spring data jpa : Projections

spring-data-jpa/src/main/java/com/example/springdatajpa/repository/MemberRepository.java

@@ -68,4 +68,8 @@ public interface MemberRepository extends JpaRepository<Member, Long>, MemberRep
 
     @Lock(LockModeType.PESSIMISTIC_WRITE)
     Member findLockByUsername(String username);
+
+//    List<UsernameOnlyDto> findProjectionsByUsername(@Param("username") String username);
+    <T> List<T> findProjectionsByUsername(@Param("username") String username, Class<T> type);
+
 }

spring-data-jpa/src/main/java/com/example/springdatajpa/repository/NestedClosedProjections.java

@@ -0,0 +1,11 @@
+package com.example.springdatajpa.repository;
+
+public interface NestedClosedProjections {
+
+    String getUsername();
+    TeamInfo getTeam();
+
+    interface TeamInfo {
+        String getName();
+    }
+}

spring-data-jpa/src/main/java/com/example/springdatajpa/repository/UsernameOnly.java

@@ -0,0 +1,9 @@
+package com.example.springdatajpa.repository;
+
+import org.springframework.beans.factory.annotation.Value;
+
+public interface UsernameOnly {
+
+//    @Value("#{target.username + ' ' + target.age}")
+    String getUsername();
+}

spring-data-jpa/src/main/java/com/example/springdatajpa/repository/UsernameOnlyDto.java

@@ -0,0 +1,14 @@
+package com.example.springdatajpa.repository;
+
+public class UsernameOnlyDto {
+
+    private final String username;
+
+    public UsernameOnlyDto(String username) {
+        this.username = username;
+    }
+
+    public String getUsername() {
+        return username;
+    }
+}

spring-data-jpa/src/test/java/com/example/springdatajpa/repository/MemberRepositoryTest.java

@@ -358,5 +358,30 @@ public class MemberRepositoryTest {
 
         assertEquals("member1", result.get(0).getUsername());
     }
+
+    @Test
+    void projections() {
+        // given
+        Team teamA = new Team("teamA");
+        em.persist(teamA);
+
+        Member member1 = new Member("member1", 0, teamA);
+        Member member2 = new Member("member2", 0, teamA);
+        em.persist(member1);
+        em.persist(member2);
+
+        em.flush();
+        em.clear();
+
+        // when
+        List<NestedClosedProjections> nestedClosedProjections = memberRepository.findProjectionsByUsername("member1", NestedClosedProjections.class);
+        List<UsernameOnlyDto> usernameOnlyDtos = memberRepository.findProjectionsByUsername("member1", UsernameOnlyDto.class);
+        List<UsernameOnly> usernameOnlyList = memberRepository.findProjectionsByUsername("member1", UsernameOnly.class);
+
+        for (NestedClosedProjections nestedClosedProjection : nestedClosedProjections) {
+            System.out.println("nestedClosedProjections.getUsername() = " + nestedClosedProjection.getUsername());
+            System.out.println("nestedClosedProjections.getTeam().getName() = " + nestedClosedProjection.getTeam().getName());
+        }
+    }
 }