.
-	 * @return
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	@SuppressWarnings("unchecked")

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

@@ -91,7 +91,7 @@ abstract class GeoConverters {
 	/**
 	 * Returns the geo converters to be registered.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	@SuppressWarnings("unchecked")
 	public static Collection<? extends Object> getConvertersToRegister() {
@@ -817,8 +817,8 @@ abstract class GeoConverters {
 	/**
 	 * Converts a coordinate pairs nested in in {@link BasicDBList} into {@link GeoJsonPoint}s.
 	 *
-	 * @param listOfCoordinatePairs
-	 * @return
+	 * @param listOfCoordinatePairs must not be {@literal null}.
+	 * @return never {@literal null}.
 	 * @since 1.7
 	 */
 	@SuppressWarnings("unchecked")
@@ -841,8 +841,8 @@ abstract class GeoConverters {
 	/**
 	 * Converts a coordinate pairs nested in in {@link BasicDBList} into {@link GeoJsonPolygon}.
 	 *
-	 * @param dbList
-	 * @return
+	 * @param dbList must not be {@literal null}.
+	 * @return never {@literal null}.
 	 * @since 1.7
 	 */
 	static GeoJsonPolygon toGeoJsonPolygon(List dbList) {

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

@@ -90,8 +90,9 @@ import com.mongodb.DBRef;
  * @author Christoph Strobl
  * @author Jordi Llach
  * @author Mark Paluch
+ * @author Heesu Jung
  */
-public class MappingMongoConverter extends AbstractMongoConverter implements ApplicationContextAware, ValueResolver {
+public class MappingMongoConverter extends AbstractMongoConverter implements ApplicationContextAware {
 
 	private static final String INCOMPATIBLE_TYPES = "Cannot convert %1$s of type %2$s into an instance of %3$s! Implement a custom Converter<%2$s, %3$s> and register it with the CustomConversions. Parent object was: %4$s";
 	private static final String INVALID_TYPE_TO_READ = "Expected to read Document %s into type %s but didn't find a PersistentEntity for the latter!";
@@ -131,7 +132,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		this.idMapper = new QueryMapper(this);
 
 		this.spELContext = new SpELContext(DocumentPropertyAccessor.INSTANCE);
-		this.dbRefProxyHandler = new DefaultDbRefProxyHandler(spELContext, mappingContext, MappingMongoConverter.this);
+		this.dbRefProxyHandler = new DefaultDbRefProxyHandler(spELContext, mappingContext,
+				MappingMongoConverter.this::getValueInternal);
 	}
 
 	/**
@@ -226,11 +228,9 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 	@Nullable
 	@SuppressWarnings("unchecked")
-	private <S extends Object> S read(TypeInformation<S> type, @Nullable Bson bson, ObjectPath path) {
+	private <S extends Object> S read(TypeInformation<S> type, Bson bson, ObjectPath path) {
 
-		if (null == bson) {
-			return null;
-		}
+		Assert.notNull(bson, "Bson must not be null!");
 
 		TypeInformation<? extends S> typeToUse = typeMapper.readType(bson, type);
 		Class<? extends S> rawType = typeToUse.getType();
@@ -431,7 +431,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			SpELExpressionEvaluator evaluator) {
 
 		return new DefaultDbRefResolverCallback(documentAccessor.getDocument(), currentPath, evaluator,
-				MappingMongoConverter.this);
+				MappingMongoConverter.this::getValueInternal);
 	}
 
 	private void readAssociation(Association<MongoPersistentProperty> association, PersistentPropertyAccessor<?> accessor,
@@ -473,7 +473,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	 * Root entry method into write conversion. Adds a type discriminator to the {@link Document}. Shouldn't be called for
 	 * nested conversions.
 	 *
-	 * @see org.springframework.data.mongodb.core.convert.MongoWriter#write(java.lang.Object, com.mongodb.Document)
+	 * @see org.springframework.data.mongodb.core.convert.MongoWriter#write(java.lang.Object, java.lang.Object)
 	 */
 	public void write(Object obj, Bson bson) {
 
@@ -1047,7 +1047,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.convert.ValueResolver#getValueInternal(org.springframework.data.mongodb.core.mapping.MongoPersistentProperty, com.mongodb.Document, org.springframework.data.mapping.model.SpELExpressionEvaluator, java.lang.Object)
 	 */
-	@Override
+	@Nullable
 	public Object getValueInternal(MongoPersistentProperty prop, Bson bson, SpELExpressionEvaluator evaluator,
 			ObjectPath path) {
 		return new MongoDbPropertyValueProvider(bson, evaluator, path).getPropertyValue(prop);
@@ -1261,9 +1261,16 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		if (conversions.isSimpleType(obj.getClass())) {
-			// Doesn't need conversion
-			return getPotentiallyConvertedSimpleWrite(obj,
-					typeInformation != null ? typeInformation.getType() : Object.class);
+
+			Class<?> conversionTargetType;
+
+			if (typeInformation != null && conversions.isSimpleType(typeInformation.getType())) {
+				conversionTargetType = typeInformation.getType();
+			} else {
+				conversionTargetType = Object.class;
+			}
+
+			return getPotentiallyConvertedSimpleWrite(obj, conversionTargetType);
 		}
 
 		if (obj instanceof List) {
@@ -1491,7 +1498,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				}
 
 				DbRefResolverCallback callback = new DefaultDbRefResolverCallback(accessor.getDocument(), path, evaluator,
-						MappingMongoConverter.this);
+						MappingMongoConverter.this::getValueInternal);
 
 				DBRef dbref = rawRefValue instanceof DBRef ? (DBRef) rawRefValue : null;
 				return (T) dbRefResolver.resolveDbRef(property, dbref, callback, dbRefProxyHandler);
@@ -1573,7 +1580,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 	@Nullable
 	private <T> T readAndConvertDBRef(@Nullable DBRef dbref, TypeInformation<?> type, ObjectPath path,
-			final Class<?> rawType) {
+			@Nullable Class<?> rawType) {
 
 		List<T> result = bulkReadAndConvertDBRefs(Collections.singletonList(dbref), type, path, rawType);
 		return CollectionUtils.isEmpty(result) ? null : result.iterator().next();
@@ -1596,7 +1603,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 	@SuppressWarnings("unchecked")
 	private <T> List<T> bulkReadAndConvertDBRefs(List<DBRef> dbrefs, TypeInformation<?> type, ObjectPath path,
-			final Class<?> rawType) {
+			@Nullable Class<?> rawType) {
 
 		if (CollectionUtils.isEmpty(dbrefs)) {
 			return Collections.emptyList();
@@ -1611,16 +1618,19 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 		for (Document document : referencedRawDocuments) {
 
+			T target = null;
 			if (document != null) {
-				maybeEmitEvent(new AfterLoadEvent<>(document, (Class<T>) rawType, collectionName));
-			}
 
-			final T target = (T) read(type, document, path);
-			targeList.add(target);
+				maybeEmitEvent(
+						new AfterLoadEvent<>(document, (Class<T>) (rawType != null ? rawType : Object.class), collectionName));
+				target = (T) read(type, document, path);
+			}
 
 			if (target != null) {
 				maybeEmitEvent(new AfterConvertEvent<>(document, target, collectionName));
 			}
+
+			targeList.add(target);
 		}
 
 		return targeList;

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

@@ -119,16 +119,16 @@ public interface MongoConverter
 	/**
 	 * Converts the given raw id value into either {@link ObjectId} or {@link String}.
 	 *
-	 * @param id
-	 * @param targetType
+	 * @param id can be {@literal null}.
+	 * @param targetType must not be {@literal null}.
 	 * @return {@literal null} if source {@literal id} is already {@literal null}.
 	 * @since 2.2
 	 */
 	@Nullable
 	default Object convertId(@Nullable Object id, Class<?> targetType) {
 
-		if (id == null) {
-			return null;
+		if (id == null || ClassUtils.isAssignableValue(targetType, id)) {
+			return id;
 		}
 
 		if (ClassUtils.isAssignable(ObjectId.class, targetType)) {

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

@@ -110,7 +110,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public String convert(ObjectId id) {
-			return id.toString();
+			return id == null ? null : id.toString();
 		}
 	}
 
@@ -136,7 +136,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public BigInteger convert(ObjectId source) {
-			return new BigInteger(source.toString(), 16);
+			return source == null ? null : new BigInteger(source.toString(), 16);
 		}
 	}
 
@@ -149,7 +149,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public ObjectId convert(BigInteger source) {
-			return new ObjectId(source.toString(16));
+			return source == null ? null : new ObjectId(source.toString(16));
 		}
 	}
 
@@ -157,7 +157,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public String convert(BigDecimal source) {
-			return source.toString();
+			return source == null ? null : source.toString();
 		}
 	}
 
@@ -168,7 +168,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public Decimal128 convert(BigDecimal source) {
-			return new Decimal128(source);
+			return source == null ? null : new Decimal128(source);
 		}
 	}
 
@@ -195,7 +195,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public String convert(BigInteger source) {
-			return source.toString();
+			return source == null ? null : source.toString();
 		}
 	}
 
@@ -211,7 +211,7 @@ abstract class MongoConverters {
 		INSTANCE;
 
 		public String convert(URL source) {
-			return source.toString();
+			return source == null ? null : source.toString();
 		}
 	}
 
@@ -224,7 +224,7 @@ abstract class MongoConverters {
 		public URL convert(String source) {
 
 			try {
-				return new URL(source);
+				return source == null ? null : new URL(source);
 			} catch (MalformedURLException e) {
 				throw new ConversionFailedException(SOURCE, TARGET, source, e);
 			}
@@ -238,6 +238,11 @@ abstract class MongoConverters {
 
 		@Override
 		public String convert(Document source) {
+
+			if (source == null) {
+				return null;
+			}
+
 			return source.toJson();
 		}
 	}
@@ -253,7 +258,7 @@ abstract class MongoConverters {
 
 		@Override
 		public String convert(Term source) {
-			return source.getFormatted();
+			return source == null ? null : source.getFormatted();
 		}
 	}
 
@@ -268,7 +273,7 @@ abstract class MongoConverters {
 		@Override
 		public NamedMongoScript convert(Document source) {
 
-			if(source.isEmpty()) {
+			if (source == null) {
 				return null;
 			}
 
@@ -290,6 +295,10 @@ abstract class MongoConverters {
 		@Override
 		public Document convert(NamedMongoScript source) {
 
+			if (source == null) {
+				return new Document();
+			}
+
 			Document document = new Document();
 
 			document.put("_id", source.getName());
@@ -316,7 +325,7 @@ abstract class MongoConverters {
 		 */
 		@Override
 		public String convert(Currency source) {
-			return source.getCurrencyCode();
+			return source == null ? null : source.getCurrencyCode();
 		}
 	}
 
@@ -452,7 +461,7 @@ abstract class MongoConverters {
 
 		@Override
 		public AtomicLong convert(Long source) {
-			return new AtomicLong(source);
+			return source != null ? new AtomicLong(source) : null;
 		}
 	}
 
@@ -468,7 +477,7 @@ abstract class MongoConverters {
 
 		@Override
 		public AtomicInteger convert(Integer source) {
-			return new AtomicInteger(source);
+			return source != null ? new AtomicInteger(source) : null;
 		}
 	}
 
@@ -483,6 +492,7 @@ abstract class MongoConverters {
 
 		INSTANCE;
 
+		@Nullable
 		@Override
 		public byte[] convert(Binary source) {
 			return source.getData();
@@ -500,6 +510,7 @@ abstract class MongoConverters {
 
 		INSTANCE;
 
+		@Nullable
 		@Override
 		public Instant convert(BsonTimestamp source) {
 			return Instant.ofEpochSecond(source.getTime(), 0);

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

@@ -24,7 +24,6 @@ import org.bson.BsonValue;
 import org.bson.Document;
 import org.bson.conversions.Bson;
 import org.bson.types.ObjectId;
-
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.data.domain.Example;
@@ -175,7 +174,7 @@ public class QueryMapper {
 		}
 
 		Document mappedSort = new Document();
-		for(Map.Entry<String,Object> entry : BsonUtils.asMap(sortObject).entrySet()) {
+		for (Map.Entry<String, Object> entry : BsonUtils.asMap(sortObject).entrySet()) {
 
 			Field field = createPropertyField(entity, entry.getKey(), mappingContext);
 			mappedSort.put(field.getMappedKey(), entry.getValue());
@@ -420,7 +419,7 @@ public class QueryMapper {
 			return false;
 		}
 
-		Class<? extends Object> type = value.getClass();
+		Class<?> type = value.getClass();
 		MongoPersistentProperty property = documentField.getProperty();
 
 		if (property.getActualType().isAssignableFrom(type)) {
@@ -444,7 +443,7 @@ public class QueryMapper {
 	protected Object convertSimpleOrDocument(Object source, @Nullable MongoPersistentEntity<?> entity) {
 
 		if (source instanceof Example) {
-			return exampleMapper.getMappedExample((Example) source, entity);
+			return exampleMapper.getMappedExample((Example<?>) source, entity);
 		}
 
 		if (source instanceof List) {
@@ -817,7 +816,7 @@ public class QueryMapper {
 		protected final String name;
 
 		/**
-		 * Creates a new {@link DocumentField} without meta-information but the given name.
+		 * Creates a new {@link Field} without meta-information but the given name.
 		 *
 		 * @param name must not be {@literal null} or empty.
 		 */
@@ -828,7 +827,7 @@ public class QueryMapper {
 		}
 
 		/**
-		 * Returns a new {@link DocumentField} with the given name.
+		 * Returns a new {@link Field} with the given name.
 		 *
 		 * @param name must not be {@literal null} or empty.
 		 * @return
@@ -923,6 +922,8 @@ 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 String INVALID_ASSOCIATION_REFERENCE = "Invalid path reference %s! Associations can only be pointed to directly or via their id property!";
 
 		private final MongoPersistentEntity<?> entity;
@@ -964,7 +965,7 @@ public class QueryMapper {
 			this.entity = entity;
 			this.mappingContext = context;
 
-			this.path = getPath(name);
+			this.path = getPath(removePlaceholders(POSITIONAL_PARAMETER_PATTERN, name), property);
 			this.property = path == null ? property : path.getLeafProperty();
 			this.association = findAssociation();
 		}
@@ -1072,16 +1073,22 @@ public class QueryMapper {
 		}
 
 		/**
-		 * Returns the {@link PersistentPropertyPath} for the given <code>pathExpression</code>.
+		 * Returns the {@link PersistentPropertyPath} for the given {@code pathExpression}.
 		 *
 		 * @param pathExpression
 		 * @return
 		 */
 		@Nullable
-		private PersistentPropertyPath<MongoPersistentProperty> getPath(String pathExpression) {
+		private PersistentPropertyPath<MongoPersistentProperty> getPath(String pathExpression,
+				@Nullable MongoPersistentProperty sourceProperty) {
 
-			String rawPath = pathExpression.replaceAll("\\.\\d+", "") //
-					.replaceAll(POSITIONAL_OPERATOR.pattern(), "");
+			String rawPath = removePlaceholders(POSITIONAL_OPERATOR,
+					removePlaceholders(DOT_POSITIONAL_PATTERN, pathExpression));
+
+			if (sourceProperty != null && sourceProperty.getOwner().equals(entity)) {
+				return mappingContext
+						.getPersistentPropertyPath(PropertyPath.from(sourceProperty.getName(), entity.getTypeInformation()));
+			}
 
 			PropertyPath path = forName(rawPath);
 			if (path == null || isPathToJavaLangClassProperty(path)) {
@@ -1158,7 +1165,7 @@ public class QueryMapper {
 		 * @return
 		 */
 		protected Converter<MongoPersistentProperty, String> getPropertyConverter() {
-			return new PositionParameterRetainingPropertyKeyConverter(name);
+			return new PositionParameterRetainingPropertyKeyConverter(name, mappingContext);
 		}
 
 		/**
@@ -1169,7 +1176,15 @@ public class QueryMapper {
 		 * @since 1.7
 		 */
 		protected Converter<MongoPersistentProperty, String> getAssociationConverter() {
-			return new AssociationConverter(getAssociation());
+			return new AssociationConverter(name, getAssociation());
+		}
+
+		protected MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> getMappingContext() {
+			return mappingContext;
+		}
+
+		private static String removePlaceholders(Pattern pattern, String raw) {
+			return pattern.matcher(raw).replaceAll("");
 		}
 
 		/**
@@ -1180,8 +1195,9 @@ public class QueryMapper {
 
 			private final KeyMapper keyMapper;
 
-			public PositionParameterRetainingPropertyKeyConverter(String rawKey) {
-				this.keyMapper = new KeyMapper(rawKey);
+			public PositionParameterRetainingPropertyKeyConverter(String rawKey,
+					MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> ctx) {
+				this.keyMapper = new KeyMapper(rawKey, ctx);
 			}
 
 			/*
@@ -1223,7 +1239,8 @@ public class QueryMapper {
 
 			private final Iterator<String> iterator;
 
-			public KeyMapper(String key) {
+			public KeyMapper(String key,
+					MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {
 
 				this.iterator = Arrays.asList(key.split("\\.")).iterator();
 				this.iterator.next();
@@ -1243,6 +1260,7 @@ public class QueryMapper {
 				while (inspect) {
 
 					String partial = iterator.next();
+
 					boolean isPositional = (isPositionalParameter(partial) && (property.isMap() || property.isCollectionLike()));
 
 					if (isPositional) {
@@ -1255,7 +1273,7 @@ public class QueryMapper {
 				return mappedName.toString();
 			}
 
-			private static boolean isPositionalParameter(String partial) {
+			static boolean isPositionalParameter(String partial) {
 
 				if ("$".equals(partial)) {
 					return true;
@@ -1283,6 +1301,7 @@ public class QueryMapper {
 	 */
 	protected static class AssociationConverter implements Converter<MongoPersistentProperty, String> {
 
+		private final String name;
 		private final MongoPersistentProperty property;
 		private boolean associationFound;
 
@@ -1291,10 +1310,11 @@ public class QueryMapper {
 		 *
 		 * @param association must not be {@literal null}.
 		 */
-		public AssociationConverter(Association<MongoPersistentProperty> association) {
+		public AssociationConverter(String name, Association<MongoPersistentProperty> association) {
 
 			Assert.notNull(association, "Association must not be null!");
 			this.property = association.getInverse();
+			this.name = name;
 		}
 
 		/*
@@ -1312,6 +1332,12 @@ public class QueryMapper {
 				associationFound = true;
 			}
 
+			if (associationFound) {
+				if (name.endsWith("$") && property.isCollectionLike()) {
+					return source.getFieldName() + ".$";
+				}
+			}
+
 			return source.getFieldName();
 		}
 	}

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

@@ -272,6 +272,7 @@ public class UpdateMapper extends QueryMapper {
 	 *
 	 * @author Thomas Darimont
 	 * @author Oliver Gierke
+	 * @author Christoph Strobl
 	 */
 	private static class MetadataBackedUpdateField extends MetadataBackedField {
 
@@ -289,7 +290,7 @@ public class UpdateMapper extends QueryMapper {
 		public MetadataBackedUpdateField(MongoPersistentEntity<?> entity, String key,
 				MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {
 
-			super(key.replaceAll("\\.\\$(\\[.*\\])?", ""), entity, mappingContext);
+			super(key, entity, mappingContext);
 			this.key = key;
 		}
 
@@ -308,7 +309,7 @@ public class UpdateMapper extends QueryMapper {
 		 */
 		@Override
 		protected Converter<MongoPersistentProperty, String> getPropertyConverter() {
-			return new PositionParameterRetainingPropertyKeyConverter(key);
+			return new PositionParameterRetainingPropertyKeyConverter(key, getMappingContext());
 		}
 
 		/*
@@ -317,7 +318,7 @@ public class UpdateMapper extends QueryMapper {
 		 */
 		@Override
 		protected Converter<MongoPersistentProperty, String> getAssociationConverter() {
-			return new UpdateAssociationConverter(getAssociation(), key);
+			return new UpdateAssociationConverter(getMappingContext(), getAssociation(), key);
 		}
 
 		/**
@@ -334,10 +335,12 @@ public class UpdateMapper extends QueryMapper {
 			 *
 			 * @param association must not be {@literal null}.
 			 */
-			public UpdateAssociationConverter(Association<MongoPersistentProperty> association, String key) {
+			public UpdateAssociationConverter(
+					MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext,
+					Association<MongoPersistentProperty> association, String key) {
 
-				super(association);
-				this.mapper = new KeyMapper(key);
+				super(key, association);
+				this.mapper = new KeyMapper(key, mappingContext);
 			}
 
 			/*

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

@@ -19,12 +19,14 @@ import org.bson.Document;
 import org.bson.conversions.Bson;
 import org.springframework.data.mapping.model.SpELExpressionEvaluator;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+import org.springframework.lang.Nullable;
 
 /**
  * Internal API to trigger the resolution of properties.
  *
  * @author Oliver Gierke
  * @author Christoph Strobl
+ * @author Mark Paluch
  */
 interface ValueResolver {
 
@@ -35,9 +37,9 @@ interface ValueResolver {
 	 * @param prop
 	 * @param bson
 	 * @param evaluator
-	 * @param parent
+	 * @param path
 	 * @return
 	 */
-	Object getValueInternal(MongoPersistentProperty prop, Bson bson, SpELExpressionEvaluator evaluator,
-			ObjectPath path);
+	@Nullable
+	Object getValueInternal(MongoPersistentProperty prop, Bson bson, SpELExpressionEvaluator evaluator, ObjectPath path);
 }

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

@@ -16,7 +16,7 @@
 package org.springframework.data.mongodb.core.geo;
 
 /**
- * Interface definition for structures defined in GeoJSON ({@link https://geojson.org/}) format.
+ * Interface definition for structures defined in <a href="https://geojson.org/>GeoJSON</a> format.
  *
  * @author Christoph Strobl
  * @since 1.7

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

@@ -38,7 +38,7 @@ public class GeoJsonGeometryCollection implements GeoJson<Iterable<GeoJson<?>>>
 	/**
 	 * Creates a new {@link GeoJsonGeometryCollection} for the given {@link GeoJson} instances.
 	 *
-	 * @param geometries
+	 * @param geometries must not be {@literal null}.
 	 */
 	public GeoJsonGeometryCollection(List<GeoJson<?>> geometries) {
 

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

@@ -58,7 +58,7 @@ public class Sphere implements Shape {
 	/**
 	 * Creates a Sphere around the given center {@link Point} with the given radius.
 	 *
-	 * @param center
+	 * @param center must not be {@literal null}.
 	 * @param radius
 	 */
 	public Sphere(Point center, double radius) {
@@ -68,7 +68,7 @@ public class Sphere implements Shape {
 	/**
 	 * Creates a Sphere from the given {@link Circle}.
 	 *
-	 * @param circle
+	 * @param circle must not be {@literal null}.
 	 */
 	public Sphere(Circle circle) {
 		this(circle.getCenter(), circle.getRadius());
@@ -86,7 +86,7 @@ public class Sphere implements Shape {
 	/**
 	 * Returns the radius of the {@link Circle}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public Distance getRadius() {
 		return radius;
@@ -136,7 +136,7 @@ public class Sphere implements Shape {
 	 * Returns the {@link Shape} as a list of usually {@link Double} or {@link List}s of {@link Double}s. Wildcard bound
 	 * to allow implementations to return a more concrete element type.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public List<? extends Object> asList() {
 		return Arrays.asList(Arrays.asList(center.getX(), center.getY()), this.radius.getValue());
@@ -145,7 +145,7 @@ public class Sphere implements Shape {
 	/**
 	 * Returns the command to be used to create the {@literal $within} criterion.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public String getCommand() {
 		return COMMAND;

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

@@ -74,7 +74,7 @@ public @interface CompoundIndex {
 	 * }
 	 * </pre>
 	 *
-	 * @return
+	 * @return empty String by default.
 	 */
 	String def() default "";
 
@@ -82,13 +82,13 @@ public @interface CompoundIndex {
 	 * It does not actually make sense to use that attribute as the direction has to be defined in the {@link #def()}
 	 * attribute actually.
 	 *
-	 * @return
+	 * @return {@link IndexDirection#ASCENDING} by default.
 	 */
 	@Deprecated
 	IndexDirection direction() default IndexDirection.ASCENDING;
 
 	/**
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-unique/">https://docs.mongodb.org/manual/core/index-unique/</a>
 	 */
@@ -97,14 +97,14 @@ public @interface CompoundIndex {
 	/**
 	 * If set to true index will skip over any document that is missing the indexed field.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-sparse/">https://docs.mongodb.org/manual/core/index-sparse/</a>
 	 */
 	boolean sparse() default false;
 
 	/**
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-creation/#index-creation-duplicate-dropping">https://docs.mongodb.org/manual/core/index-creation/#index-creation-duplicate-dropping</a>
 	 * @deprecated since 2.1. No longer supported by MongoDB as of server version 3.0.
@@ -148,7 +148,7 @@ public @interface CompoundIndex {
 	 * db.hybrid.createIndex( { h1: 1, h2: 1 } , { name: "compound_index" } )
 	 * </pre>
 	 *
-	 * @return
+	 * @return empty String by default.
 	 */
 	String name() default "";
 
@@ -156,7 +156,7 @@ public @interface CompoundIndex {
 	 * If set to {@literal true} then MongoDB will ignore the given index name and instead generate a new name. Defaults
 	 * to {@literal false}.
 	 *
-	 * @return
+	 * @return {@literal false} by default
 	 * @since 1.5
 	 */
 	boolean useGeneratedName() default false;
@@ -164,7 +164,7 @@ public @interface CompoundIndex {
 	/**
 	 * If {@literal true} the index will be created in the background.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/indexes/#background-construction">https://docs.mongodb.org/manual/core/indexes/#background-construction</a>
 	 */

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

@@ -72,7 +72,7 @@ public @interface GeoSpatialIndexed {
 	 * </code>
 	 * </pre>
 	 *
-	 * @return
+	 * @return empty {@link String} by default.
 	 */
 	String name() default "";
 
@@ -80,7 +80,7 @@ public @interface GeoSpatialIndexed {
 	 * If set to {@literal true} then MongoDB will ignore the given index name and instead generate a new name. Defaults
 	 * to {@literal false}.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @since 1.5
 	 */
 	boolean useGeneratedName() default false;
@@ -88,21 +88,21 @@ public @interface GeoSpatialIndexed {
 	/**
 	 * Minimum value for indexed values.
 	 *
-	 * @return
+	 * @return {@literal -180} by default.
 	 */
 	int min() default -180;
 
 	/**
 	 * Maximum value for indexed values.
 	 *
-	 * @return
+	 * @return {@literal +180} by default.
 	 */
 	int max() default 180;
 
 	/**
 	 * Bits of precision for boundary calculations.
 	 *
-	 * @return
+	 * @return {@literal 26} by default.
 	 */
 	int bits() default 26;
 
@@ -110,7 +110,7 @@ public @interface GeoSpatialIndexed {
 	 * The type of the geospatial index. Default is {@link GeoSpatialIndexType#GEO_2D}
 	 *
 	 * @since 1.4
-	 * @return
+	 * @return {@link GeoSpatialIndexType#GEO_2D} by default.
 	 */
 	GeoSpatialIndexType type() default GeoSpatialIndexType.GEO_2D;
 
@@ -118,7 +118,7 @@ public @interface GeoSpatialIndexed {
 	 * The bucket size for {@link GeoSpatialIndexType#GEO_HAYSTACK} indexes, in coordinate units.
 	 *
 	 * @since 1.4
-	 * @return
+	 * @return {@literal 1.0} by default.
 	 */
 	double bucketSize() default 1.0;
 
@@ -126,7 +126,7 @@ public @interface GeoSpatialIndexed {
 	 * The name of the additional field to use for {@link GeoSpatialIndexType#GEO_HAYSTACK} indexes
 	 *
 	 * @since 1.4
-	 * @return
+	 * @return empty {@link String} by default.
 	 */
 	String additionalField() default "";
 }

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

@@ -59,7 +59,7 @@ public class GeospatialIndex implements IndexDefinition {
 
 	/**
 	 * @param name must not be {@literal null} or empty.
-	 * @return
+	 * @return this.
 	 */
 	public GeospatialIndex named(String name) {
 
@@ -69,7 +69,7 @@ public class GeospatialIndex implements IndexDefinition {
 
 	/**
 	 * @param min
-	 * @return
+	 * @return this.
 	 */
 	public GeospatialIndex withMin(int min) {
 		this.min = Integer.valueOf(min);
@@ -78,7 +78,7 @@ public class GeospatialIndex implements IndexDefinition {
 
 	/**
 	 * @param max
-	 * @return
+	 * @return this.
 	 */
 	public GeospatialIndex withMax(int max) {
 		this.max = Integer.valueOf(max);
@@ -87,7 +87,7 @@ public class GeospatialIndex implements IndexDefinition {
 
 	/**
 	 * @param bits
-	 * @return
+	 * @return this.
 	 */
 	public GeospatialIndex withBits(int bits) {
 		this.bits = Integer.valueOf(bits);
@@ -96,7 +96,7 @@ public class GeospatialIndex implements IndexDefinition {
 
 	/**
 	 * @param type must not be {@literal null}.
-	 * @return
+	 * @return this.
 	 */
 	public GeospatialIndex typed(GeoSpatialIndexType type) {
 
@@ -108,7 +108,7 @@ public class GeospatialIndex implements IndexDefinition {
 
 	/**
 	 * @param bucketSize
-	 * @return
+	 * @return this.
 	 */
 	public GeospatialIndex withBucketSize(double bucketSize) {
 		this.bucketSize = bucketSize;
@@ -116,8 +116,8 @@ public class GeospatialIndex implements IndexDefinition {
 	}
 
 	/**
-	 * @param fieldName.
-	 * @return
+	 * @param fieldName
+	 * @return this.
 	 */
 	public GeospatialIndex withAdditionalField(String fieldName) {
 		this.additionalField = fieldName;
@@ -128,7 +128,7 @@ public class GeospatialIndex implements IndexDefinition {
 	 * Only index the documents in a collection that meet a specified {@link IndexFilter filter expression}.
 	 *
 	 * @param filter can be {@literal null}.
-	 * @return
+	 * @return this.
 	 * @see <a href=
 	 *      "https://docs.mongodb.com/manual/core/index-partial/">https://docs.mongodb.com/manual/core/index-partial/</a>
 	 * @since 1.10
@@ -146,7 +146,7 @@ public class GeospatialIndex implements IndexDefinition {
 	 * index.
 	 *
 	 * @param collation can be {@literal null}.
-	 * @return
+	 * @return this.
 	 * @since 2.0
 	 */
 	public GeospatialIndex collation(@Nullable Collation collation) {
@@ -155,6 +155,11 @@ public class GeospatialIndex implements IndexDefinition {
 		return this;
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.index.IndexDefinition#getIndexKeys()
+	 */
+	@Override
 	public Document getIndexKeys() {
 
 		Document document = new Document();
@@ -184,6 +189,11 @@ public class GeospatialIndex implements IndexDefinition {
 		return document;
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.index.IndexDefinition#getIndexOptions()
+	 */
+	@Override
 	public Document getIndexOptions() {
 
 		Document document = new Document();

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

@@ -40,7 +40,7 @@ public class HashedIndex implements IndexDefinition {
 	 * Creates a new {@link HashedIndex} for the given field.
 	 *
 	 * @param field must not be {@literal null} nor empty.
-	 * @return
+	 * @return new instance of {@link HashedIndex}.
 	 */
 	public static HashedIndex hashed(String field) {
 		return new HashedIndex(field);

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

@@ -73,7 +73,7 @@ public class Index implements IndexDefinition {
 	/**
 	 * Reject all documents that contain a duplicate value for the indexed field.
 	 *
-	 * @return
+	 * @return this.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-unique/">https://docs.mongodb.org/manual/core/index-unique/</a>
 	 */
@@ -85,7 +85,7 @@ public class Index implements IndexDefinition {
 	/**
 	 * Skip over any document that is missing the indexed field.
 	 *
-	 * @return
+	 * @return this.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-sparse/">https://docs.mongodb.org/manual/core/index-sparse/</a>
 	 */
@@ -97,7 +97,7 @@ public class Index implements IndexDefinition {
 	/**
 	 * Build the index in background (non blocking).
 	 *
-	 * @return
+	 * @return this.
 	 * @since 1.5
 	 */
 	public Index background() {
@@ -110,7 +110,7 @@ public class Index implements IndexDefinition {
 	 * Specifies TTL in seconds.
 	 *
 	 * @param value
-	 * @return
+	 * @return this.
 	 * @since 1.5
 	 */
 	public Index expire(long value) {
@@ -135,8 +135,8 @@ public class Index implements IndexDefinition {
 	 * Specifies TTL with given {@link TimeUnit}.
 	 *
 	 * @param value
-	 * @param unit
-	 * @return
+	 * @param unit must not be {@literal null}.
+	 * @return this.
 	 * @since 1.5
 	 */
 	public Index expire(long value, TimeUnit unit) {
@@ -150,7 +150,7 @@ public class Index implements IndexDefinition {
 	 * Only index the documents in a collection that meet a specified {@link IndexFilter filter expression}.
 	 *
 	 * @param filter can be {@literal null}.
-	 * @return
+	 * @return this.
 	 * @see <a href=
 	 *      "https://docs.mongodb.com/manual/core/index-partial/">https://docs.mongodb.com/manual/core/index-partial/</a>
 	 * @since 1.10
@@ -168,7 +168,7 @@ public class Index implements IndexDefinition {
 	 * index.
 	 *
 	 * @param collation can be {@literal null}.
-	 * @return
+	 * @return this.
 	 * @since 2.0
 	 */
 	public Index collation(@Nullable Collation collation) {
@@ -192,6 +192,10 @@ public class Index implements IndexDefinition {
 		return document;
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.index.IndexDefinition#getIndexOptions()
+	 */
 	public Document getIndexOptions() {
 
 		Document document = new Document();

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

@@ -24,7 +24,17 @@ import org.bson.Document;
  */
 public interface IndexDefinition {
 
+	/**
+	 * Get the {@link Document} containing properties covered by the index.
+	 *
+	 * @return never {@literal null}.
+	 */
 	Document getIndexKeys();
 
+	/**
+	 * Get the index properties such as {@literal unique},...
+	 *
+	 * @return never {@literal null}.
+	 */
 	Document getIndexOptions();
 }

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

@@ -81,7 +81,7 @@ public final class IndexField {
 	 * Creates a geo {@link IndexField} for the given key.
 	 *
 	 * @param key must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link IndexField}.
 	 */
 	public static IndexField geo(String key) {
 		return new IndexField(key, null, Type.GEO);

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

@@ -29,7 +29,7 @@ public interface IndexFilter {
 	/**
 	 * Get the raw (unmapped) filter expression.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	Document getFilterObject();
 

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

@@ -68,8 +68,8 @@ public class IndexInfo {
 	/**
 	 * Creates new {@link IndexInfo} parsing required properties from the given {@literal sourceDocument}.
 	 *
-	 * @param sourceDocument
-	 * @return
+	 * @param sourceDocument never {@literal null}.
+	 * @return new instance of {@link IndexInfo}.
 	 * @since 1.10
 	 */
 	public static IndexInfo indexInfoOf(Document sourceDocument) {
@@ -135,7 +135,7 @@ public class IndexInfo {
 	}
 
 	/**
-	 * @param sourceDocument
+	 * @param sourceDocument never {@literal null}.
 	 * @return the {@link String} representation of the partial filter {@link Document}.
 	 * @since 2.1.11
 	 */

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

@@ -52,7 +52,7 @@ public interface IndexResolver {
 	 * Find and create {@link IndexDefinition}s for properties of given {@link TypeInformation}. {@link IndexDefinition}s
 	 * are created for properties and types with {@link Indexed}, {@link CompoundIndexes} or {@link GeoSpatialIndexed}.
 	 *
-	 * @param typeInformation
+	 * @param typeInformation must not be {@literal null}.
 	 * @return Empty {@link Iterable} in case no {@link IndexDefinition} could be resolved for type.
 	 */
 	Iterable<? extends IndexDefinition> resolveIndexFor(TypeInformation<?> typeInformation);
@@ -61,7 +61,7 @@ public interface IndexResolver {
 	 * Find and create {@link IndexDefinition}s for properties of given {@link TypeInformation}. {@link IndexDefinition}s
 	 * are created for properties and types with {@link Indexed}, {@link CompoundIndexes} or {@link GeoSpatialIndexed}.
 	 *
-	 * @param entityType
+	 * @param entityType must not be {@literal null}.
 	 * @return Empty {@link Iterable} in case no {@link IndexDefinition} could be resolved for type.
 	 * @see 2.2
 	 */

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

@@ -39,25 +39,30 @@ public @interface Indexed {
 	/**
 	 * If set to true reject all documents that contain a duplicate value for the indexed field.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-unique/">https://docs.mongodb.org/manual/core/index-unique/</a>
 	 */
 	boolean unique() default false;
 
+	/**
+	 * The index sort direction.
+	 * 
+	 * @return {@link IndexDirection#ASCENDING} by default.
+	 */
 	IndexDirection direction() default IndexDirection.ASCENDING;
 
 	/**
 	 * If set to true index will skip over any document that is missing the indexed field.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-sparse/">https://docs.mongodb.org/manual/core/index-sparse/</a>
 	 */
 	boolean sparse() default false;
 
 	/**
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/index-creation/#index-creation-duplicate-dropping">https://docs.mongodb.org/manual/core/index-creation/#index-creation-duplicate-dropping</a>
 	 * @deprecated since 2.1. No longer supported by MongoDB as of server version 3.0.
@@ -105,7 +110,7 @@ public @interface Indexed {
 	 * </code>
 	 * </pre>
 	 *
-	 * @return
+	 * @return empty String by default.
 	 */
 	String name() default "";
 
@@ -113,7 +118,7 @@ public @interface Indexed {
 	 * If set to {@literal true} then MongoDB will ignore the given index name and instead generate a new name. Defaults
 	 * to {@literal false}.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @since 1.5
 	 */
 	boolean useGeneratedName() default false;
@@ -121,7 +126,7 @@ public @interface Indexed {
 	/**
 	 * If {@literal true} the index will be created in the background.
 	 *
-	 * @return
+	 * @return {@literal false} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/core/indexes/#background-construction">https://docs.mongodb.org/manual/core/indexes/#background-construction</a>
 	 */
@@ -130,7 +135,7 @@ public @interface Indexed {
 	/**
 	 * Configures the number of seconds after which the collection should expire. Defaults to -1 for no expiry.
 	 *
-	 * @return
+	 * @return {@literal -1} by default.
 	 * @see <a href=
 	 *      "https://docs.mongodb.org/manual/tutorial/expire-data/">https://docs.mongodb.org/manual/tutorial/expire-data/</a>
 	 */

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

@@ -77,7 +77,7 @@ public class MongoPersistentEntityIndexCreator implements ApplicationListener<Ma
 	 * {@link MongoDbFactory}.
 	 *
 	 * @param mappingContext must not be {@literal null}.
-	 * @param mongoDbFactory must not be {@literal null}.
+	 * @param indexOperationsProvider must not be {@literal null}.
 	 * @param indexResolver must not be {@literal null}.
 	 */
 	public MongoPersistentEntityIndexCreator(MongoMappingContext mappingContext,