休眠异常(一对多关系):引用未知实体

Question

我正在使用Hibernate。

比如说，一个Person只能有一个Book。但每个Book可以由多个Person s保留。

我正在实现这种关系，并且我正在为这种关系使用一个联合表*person_book*。

以下是我所做的：

Person类：

@Entity
@Table(name = "person")
public class Person {
   private int pid;
   private Book book;
  @ManyToOne(cascade = {CascadeType.ALL})
  @JoinTable(name="person_book",
             joinColumns={@JoinColumn(name="pid")},
                inverseJoinColumns={@JoinColumn(name="bid")})
   public Book getBook() {
    return book;
   }
   public void setBook(Book book) {
        this.book = book;
   }
   //Setter & Getter for pid 
   ...
}

Book类：

@Entity
@Table(name = "book")
public class Book {
   private int bid;
   private Set<Person> persons = new HashSet<Person>();
  @OneToMany(cascade = {CascadeType.ALL})
  @JoinTable(name="person_book",
                joinColumns={@JoinColumn(name="bid")},
                inverseJoinColumns={@JoinColumn(name="pid")})
  public Set<Person> getPersons() {
     return persons;
  }
  public void setPersons(Set<Person> persons) {
    this.persons = persons;
  }
  //Setter & Getter for bid
  ...
}

在休眠配置中，我有：

<hibernate-configuration>
  <session-factory>
    ...
    <mapping class="com.my.db.model.Person" />
    <mapping class="com.my.db.model.Book" />
  </session-factory>
</hibernate-configuration>

当我开始我的tomecat时，我经常得到以下Hibernate异常：

 org.hibernate.AnnotationException: @OneToOne or @ManyToOne on com.my.db.model.Person.book references an unknown entity: com.my.db.model.Book
    at org.hibernate.cfg.ToOneFkSecondPass.doSecondPass(ToOneFkSecondPass.java:109)
    at org.hibernate.cfg.Configuration.processEndOfQueue(Configuration.java:1557)
    at org.hibernate.cfg.Configuration.processFkSecondPassInOrder(Configuration.java:1478)

它抱怨书是一个未知的实体。为什么？我哪里做错了？我觉得问题很可能是关于hibernate注释。。。

（请不要建议我不要使用联接表。谢谢。）

Answer 1

如果你仔细观察你的堆栈，你会看到

Error :org.hibernate.AnnotationException: @OneToOne or @ManyToOne on com.my.db.model.Person.book references an unknown entity: com.my.db.model.Book

所以这个领域

@ManyToOne(cascade = {CascadeType.ALL})
  @JoinTable(name="person_book",
             joinColumns={@JoinColumn(name="pid")},
                inverseJoinColumns={@JoinColumn(name="bid")})
   public Book getBook() {
    return book;
   }

是造成问题的原因。@ManyToOne有一个参数targetEntity，它说：

(Optional) The entity class that is the target of the association. Defaults to the type of the field or property that stores the association.

从而将您的表结构更新为

表格结构

CREATE TABLE `person` (
  `id` INT(3) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) CHARACTER SET latin1 NOT NULL,
   KEY `fk_book_id` 
  CONSTRAINT `fk_book_id` FOREIGN KEY (`fk_book_id`) REFERENCES `book` (`id`)   
  PRIMARY KEY (`id`)
) ENGINE=INNODB
CREATE TABLE `book` (
  `id` INT(3) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) CHARACTER SET latin1 NOT NULL,
  PRIMARY KEY (`id`),
) ENGINE=INNODB

实体冲突

@Entity
@Table(name = "person")
public class Person {
   private int pid;
   private Book book;
  @ManyToOne()
  @JoinColumn(name = "fk_book_id", nullable = false)
  public Book getBook() {
    return book;
  }
  //Setter & Getter for pid 
   ...
}
@Entity
@Table(name = "book")
public class Book {
   private int bid;
   private Set<Person> persons = new HashSet<Person>();
  @OneToMany(mappedBy = "book")
  public Set<Person> getPersons() {
     return persons;
  }
  //Setter & Getter for bid
  ...
}

希望这将解决您的问题