【SSH三大框架】Hibernate基础第十篇：inverse属性详解

inverse经常用在双向的1—N关联之中，也可以用在N—N的关联，这里举例子用双向的1—N关联

还是用部门和员工两个，关于两个的javabean不再写。

首先，我们看下员工的映射文件：

<?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="cn.itcast.hibernate.domain">  
    <class name="Employee" table="tb_Employee">  
        <id name="id">  
            <generator class="native"/>   
        </id>  
        <property name="name" />  
        <many-to-one name="depart" column="depart_id" />  
    </class>  
      
</hibernate-mapping>  

<?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="cn.itcast.hibernate.domain">  
  
    <class name="Department" table="tb_Department">  
        <id name="id">  
            <generator class="native"/>   
        </id>  
        <property name="name" />  
        
        <set name="emps">
        	<key column="depart_id" />
        	<one-to-many class="Employee" />
        </set>
    </class>  
      
</hibernate-mapping>  

package cn.itcast.hibernate;

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

import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.Transaction;

import cn.itcast.hibernate.domain.Department;
import cn.itcast.hibernate.domain.Employee;

public class Many2One {  
    public static void main(String[] arg){  
        Department depart = add();  
    }  
      
    static Department add(){  
        Session s = null;  
        Transaction tx = null;  
        try{  
            Department depart = new Department(); 
            Department depart1 = new Department(); 
            depart.setName("depart name");  
              
            Employee emp1 = new Employee();  
            emp1.setDepart(depart); //建立两个对象的关联关系  
            emp1.setName("emp name");  
            
            
            Employee emp2 = new Employee();  
            emp2.setDepart(depart); //建立两个对象的关联关系  
            emp2.setName("emp name");  
            
    
            Set<Employee> emps = new HashSet<Employee>();
            
            emps.add(emp1);
            emps.add(emp2);
            depart.setEmps(emps);
            
            s = HibernateUtil.getSession();  
            tx = s.beginTransaction();  
           
            s.save(depart);
            s.save(emp1);
            s.save(emp2);
            System.out.println(depart.getEmps().size());
             
            
            tx.commit();  
            return depart;  
        }finally{  
            if(s!=null){  
                s.close();  
            }  
        }  
    }  
}  

我们看一下MyEclipse打印出来的sql语句：

Hibernate: insert into tb_Department (name) values (?)
Hibernate: insert into tb_Employee (name, depart_id) values (?, ?)
Hibernate: insert into tb_Employee (name, depart_id) values (?, ?)
2
Hibernate: update tb_Employee set depart_id=? where id=?
Hibernate: update tb_Employee set depart_id=? where id=?

当双向1—N关联的时候，1的一端想要维护关系，N的一端也想要维护关系，在1的一端先插入，N的一端后插入之后，还要为N的一端外键再更新成为1的一端的id。所以会多出两条update语句。

但是，如果我们在部门映射文件中的set标签加入inverse="true"的属性后，这两条update语句就会消失，我们修改下部门映射文件：

<?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="cn.itcast.hibernate.domain">  
  
    <class name="Department" table="tb_Department">  
        <id name="id">  
            <generator class="native"/>   
        </id>  
        <property name="name" />  
        
        <set name="emps" inverse="true" >
        	<key column="depart_id" />
        	<one-to-many class="Employee" />
        </set>
    </class>  
      
</hibernate-mapping>  

Hibernate: insert into tb_Department (name) values (?)
Hibernate: insert into tb_Employee (name, depart_id) values (?, ?)
Hibernate: insert into tb_Employee (name, depart_id) values (?, ?)
2

这是因为1的一端不再控制关联关系。

inverse=“true”表明1的一端不再控制关联关系