[JPA_Basic] 프로젝션 - 여러 값 조회

이 글은 김영한의 [자바 ORM 표준 JPA 프로그래밍 - 기본편]을 수강하며 정리한 글입니다.

 

👉 기본 환경

- Language: Java

- DB: H2 Database

- IDE: IntelliJ

 

 

Query: SELECT m.name, m.age FROM Member m

 

1. Query 타입 조회

 
public class Main {
    public static void main(String[] args) {
 
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
 
        EntityManager em = emf.createEntityManager();
 
        EntityTransaction tx = em.getTransaction();
        tx.begin(); // transaction 시작 선언
 
        try {
            Member member1 = new Member();
            member1.setName("Member1");
            em.persist(member1);
 
            Member member2 = new Member();
            member2.setName("Member2");
            em.persist(member2);
 
            // Scalar Type Projection
            List list = em.createQuery("select distinct m.name, m.age from Member m")
                    .getResultList();
 
            for(Object o : list){
                Object[] result = (Object[]) o;
                System.out.println("name: " + result[0]);
            }
 
            tx.commit(); // transaction 종료 후 commit
        } catch (Exception e) {
            tx.rollback(); // 문제가 생길 경우, rollback 진행
            e.printStackTrace();
        } finally {
            em.close(); // tx에 문제가 생기더라도 em 반드시 종료
        }
 
        emf.close();
 
    }
}
 

 

 

2. Object[] 타입 조회

 
public class Main {
    public static void main(String[] args) {
 
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
 
        EntityManager em = emf.createEntityManager();
 
        EntityTransaction tx = em.getTransaction();
        tx.begin(); // transaction 시작 선언
 
        try {
            Member member1 = new Member();
            member1.setName("Member1");
            em.persist(member1);
 
            Member member2 = new Member();
            member2.setName("Member2");
            em.persist(member2);
 
            // Scalar Type Projection
            List<Object[]> list = em.createQuery("select distinct m.name, m.age from Member m")
                    .getResultList();
 
            for(Object[] o : list){
                System.out.println("name: " + o[0]);
            }
 
            tx.commit(); // transaction 종료 후 commit
        } catch (Exception e) {
            tx.rollback(); // 문제가 생길 경우, rollback 진행
            e.printStackTrace();
        } finally {
            em.close(); // tx에 문제가 생기더라도 em 반드시 종료
        }
 
        emf.close();
 
    }
}
 

 

 

3. new 명령어 조회

MemberDTO

public class MemberDTO {
 
    private String name;
    private int age;
 
    public MemberDTO(String name, int age) {
        this.name = name;
        this.age = age;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
}
 

 

Main

 
public class Main {
    public static void main(String[] args) {
 
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
 
        EntityManager em = emf.createEntityManager();
 
        EntityTransaction tx = em.getTransaction();
        tx.begin(); // transaction 시작 선언
 
        try {
            Member member1 = new Member();
            member1.setName("Member1");
            em.persist(member1);
 
            Member member2 = new Member();
            member2.setName("Member2");
            em.persist(member2);
 
            // Scalar Type Projection
            List<MemberDTO> resultList = em.createQuery("select new jpql.MemberDTO(m.name, m.age) from Member m", MemberDTO.class)
                    .getResultList();
 
            MemberDTO memberDTO = resultList.get(0);
            System.out.println(memberDTO.getName());
 
            tx.commit(); // transaction 종료 후 commit
        } catch (Exception e) {
            tx.rollback(); // 문제가 생길 경우, rollback 진행
            e.printStackTrace();
        } finally {
            em.close(); // tx에 문제가 생기더라도 em 반드시 종료
        }
 
        emf.close();
 
    }
}
 

⭐ 권장

    - 단, 패키지명을 포함한 전체 클래스 명 입력이 필요(가독성 저하)

    - 순서와 타입이 일치하는 생성자 필요