Projections

• 엔티티 대신에 DTO를 편리하게 조회할 때 사용
• 전체 엔티티가 아니라 딱 회원의 이름만 조회하고 싶은 경우!

 

 

1. closed Projection

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• 인터페이스만 생성하면 구현체는 SpringDataJpa가 만듦.

public interface UserNameOnly {

    String getUserName();
}

• 사용 방법 : 아래와 같이 return type으로 인터페이스를 지정해주면 됨.

@Test
    public void projections(){
        Team teamA = new Team("teamA");
        em.persist(teamA);

        Member m1 = new Member("m1", 0, teamA);
        Member m2 = new Member("m2", 0, teamA);
        em.persist(m1);
        em.persist(m2);

        em.flush();
        em.clear();

        //when
        List<UserNameOnly> result = memberRepository.findProjectionsByUserName("m1");
        for (UserNameOnly userNameOnly : result) {
            System.out.println("userNameOnly = " + userNameOnly);
        }
    }

• 반환 타입을 보면 스프링이 구현체를 만들어서 넣어준 것을 확인 가능.

    select
        member0_.user_name as col_0_0_ 
    from
        member member0_ 
    where
        member0_.user_name=?
        
2021-08-31 22:55:31.844  INFO 34388 --- [           main] p6spy                                    : #1630418131844 | took 1ms | statement | connection 103| url jdbc:h2:tcp://localhost/~/datajpa
select member0_.user_name as col_0_0_ from member member0_ where member0_.user_name=?
select member0_.user_name as col_0_0_ from member member0_ where member0_.user_name='m1';
userNameOnly = org.springframework.data.jpa.repository.query.AbstractJpaQuery$TupleConverter$TupleBackedMap@258ac1e6

 

 

 

2. open Projection

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• DB에서 엔티티 전체를 가져와서 애플리케이션에서 원하는 데이터만 찍을 수 있음

public interface UserNameOnly {

    @Value("#{target.userName + ' ' + target.age}") //
    String getUserName();
}

 

 

3. Class 기반 projection

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@Getter
public class UserNameOnlyDto {

    private String userName;

    public UserNameOnlyDto(String userName) {
        this.userName = userName;
    }
}

• 파라미터 명(userName)을 가지고 분석함.
• 해당 파라미터를 가져와서 생성자를 통해 DTO반환
• Repository

List<UserNameOnlyDto> findProjectionsByUserName(@Param("userName") String userName);

• Test

  List<UserNameOnlyDto> result = memberRepository.findProjectionsByUserName("m1");
        for (UserNameOnlyDto userNameOnly : result) {
            System.out.println("userNameOnly = " + userNameOnly);
        }

 

 

4. 동적 projection

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• Generic으로 넘기면 됨.
• Repository

<T> List<T> findProjectionsByUsername(String username, Class<T> type);

 

 

5. 중첩 구조

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• 멤버랑 연관된 팀 까지 가져오기

public interface NestedClosedProjections {
    
    String getUserName(); //root --> 최적화 해서 가져옴
    TeamInfo getTeam(); //최적화 안하고 다가져옴
    
    interface TeamInfo{
        String getName();
    }
}

• Repository

List<NestedClosedProjections> findProjectionsByUserName(@Param("userName") String userName);

• Test

//when
        List<NestedClosedProjections> result = memberRepository.findProjectionsByUserName("m1");
        for (NestedClosedProjections userNameOnly : result) {
            System.out.println("userNameOnly = " + userNameOnly);
        }

• root인 user는 최적화해서 UserName만 가져옴.
• team은 outer join으로 가져옴

    select
        member0_.user_name as col_0_0_,
        team1_.team_id as col_1_0_,
        team1_.team_id as team_id1_2_,
        team1_.name as name2_2_ 
    from
        member member0_ 
    left outer join
        team team1_ 
            on member0_.team_id=team1_.team_id 
    where
        member0_.user_name=?

 

 

 

6. 정리

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• 프로젝션 대상이 root 엔티티면, JPQL SELECT 절 최적화 가능
• 프로젝션 대상이 ROOT가 아니면
  
  • LEFT OUTER JOIN 처리
  • 모든 필드를 SELECT해서 엔티티로 조회한 다음에 계산
• 프로젝션 대상이 root 엔티티면 유용하다.
• 프로젝션 대상이 root 엔티티를 넘어가면 JPQL SELECT 최적화가 안된다!
• 실무의 복잡한 쿼리를 해결하기에는 한계가 있다.
• 실무에서는 단순할 때만 사용하고, 조금만 복잡해지면 QueryDSL을 사용하자

 

7. GitHub : 210831 Projection

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GitHub - bsh6463/SpringDataJpa