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

Question

I have 2 tables: genres(genre_id, genre_name) and movies (movie_id, movie_name, movie_score, genre_id). genre_id_fk from movies referenced to genre_id in genres.

@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<>();
}

and 2nd entity for movies

@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;
}

when i am trying to print it in console with this code:

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);
}

receiving Exception in thread "main" java.lang.StackOverflowError only removing from toString() in movie class field "genres" can fix it. But is it possible to avoid it? And same problem with spring boot application

@RestController
public class GenreController {

@Autowired
private GenreService genreService;

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

here service

@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;
}
}

and receiving this problem like: [{"id":1,"name":"Thriller","movies":[{"id":1,"name":"Any, are you ok?","score":5.45,"genre":{"id":1,"name":"Thriller","movies":[{"id":1,"name":"Any, are you ok?","score":5.45,"genre":{"id":1,"name":"Thriller","movies":[{"id":1,"name":"Any, are you ok?","score":5.45,"genre":.... and to infinity with sof exception. Console i can fix just with ignoring field in toString() method. But how to fix this problem in webapplication?

here hibernate debug log when console print

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

tell me where i am doing wrong and how to fix or google this problem? just not print this object by localhost:8080/genre/id? make something particular print or what?

Answer 1

It seems like you have an infinite recursion when trying toString your Genre entities. Your code first loads your Genre entity by id, and then calls Genre.toString() . Because you have @OneToMany relationship with Movie s, it lazy loads the list and then calls Movie.toString() for every movie related to the genre. Then for every movie you have a @ManyToOne relationship back with Genre. And here lies the problem. It will call again Genre.toString() for each movie in the list.

Possible solutions

1. If you only want to simply print it in console, do not include movies list in Genre.toString()
2. If you are using Jackson, add @JsonBackReference to your @ManyToOne relationship in Movie , this way Jackson will ignore it when mapping to Json Annotation documentation here
3. If using DTOs, just do not include Movie property in your DTO.

Hope this helps.