feat(add): 즉시로딩과 지연로딩

README.md

@@ -117,3 +117,11 @@ ERROR: NULL not allowed for column "ID"; SQL statement:
   - 프록시 객체는 원본 엔티티를 상속받음, 타입 체크시 주의필요 (`instance of`로 확인, `==` 으로 비교하면 안됨)
   - 영속성 컨텍스트에 엔티티가 이미 있으면 `em.getReference()`를 호출해도 실제 엔티티 반환
   - 프록시 `==` 엔티티 항상 `true`
+
+---
+
+### 즉시로딩과 지연로딩 
+
+- 가급적 지연 로딩만 사용(특히 실무에서)
+- 즉시로딩은 `JPQL`에서 N + 1 문제를 일으킨다
+- `@ManyToOne`, `@OneToOne`은 기본이 즉시 로딩 -> `LAZY`로 설정 

src/main/java/hellojpa/JpaMain.java

@@ -4,6 +4,7 @@
 import javax.persistence.EntityManagerFactory;
 import javax.persistence.EntityTransaction;
 import javax.persistence.Persistence;
+import java.util.List;
 
 public class JpaMain {
 
@@ -16,17 +17,28 @@ public static void main(String[] args) {
 		tx.begin();
 
 		try {
+			Team team = new Team();
+			team.setName("teamA");
+			em.persist(team);
+
+			Team team2 = new Team();
+			team2.setName("teamB");
+			em.persist(team2);
+
 			Member member = new Member();
 			member.setUsername("hello");
-
+			member.addTeam(team);
 			em.persist(member);
 
+			Member member2 = new Member();
+			member2.setUsername("hello2");
+			member2.addTeam(team2);
+			em.persist(member2);
+
 			em.flush();
 			em.clear();
 
-			Member findMember = em.find(Member.class, member.getId());
-			System.out.println("findMember.getUsername() = " + findMember.getUsername());
-			System.out.println("findMember.getId() = " + findMember.getId());
+			List<Member> members = em.createQuery("select m from Member m join fetch m.team", Member.class).getResultList();
 
 			tx.commit();
 		} catch (Exception e) {