相对于上文的多对一关系，这里又说明下一对多的关联关系。

在上文中，我们描述了多对一的关系，在关系数据库中也是多对一的关系，并且还是一对多的关系。但是，仅仅如此是不够的，Hibernate是一种面向对象的结构，在Hibernate中仍然是多对一的关系，但是没有一对多，所以我们需要额外添加一对多的关系。

部门实体类：Department.java

package cn.itcast.hibernate.domain;

import java.util.Set;

public class Department {  
    private int id;  
    private String name;  
    private Set<Employee> emps;
    
    public Set<Employee> getEmps() {
		return emps;
	}
	public void setEmps(Set<Employee> emps) {
		this.emps = emps;
	}
	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;  
    }  
}  

部门映射关系：Department.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="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>  

在映射文件中的set标签，我们定义了一个key标签，这是外键，必须与员工类的外键相同。

package cn.itcast.hibernate.domain;

public class Employee {  
    private int id;  
    private String name;  
    private Department depart;  
      
    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;  
    }  
    public Department getDepart() {  
        return depart;  
    }  
    public void setDepart(Department depart) {  
        this.depart = depart;  
    }  
}  

<?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>  

public class Many2One {  
    public static void main(String[] arg){  
        Department depart = add();  
        query(depart.getId()); 
    }  
      
    static Department add(){  
        Session s = null;  
        Transaction tx = null;  
        try{  
            Department depart = 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");  
              
            s = HibernateUtil.getSession();  
            tx = s.beginTransaction();  
            s.save(depart);
            s.save(emp1);   //emp对象和depart对象保存的先后顺序不同也会有一定的差异  
            s.save(emp2); //如果员工(emp)先插入，部门(depart)后插入，会生成三条数据库语句，  
                            //因为在两者都插入之后，emp的depart_id字段还未插入，所以还要有最后一条语句插入  
            tx.commit();  
            return depart;  
        }finally{  
            if(s!=null){  
                s.close();  
            }  
        }  
    }  
      
    static Department query(int departId){  
        Session s = null;  
        Transaction tx = null;  
        try{  
            s = HibernateUtil.getSession();  
            tx = s.beginTransaction();  
            Department depart = (Department)s.get(Department.class, departId); //根据ID查询  
            System.out.println("emp size:"+depart.getEmps().size());
            //Hibernate.initialize(depart.getEmps());
            tx.commit();  
            return depart;  
        }finally{  
            if(s!=null){  
                s.close();  
            }  
        }  
    }  
}  

【SSH三大框架】Hibernate基础第七篇：一对多关联关系的操作