JPA 공부중인데 왜 NullPointException 이 나는걸까요 ㅠㅠ??

Question

안녕하세요!JPA를 공부하고있는데아래 부분 NullPointException이 발생하여 질문 드립니다.@Entity @Getter @Builder @AllArgsConstructor @NoArgsConstructor(access = AccessLevel.PROTECTED) @ToString(of = {"id", "username", "age"}) public class Member { @Id @GeneratedValue @Column(name = "member_id") private long id; private int age; private String username; @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "team_id") private Team team; public void changeTeam(Team team) { this.team = team; Member member = this; List members = team.getMembers(); // NullPointException members.add(member); // NullPointException } } @Entity @Builder @AllArgsConstructor @NoArgsConstructor(access = AccessLevel.PROTECTED) @ToString(of = {"id", "name"}) @Getter public class Team { @Id @GeneratedValue @Column(name = "team_id") private long id; private String name; @OneToMany(mappedBy = "team") private List members = new ArrayList<>(); } @Controller public class TestController { @PersistenceContext EntityManager em; @RequestMapping("/test") @Transactional public void test(){ Team team = Team.builder().name("develop").build(); em.persist(team); Member member = Member.builder() .username("gbitkim") .team(team) .build(); member.changeTeam(team); em.persist(member); } } java.lang.NullPointerException: null at study.datajpa.entity.Member.changeTeam(Member.java:32) ~[classes/:na] at study.datajpa.TestController.test(TestController.java:29) ~[classes/:na] at study.datajpa.TestController$$FastClassBySpringCGLIB$$54b04f50.invoke() ~[classes/:na] at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.3.27.jar:5.3.27] at 이해가 안되는게.. ㅠㅠList tests = new ArrayList<>();test.add(new Test());이런식으로 데이터를 넣으니까 될 줄 알았는데... public void changeTeam(Team team) { this.team = team; Member member = this; List members = team.getMembers(); // NullPointException members.add(member); // NullPointException }왜 저기서 NullPointException 이 발생하는걸까요..???team.getMembers는 new ArrayList()로 초기화를 하였는대도 말이죠..

개못남 · Answer

TestController 에서 Team team = Team.builder().name("develop").build();이 team 이 null 인거 아닐까요?team 을 매개변수로 넘기는데 null 인 상태인데 getMembers() 호출하니까 NPE 터지구요.

okkkkkkkkk · Answer

Build패턴으로 작성된 경우에면 null로 들어가네요...

okkkkkkkkk · Answer

확인해보니 , Build패턴의 경우 초기화 설정을 따로 해줘야한다고 하네요..Build패턴 쓰니 저런식으로 설정해주면 지저분해보여 Constructor를 선언해서 사용해야겠습니다..@Builder public class Team { // ... @Builder.Default @OneToMany(mappedBy = "team") private List members = new ArrayList<>(); // ... }

JPA 공부중인데 왜 NullPointException 이 나는걸까요 ㅠㅠ??

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