Question

查找以前保存的1对多实体。标题是：

Book.java：

@Entity
@Table(name = "Book")
public class Book {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long id;

    @OneToMany(fetch=FetchType.LAZY, cascade=CascadeType.ALL, mappedBy="book", orphanRemoval=true)
    private Collection<Author> authors;

    public Collection<Author> getAuthors() {
        return authors;
    }
        ...

        @Override
    public String toString() {
        return "Book [id=" + id + ", authors=" + authors + ", title=" + title
                + ", isbn=" + isbn + "]";
    }
}

Author.java：

@Entity
@Table(name = "Authors")
public class Author {

    @Id
    @GeneratedValue
    @Column(name="id")
    private Long id;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="book_id")
    private Book book;

        ...

    @Override
    public String toString() {
         return "Author [id=" + id + ", book=" + book + ", firstName="
            + firstName + ", lastName=" + lastName + "]";
    }
}

忽略数据模型的可疑基数，查找代码为

long bookID = 10; // saved entity ID
entityManager.getTransaction().begin();
Book savedBook = entityManager.find(Book.class, bookID);
System.out.println("Book " + savedBook);
entityManager.getTransaction().commit();

给出输出

Book Book [id=10, authors=[], title=Treasure Island, isbn=123456]

为什么作者空洞的想法？

Retrieving 11...

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.Long.toString(Long.java:240)
    at java.lang.Long.toString(Long.java:100)
    at java.lang.String.valueOf(String.java:2946)
    at java.lang.Long.toString(Long.java:733)
    at java.lang.String.valueOf(String.java:2827)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at org.hibstore.domain.Book.toString(Book.java:64)
    at java.lang.String.valueOf(String.java:2827)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at org.hibstore.domain.Author.toString(Author.java:67)
    at java.lang.String.valueOf(String.java:2827)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at java.util.AbstractCollection.toString(AbstractCollection.java:422)
    at org.hibernate.collection.internal.PersistentBag.toString(PersistentBag.java:501)
    at java.lang.String.valueOf(String.java:2827)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at org.hibstore.domain.Book.toString(Book.java:64)
    at java.lang.String.valueOf(String.java:2827)

Answer 1

插入记录时可能出现问题。您需要将父级设置为子级，并将子级添加到父级，如book.addAuthor(author);和author.setBook(book)，因为孩子正在这里推动这种关系。

Answer 2

此问题的另一个解决方案可能是设置实体作者＆gt;将@ManyToOne的书籍字段映射到@ManyToOne(fetch=FetchType.LAZY, cascade = CascadeType.PERSIST)

JPA OneToMany集合作为Empty返回

2 个答案: