ManyToOne FetchType.LAZY 不起作用。 null 获取列表时总是返回

Question

我正在使用 JPA 和 Spring 引导。 我有实体：

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column(nullable = false, unique = true)
    private String title;

    @Column(nullable = false)
    private String author;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "bookshelf_id", referencedColumnName = "id", nullable = true)
    private Bookshelf bookshelf;

    // getter & setter
}

@Entity
public class Bookshelf {
    @Id
    @GeneratedValue
    private long id;

    @Column(nullable = false)
    private String location;

    @OneToMany(mappedBy = "bookshelf", fetch = FetchType.LAZY)
    private List<Book> books;

    // getter & setter
}

然后在我的测试中，我尝试检索放在书架上的书：

        Bookshelf shelf = new Bookshelf("room A");
        Book book = new Book("a book", "chris");
        book.setBookshelf(shelf);;
        entityManager.persist(book);
        entityManager.persist(shelf);
        entityManager.flush();

        Bookshelf persistedShelf = bookshelfRepository.findById(shelf.getId()).get();
        Book persistedBook = bookRepository.findById(book.getId()).get();

        book = null;
        shelf = null;

        shelf = persistedBook.getBookshelf();
        int count = persistedShelf.getBooks().size();
        assertEquals(1, count);

测试失败，因为 getBooks() 总是在此行中返回 null：

        int count = persistedShelf.getBooks().size();

测试日志显示只执行了两条 SQL 命令：

Hibernate: insert into bookshelf (location, id) values (?, ?)
Hibernate: insert into book (author, bookshelf_id, title, id) values (?, ?, ?, ?)

可能是什么问题？

Answer 1

尝试设置从书架到书的反比关系：

Bookshelf shelf = new Bookshelf("room A");
Book book = new Book("a book", "chris");
book.setBookshelf(shelf);

// new code here:
List<Book> books = new ArrayList<>();
books.add(book);
shelf.setBooks(books);

entityManager.persist(book);
entityManager.persist(shelf);
entityManager.flush();

一般来说，您负责管理 JPA/Hibernate 中关系的双方。