多对多其实是个很复杂的关系，hibernate在进行处理的时候借助中间表或者中间类。中间表是在映射文件的关联标签(比如集合标签<set>)中由table属性指定的由hibernate自动生成的表，它只有两个字段，分别由<key>和<many-to-many>标签的table属性指定，作为外键分别用来指向关联双方表的主键。中间类就是把我们的中间表抽象生成一个实体类，在映射的时候分别和两个关联类构成一对多的关系，即演变成两个一对多来处理。
我习惯使用中间表的方式。
1.实体模型：
2.关系模型：
3.实体类：
Role.java
public
class Role {
private Integer id;
private String name;
//一系列的setter.getter方法
@Override
public String toString() {
return "Role:" + name;
}
}
private Integer id;
private String name;
//一系列的setter.getter方法
@Override
public String toString() {
return "Role:" + name;
}
}
Player.java
public
class Player {
private Integer id;
private String name;
private Set<Role> roles;
//一系列的setter.getter方法
@Override
public String toString() {
return "Player:" + name;
}
}
private Integer id;
private String name;
private Set<Role> roles;
//一系列的setter.getter方法
@Override
public String toString() {
return "Player:" + name;
}
}
4.映射文件：
Role.hbm.xml
<
class
name
="com.sxt.hibernate.many2many.entity.Role"
table
="sxt_hibernate_role"
>
< id name ="id" length ="4" >
< generator class ="native" > </ generator >
</ id >
< property name ="name" length ="10" > </ property >
</ class >
< id name ="id" length ="4" >
< generator class ="native" > </ generator >
</ id >
< property name ="name" length ="10" > </ property >
</ class >
Player.hbm.xml
<
class
name
="com.sxt.hibernate.many2many.entity.Player"
table
="sxt_hibernate_player"
>
< id name ="id" length ="4" >
< generator class ="native" > </ generator >
</ id >
< property name ="name" length ="10" > </ property >
<!-- table="sxt_hibernate_user_role"含义,用来指定中间表 -->
< set name ="roles" table ="sxt_hibernate_player_role" cascade ="save-update" >
<!-- <key column="user_id">含义,指定中间表中用来指向本表的外键 -->
< key column ="player_id" > </ key >
<!-- column含义,用来指定中间表中用来指向另一端表的外键 -->
< many-to-many class ="com.sxt.hibernate.many2many.entity.Role" column ="role_id" > </ many-to-many >
</ set >
</ class >
< id name ="id" length ="4" >
< generator class ="native" > </ generator >
</ id >
< property name ="name" length ="10" > </ property >
<!-- table="sxt_hibernate_user_role"含义,用来指定中间表 -->
< set name ="roles" table ="sxt_hibernate_player_role" cascade ="save-update" >
<!-- <key column="user_id">含义,指定中间表中用来指向本表的外键 -->
< key column ="player_id" > </ key >
<!-- column含义,用来指定中间表中用来指向另一端表的外键 -->
< many-to-many class ="com.sxt.hibernate.many2many.entity.Role" column ="role_id" > </ many-to-many >
</ set >
</ class >
5.hibernate配置文件：
参见上一篇。
6.测试方法：
public
static
void main(String[] args) {
Session session = HibernateUtils.getSession();
Transaction t = session.beginTransaction();
try {
/**
* 测试插入数据
*/
/* Role role1=new Role();
role1.setName("后卫");
Role role2=new Role();
role2.setName("前锋");
Role role3=new Role();
role3.setName("中锋");
Player player1=new Player();
player1.setName("姚明");
Set<Role> roles1=new HashSet<Role>();
roles1.add(role3);
player1.setRoles(roles1);
Player player2=new Player();
player2.setName("詹姆斯");
Set<Role> roles2=new HashSet<Role>();
roles2.add(role1);
roles2.add(role2);
roles2.add(role3);
player2.setRoles(roles2);
//能正确保存.每保存player后,都要级联保存它的role,并且级联插入中间表记录.
session.save(player1);
session.save(player2);*/
/**
* 测试加载数据
*/
Player player=(Player)session.load(Player. class, 1);
System.out.println(player);
for(Iterator<Role> iterator=player.getRoles().iterator();iterator.hasNext();){
System.out.println(iterator.next());
}
t.commit();
} catch (HibernateException e) {
e.printStackTrace();
t.rollback();
} finally {
HibernateUtils.closeSession(session);
}
}
}
Session session = HibernateUtils.getSession();
Transaction t = session.beginTransaction();
try {
/**
* 测试插入数据
*/
/* Role role1=new Role();
role1.setName("后卫");
Role role2=new Role();
role2.setName("前锋");
Role role3=new Role();
role3.setName("中锋");
Player player1=new Player();
player1.setName("姚明");
Set<Role> roles1=new HashSet<Role>();
roles1.add(role3);
player1.setRoles(roles1);
Player player2=new Player();
player2.setName("詹姆斯");
Set<Role> roles2=new HashSet<Role>();
roles2.add(role1);
roles2.add(role2);
roles2.add(role3);
player2.setRoles(roles2);
//能正确保存.每保存player后,都要级联保存它的role,并且级联插入中间表记录.
session.save(player1);
session.save(player2);*/
/**
* 测试加载数据
*/
Player player=(Player)session.load(Player. class, 1);
System.out.println(player);
for(Iterator<Role> iterator=player.getRoles().iterator();iterator.hasNext();){
System.out.println(iterator.next());
}
t.commit();
} catch (HibernateException e) {
e.printStackTrace();
t.rollback();
} finally {
HibernateUtils.closeSession(session);
}
}
}
本文转自NightWolves 51CTO博客，原文链接：
http://blog.51cto.com/yangfei520/276054
，如需转载请自行联系原作者
