Hibernate 注解 多对多双向关联失败 的问题求解

Question

有类A与类B多对多映射，
类A代码

[code="java"]
@Entity
@Table(name="a")
public class A {
@Id
@GeneratedValue
private Integer id;
private String name;

 public Integer getId() {
     return id;
 }

 public void setId(Integer id) {
     this.id = id;
 }

 @ManyToMany(cascade={CascadeType.MERGE,CascadeType.PERSIST}
 )
 @JoinTable(name="a_b",
            joinColumns={@JoinColumn(name="a_id")},
            inverseJoinColumns={@JoinColumn(name="b_id")}
 )
 private List<B> bs;

 public List<B> getBs() {
     return bs;
 }

 public void setBs(List<B> bs) {
     this.bs = bs;
 }

 public String getName() {
     return name;
 }

 public void setName(String name) {
     this.name = name;
 }

}[/code]
类B代码

[code="java"] @Entity
@Table(name = "b")
public class B {
@Id
@GeneratedValue
private Integer id;
private String name;

 @ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE }, mappedBy = "bs", targetEntity = A.class)
 private List<A> as;

 public Integer getId() {
     return id;
 }

 public void setId(Integer id) {
     this.id = id;
 }

 public List<A> getAs() {
     return as;
 }

 public void setAs(List<A> as) {
     this.as = as;
 }

 public String getName() {
     return name;
 }

 public void setName(String name) {
     this.name = name;
 }

}[/code]

现在的问题是：
可以通过
a.setBs(bs);
session.save(a);
来保存与b与a之间关系，

但不能通过
b.setAs(as);
session.save(b);
来保存b与a之间的关系，也就是说实际上成了单向关联

我好像找到问题所在了，是因为我先保存再

Answer 1

首先,你要清楚:在使用mappedBy后....,就成单向的了.也就是mappedBy出现的位置所在的类,这个类是被维护端,它只能被别人级联,不能去保存别人.

其次,在一对多,多对一,多对多中,要使用Set,最好不用使用List.

最后,下面是经测试在Oracle通过的代码.

[code="java"]package com;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

/**

• @author ChinoisTzc
  *
  */
  @Entity
  @Table(name="a")
  public class A {
  @Id
  private Integer id;
  private String name;

  public Integer getId() {
  return id;
  }

  public void setId(Integer id) {
  this.id = id;
  }

  @ManyToMany(cascade = CascadeType.ALL)
  @JoinTable(name="a_b",
  joinColumns={@JoinColumn(name="a_id")},
  inverseJoinColumns={@JoinColumn(name="b_id")}
  )
  private Set bs = new HashSet();

  public Set getBs() {
  return bs;
  }

  public void setBs(Set bs) {
  this.bs = bs;
  }

  public String getName() {
  return name;
  }

  public void setName(String name) {
  this.name = name;
  }

  }[/code]

[code="java"]package com;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name = "b")
public class B {
@Id
private Integer id;
private String name;

 @ManyToMany(cascade = CascadeType.ALL)
 @JoinTable(name="a_b",joinColumns=@JoinColumn(name="a_id"),inverseJoinColumns=@JoinColumn(name="b_id"))
 private Set<A> as = new HashSet<A>();

 public Integer getId() {
     return id;
 }

 public void setId(Integer id) {
     this.id = id;
 }


 public Set<A> getAs() {
    return as;
}

public void setAs(Set<A> as) {
    this.as = as;
}

public String getName() {
     return name;
 }

 public void setName(String name) {
     this.name = name;
 }

}[/code]

[code="java"]package com;

import java.util.HashSet;
import java.util.Set;

import org.hibernate.Session;

/*

• created time:上午11:44:11

• creator: Tian Zhichao ([e-mail])
  *
  */
  public class Test {

  /**

  • main()
  • 2010-4-5,上午11:44:11 */ public static void main(String[] args) { // TODO Auto-generated method stub Session session = HibernateSessionFactory.getSession(); session.beginTransaction(); A a = new A(); a.setName("a1 name"); a.setId(1); A a2 = new A(); a2.setName("a2 name"); a2.setId(2); B b = new B(); b.setName("b1 name"); b.setId(22); Set as = new HashSet(); as.add(a); as.add(a2); b.setAs(as); Set bs = new HashSet(); bs.add(b); a.setBs(bs); session.save(a); session.getTransaction().commit(); session.close(); }

}
[/code]

希望对你有帮助 ....