JPA OneToMany，在控制台或webapp中打印时如何忽略字段ManyToMany

Question

我有两个表：流派（genre\u id，genre\u name）和电影（movie\u id，movie\u name，movie\u score，genre\u id）。电影中的genre\u id\u fk引用了genres中的genre\u id。

@Entity
@Table(name = "genres", schema = "test")
public class Genre {

@Id
@Column(name = "genre_id")
private int id;
@Column(name = "genre_name")
private String name;
@OneToMany(mappedBy = "genre", fetch = FetchType.LAZY)
private List<Movie> movies = new ArrayList<>();
}

和第二个电影实体

@Entity
@Table(name = "movies", schema = "test")
public class Movie {

@Id
@GeneratedValue
@Column(name = "movie_id")
private int id;
@Column(name = "movie_name")
private String name;
@Column(name = "movie_score")
private double score;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "genre_id")
private Genre genre;
}

当我尝试使用以下代码在控制台中打印它时：

public static void main(String[] args) {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("pu");
    EntityManager em = emf.createEntityManager();
    Genre genre = em.find(Genre.class, 1);
    System.out.println(genre);
}

在线程“main”中接收异常java.lang.StackOverflow Error只有从电影类字段“genres”中的toString（）中删除才能修复它。但是有可能避免它吗？Spring启动应用程序也有同样的问题

@RestController
public class GenreController {

@Autowired
private GenreService genreService;

@RequestMapping("/test/{id}")
public List<Genre> getGenreInfo(@PathVariable int id){
    return genreService.getGenreFilms(id);
}
}

此处服务

@Service
public class GenreService {

public List<Genre> getGenreFilms(int id){
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("pu");
    EntityManager em = emf.createEntityManager();
    List<Genre> genres = new ArrayList<>();
    Genre genre = em.find(Genre.class, id);
    genres.add(genre);
    return genres;
}
}

收到这样的问题：[{“id”：1，“name”：“Thriller”，“movies”：[{“id”：1，“name”：“Any，你还好吗？”，“score”：5.45，“流派”：{“id”：1，“name”：“Thriller”，“movies”：[{“id”：1，“name”：“Any，you ok？”，“score”：5.45，“流派”：{“id”：1，“name”：“Thriller”，“movies”：[{“id”：1，“name”：“Any，you ok？”，“得分”：5.45，“流派”：。。。。除了sof之外，还有无穷大。我可以通过在toString（）方法中忽略字段来修复控制台。但是如何在webapplication中解决这个问题呢？

控制台打印时，在此Hibernate调试日志

22:15:37.154 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - 
Resolving associations for [com.company.Genre#1]
22:15:37.164 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - Done 
materializing entity [com.company.Genre#1]
22:15:37.164 [main] DEBUG 
org.hibernate.resource.jdbc.internal.ResourceRegistryStandardImpl - 
HHH000387: ResultSet's statement was not registered
22:15:37.165 [main] DEBUG 
org.hibernate.loader.entity.plan.AbstractLoadPlanBasedEntityLoader - Done 
entity load : com.company.Genre#1
22:15:37.165 [main] DEBUG 
org.hibernate.resource.jdbc.internal.LogicalConnectionManagedImpl - 
Initiating JDBC connection release from afterTransaction
22:15:37.167 [main] DEBUG org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer 
- Loading collection: [com.company.Genre.movies#1]
22:15:37.167 [main] DEBUG org.hibernate.SQL - select movies0_.genre_id as 
genre_id4_1_0_, movies0_.movie_id as movie_id1_1_0_, movies0_.movie_id as 
movie_id1_1_1_, movies0_.genre_id as genre_id4_1_1_, movies0_.movie_name as 
movie_na2_1_1_, movies0_.movie_score as movie_sc3_1_1_ from test.movies 
movies0_ where movies0_.genre_id=?
Hibernate: select movies0_.genre_id as genre_id4_1_0_, movies0_.movie_id as 
movie_id1_1_0_, movies0_.movie_id as movie_id1_1_1_, movies0_.genre_id as 
genre_id4_1_1_, movies0_.movie_name as movie_na2_1_1_, movies0_.movie_score 
as movie_sc3_1_1_ from test.movies movies0_ where movies0_.genre_id=?
22:15:37.168 [main] DEBUG 
org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl - 
Preparing collection intializer : [com.company.Genre.movies#1]
22:15:37.170 [main] DEBUG 
org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl - 
Starting ResultSet row #0
22:15:37.171 [main] DEBUG org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerIm 
pl - Found row of collection: [com.company.Genre.movies#1]
22:15:37.171 [main] DEBUG 
org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl - 
Starting ResultSet row #1
22:15:37.172 [main] DEBUG org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerIm 
pl - Found row of collection: [com.company.Genre.movies#1]
22:15:37.172 [main] DEBUG 
org.hibernate.loader.plan.exec.process.internal.ResultSetProcessorImpl - 
Starting ResultSet row #2
22:15:37.172 [main] DEBUG org.hibernate.loader.plan.exec.process.internal.CollectionReferenceInitializerIm 
pl - Found row of collection: [com.company.Genre.movies#1]
22:15:37.172 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - 
Resolving associations for [com.company.Movie#1]
22:15:37.172 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - Done 
materializing entity [com.company.Movie#1]
22:15:37.173 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - 
Resolving associations for [com.company.Movie#2]
22:15:37.173 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - Done 
materializing entity [com.company.Movie#2]
22:15:37.173 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - 
Resolving associations for [com.company.Movie#3]
22:15:37.173 [main] DEBUG org.hibernate.engine.internal.TwoPhaseLoad - Done 
materializing entity [com.company.Movie#3]
22:15:37.173 [main] DEBUG 
org.hibernate.engine.loading.internal.CollectionLoadContext - 1 collections 
were found in result set for role: com.company.Genre.movies
22:15:37.173 [main] DEBUG 
org.hibernate.engine.loading.internal.CollectionLoadContext - Collection 
fully initialized: [com.company.Genre.movies#1]
22:15:37.173 [main] DEBUG 
org.hibernate.engine.loading.internal.CollectionLoadContext - 1 collections 
initialized for role: com.company.Genre.movies
22:15:37.173 [main] DEBUG 
org.hibernate.resource.jdbc.internal.ResourceRegistryStandardImpl - 
HHH000387: ResultSet's statement was not registered
22:15:37.173 [main] DEBUG org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer 
- Done loading collection
Exception in thread "main" java.lang.StackOverflowError

告诉我哪里做错了，如何修复或用谷歌搜索这个问题？只是不按本地主机打印此对象：8080/流派/id？做一些特别的印刷品还是什么？

Answer 1

当你尝试去串你的类型实体时，你似乎有一个无限的递归。代码首先按id加载流派实体，然后调用流派。toString（）。因为您与电影有关系，所以它会延迟加载列表，然后调用电影。toString（）适用于与该类型相关的每部电影。那么，对于每一部电影，你都与流派有着千丝万缕的联系。这就是问题所在。它将再次调用流派。列表中每部电影的toString（）。

1. 如果您只想在控制台中简单地打印它，请不要在Genre.toString（）
中包含电影列表
2. 如果您使用的是Jackson，请在Movie中将@JsonBack参考添加到您的@ManyToOne关系中，这样Jackson在映射到Json时会忽略它注释留档在这里
3. 如果使用DTO，请不要在您的DTO中包含Movie属性。

希望这有帮助。