相對於上文的多對一關係，這裡又說明下一對多的關聯關係。

在上文中，我們描述了多對一的關係，在關係資料庫中也是多對一的關係，並且還是一對多的關係。但是，僅僅如此是不夠的，Hibernate是一種面向物件的結構，在Hibernate中仍然是多對一的關係，但是沒有一對多，所以我們需要額外新增一對多的關係。

部門實體類：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;  
    }  
}  
 

部門對映關係：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>  
 

在對映檔案中的set標籤，我們定義了一個key標籤，這是外來鍵，必須與員工類的外來鍵相同。

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); //建立兩個物件的關聯關係  
            emp1.setName("emp name");  
            
            Employee emp2 = new Employee();  
            emp2.setDepart(depart); //建立兩個物件的關聯關係  
            emp2.setName("emp name");  
              
            s = HibernateUtil.getSession();  
            tx = s.beginTransaction();  
            s.save(depart);
            s.save(emp1);   //emp物件和depart物件儲存的先後順序不同也會有一定的差異  
            s.save(emp2); //如果員工(emp)先插入，部門(depart)後插入，會生成三條資料庫語句，  
                            //因為在兩者都插入之後，emp的depart_id欄位還未插入，所以還要有最後一條語句插入  
            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); //根據ID查詢  
            System.out.println("emp size:"+depart.getEmps().size());
            //Hibernate.initialize(depart.getEmps());
            tx.commit();  
            return depart;  
        }finally{  
            if(s!=null){  
                s.close();  
            }  
        }  
    }  
}