SpringBoot中的@ManyToMany（無法延遲初始化角色集合：com.example.demo.Movie.actors，無法初始化代理-無會話）-有解無憂

我目前正在嘗試 Spring boot 并使用測驗專案，我遇到了 @ManyToMany-Relationships 的問題。

應該有電影，可以保存，它們可以有多種型別、演員和類似的東西。演員可以參演很多電影。

現在我可以將電影保存到資料庫中，但由于某種原因，我只能讀出簡單的資料，比如標題或制作年份。如果我嘗試將流派列印到命令列或參與的演員，我會收到以下例外：

Exception in thread "main" org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.example.demo.Movie.actors, could not initialize proxy - no Session at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:612) at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:218) at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:591) at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:149) at org.hibernate.collection.internal.PersistentBag.get(PersistentBag.java:561) at com.example.demo.MoviedbApplication.main(MoviedbApplication.java:75)

這是我的代碼：

        ConfigurableApplicationContext run = SpringApplication.run(MoviedbApplication.class, args); 
        GenreRepository genreRepository = run.getBean(GenreRepository.class);
    ActorRepository actorRepository = run.getBean(ActorRepository.class);
    AuthorRepository authorRepository = run.getBean(AuthorRepository.class);
    DirectorRepository directorRepository = run.getBean(DirectorRepository.class);
    MovieRepository movieRepository = run.getBean(MovieRepository.class);

    Movie theGodfather = new Movie();
    theGodfather.setTitle("Der Pate");
    theGodfather.setYear(1972);
    theGodfather.setOriginalTitle("The Godfather");
    theGodfather.setLengthInMinutes(175);
    theGodfather.setPlot("Der alternde Patriarch einer Verbrecherdynastie will die Herrschaft über sein geheimes Reich auf seinen widerwilligen Sohn übertragen.");
    theGodfather.setAge(16);

    List<Director> dirs = new ArrayList<>();
    dirs.add(new Director("Francis Ford Coppola"));

    List<Actor> acts = new ArrayList<>();
    acts.add(new Actor("Marlon Brando"));
    acts.add(new Actor("Al Pacino"));
    acts.add(new Actor("James Caan"));

    List<Genre> genres = new ArrayList<>();
    genres.add(new Genre("Krimi"));
    genres.add(new Genre("Drama"));

    List<Author> authors = new ArrayList<>();
    authors.add(new Author("Mario Puzo"));
    authors.add(new Author("Francis Ford Coppola"));

    theGodfather.setActors(acts);
    theGodfather.setAuthors(authors);
    theGodfather.setDirectors(dirs);
    theGodfather.setGenres(genres);
    
    
    movieRepository.save(theGodfather);

    List<Movie> der_pate = movieRepository.findByTitle("Der Pate");

    Movie movie = der_pate.get(0);


    System.out.println("Test");
    System.out.println(movie.getYear());
    System.out.println(movie.getTitle());
    System.out.println(movie.getId());
    System.out.println(movie.getActors().get(0).getName());
    System.out.println(movie.getAuthors().get(0).getName());


@Entity
@Table(name="movies")
public class Movie {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    private String title;
    private String originalTitle;
    private int year;
    private int age;
    private int lengthInMinutes;
    private String plot;
    private String linkToCover;

    @ManyToMany
    @Cascade({CascadeType.PERSIST})
    @JoinTable(
            name = "movie_genres",
            joinColumns = @JoinColumn(name = "movie_id"),
            inverseJoinColumns = @JoinColumn(name = "genre_id")
    )
    private List<Genre> genres = new ArrayList<>();

    @ManyToMany
    @Cascade({CascadeType.PERSIST})
    @JoinTable(
            name = "movie_actors",
            joinColumns = @JoinColumn(name = "movie_id"),
            inverseJoinColumns = @JoinColumn(name="actor_id")
    )
    private List<Actor> actors = new ArrayList<>();

    @ManyToMany
    @Cascade({CascadeType.PERSIST})
    @JoinTable(
            name = "movie_directors",
            joinColumns = @JoinColumn(name = "movie_id"),
            inverseJoinColumns = @JoinColumn(name = "director_id")
    )
    private List<Director> directors = new ArrayList<>();

    @ManyToMany
    @Cascade({CascadeType.PERSIST})
    @JoinTable(
            name = "movie_authors",
            joinColumns = @JoinColumn(name = "movie_id"),
            inverseJoinColumns = @JoinColumn(name = "author_id")
    )
    private List<Author> authors = new ArrayList<>();


@ManyToMany
@Cascade({CascadeType.PERSIST})
@JoinTable(
        name = "movie_genres",
        joinColumns = @JoinColumn(name = "movie_id"),
        inverseJoinColumns = @JoinColumn(name = "genre_id")
)
private List<Genre> genres = new ArrayList<>();

@ManyToMany
@Cascade({CascadeType.PERSIST})
@JoinTable(
        name = "movie_actors",
        joinColumns = @JoinColumn(name = "movie_id"),
        inverseJoinColumns = @JoinColumn(name="actor_id")
)
private List<Actor> actors = new ArrayList<>();

@ManyToMany
@Cascade({CascadeType.PERSIST})
@JoinTable(
        name = "movie_directors",
        joinColumns = @JoinColumn(name = "movie_id"),
        inverseJoinColumns = @JoinColumn(name = "director_id")
)
private List<Director> directors = new ArrayList<>();

@ManyToMany
@Cascade({CascadeType.PERSIST})
@JoinTable(
        name = "movie_authors",
        joinColumns = @JoinColumn(name = "movie_id"),
        inverseJoinColumns = @JoinColumn(name = "author_id")
)
private List<Author> authors = new ArrayList<>();

@Entity @Table(name="Actor") public class Actor {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
private String name;

@ManyToMany
@Cascade({CascadeType.PERSIST})
@JoinColumn(name = "movie_id")
private List<Movie> movies = new ArrayList<>();

public Actor(String name) {
    this.name=name;
}`

我已經嘗試過的是將演員和流派保存到相應的存盤庫中，這會導致以下例外：

detached entity passed to persist: com.example.demo.Actor; nested exception is org.hibernate.PersistentObjectException: detached entity passed to persist: com.example.demo.Actor

我還摸索著急切的獲取和東西..現在我想，我在這里錯過了一些非常重要的東西，所以我很抱歉，如果這聽起來像一個初學者的問題..

非常感謝您！

uj5u.com熱心網友回復：

它是關于獲取型別和會話管理的。如果你想訪問與你的電影相關的演員，你需要@ManyToMany(fetch = FetchType.EAGER)在這種情況下設定你的獲取型別。對于 ManyToMany 關系，默認獲取型別是 LAZY。可能，您的電影物體在從資料庫中獲取后已與會話分離。所以，即使你有movie.getActors() 行，你也會得到LazyInitializationException。有關休眠延遲加載的詳細資訊，請參閱。 https://howtodoinjava.com/hibernate/lazy-loading-in-hibernate/

uj5u.com熱心網友回復：

在您的“findByTitle”方法中，您必須加載您的 ManyToMany 關系，因為它們默認是 LAZY 獲取的。要加載集合，您可以使用 size() 方法。例如：

List<Movie> findByTitle(String title) {
   List<Movie> movies;
   //fetch movies

   //now load ManyToMany relationships
   for(var movie : movies) {
     movie.getActors().size();
     movie.getAuthors().size();
   }
   return movies;
}

我還建議在您的多對多關系中使用 Set 而不是 List

轉載請註明出處，本文鏈接：https://www.uj5u.com/ruanti/451695.html

標籤：爪哇春天弹簧靴休眠 jpa

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