【SSH进阶之路】Hibernate映射——多对多关联映射（八）

朱雀 2022-08-21 01:15 150阅读 0赞

**映射原理**

不论是单向关联还是双向关联都是通过第三张表，将两个表中的主键放到第三张做一个关联。用第三张表来解决可能会造成数据冗余的问题。

## 举例 ##

一个用户（User）对多个角色（Role），一个角色对多个用户。

## 分类 ##

### 单向的多对多关联映射（单向User--->Role） ###

**对象模型**

![SouthEast][]

**关系模型**

![SouthEast 1][]

**实例**

下面我们看一下实体类和映射文件的代码。

User

package com.liang.hibernate;
    
    import java.util.Set;
    
    public class User {
    	private int id;
    	private String name;
    	
    	private Set roles;
    	
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public Set getRoles() {
    		return roles;
    	}
    	public void setRoles(Set roles) {
    		this.roles = roles;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    	
    }

Role

package com.liang.hibernate;
    
    public class Role {
    	private int id;
    	private String name;
    	
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    }

User.hbm.xml

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
    	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
    <hibernate-mapping package="com.liang.hibernate">
    	<class name="User" table="t_user">
    		<id name="id">
    			<generator class="native"></generator>
    		</id>
    		<property name="name"></property>
    		  
    		<set name="roles" table="t_user_role">
                
    			<key column="user_id"></key>
    			<many-to-many class="Role" column="role_id"></many-to-many>
    		</set>
    	</class>
    </hibernate-mapping>

Role.hbm.xml

生成的表结构和测试数据

![SouthEast 2][]![SouthEast 3][]

多对多关联映射，在实体类中，跟[一对多关联映射][Link 1]一样，也是用集合来表示的。<set>标签中用table属性重命名中间表名称，<key>标签定义当前表的主键，用<many-to-many>标签来关联另一张表。

### 双向的多对多关联映射（双向User<--->Role） ###

**对象模型**

![SouthEast 4][]

**关系模型**

同上

**实例**

下面我们看一下实体类和映射文件的代码。

User

package com.liang.hibernate;
    import java.util.Set;
    
    public class User {
    	private int id;
    	private String name;
    	
    	private Set roles;
    	
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public Set getRoles() {
    		return roles;
    	}
    	public void setRoles(Set roles) {
    		this.roles = roles;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    	
    }

Role

package com.liang.hibernate;
    import java.util.Set;
    public class Role {
    	private int id;
    	private String name;
    	private Set users;
    	
    	public Set getUsers() {
    		return users;
    	}
    	public void setUsers(Set users) {
    		this.users = users;
    	}
    	public int getId() {
    		return id;
    	}
    	public void setId(int id) {
    		this.id = id;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    }

User.hbm.xml

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
    	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
    <hibernate-mapping package="com.liang.hibernate">
    	<class name="User" table="t_user">
    		<id name="id">
    			<generator class="native"></generator>
    		</id>
    		<property name="name"></property>
    		  
    		<set name="roles" table="t_user_role">
                
    			<key column="user_id"></key>
    			<many-to-many class="Role" column="role_id"></many-to-many>
    		</set>
    	</class>
    </hibernate-mapping>

Role.hbm.xml

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC 
    	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
    <hibernate-mapping package="com.liang.hibernate">
    	<class name="Role" table="t_role">
    		<id name="id">
    			<generator class="native"></generator>
    		</id>
    		<property name="name"></property>
    		  
    		<set name="users" table="t_user_role">
                                
    			<key column="role_id"></key>
    			<many-to-many class="User" column="user_id"></many-to-many>
    		</set>
    	</class>
    </hibernate-mapping>

生成的表结构和测试数据

![SouthEast 5][]![SouthEast 6][]

多对多双向关系中，User和Role的映射文件相同，值得注意的是生成的中间表名称必须一样，生成中间表的字段必须一样。

[SouthEast]: /images/20220731/2814db6f10b54aeda4ad09d0773ae1f7.png
[SouthEast 1]: /images/20220731/31bb4ae4c56b447396ecaaf679b572ff.png
[SouthEast 2]: https://img-blog.csdn.net/20141120194801465?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvaml1cWl5dWxpYW5n/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 3]: /images/20220731/f9d81c17f5ec4ff8b5e6f9845184adac.png
[Link 1]: http://blog.csdn.net/jiuqiyuliang/article/details/41015641
[SouthEast 4]: /images/20220731/849796571b6b4d5e9999f27fff24d9eb.png
[SouthEast 5]: https://img-blog.csdn.net/20141120194821886?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvaml1cWl5dWxpYW5n/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast
[SouthEast 6]: /images/20220731/7abe322d7b894d048b702ec263a67657.png