Hibernate中的关系映射，最常见的关系映射之一就是一对多关系映射例如学生与班级的关系，一个班级对应多个学生。如图：

      Hibernate中如何来映射这两个的关系呢？

1、创建实体类Classes和实体类Student

Classes.java

package com.bjpowernode.hibernate;import java.util.Set;public class Classes {		private int id;		private String name;		private Set students;	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 Set getStudents() {		return students;	}	public void setStudents(Set students) {		this.students = students;	}	}

Student.java

package com.bjpowernode.hibernate;public class Student {	private int id;		private String name;		private Classes classes;	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 Classes getClasses() {		return classes;	}	public void setClasses(Classes classes) {		this.classes = classes;	}}

2、创建映射文件Classes.hbm.xml和Student.hbm.xml

Classes.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>	<class name="com.bjpowernode.hibernate.Classes" table="t_classes">		<id name="id">			<generator class="native"/>		</id>		<property name="name"/>		<set name="students" inverse="true">			<!-- 			<key column="classesid" not-null="true"/>			 -->			 <key column="classesid"/>			<one-to-many class="com.bjpowernode.hibernate.Student"/>		</set>	</class></hibernate-mapping>

Student.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>	<class name="com.bjpowernode.hibernate.Student" table="t_student">		<id name="id">			<generator class="native"/>		</id>		<property name="name"/>		<many-to-one name="classes" column="classesid"/>	</class></hibernate-mapping>

3、效果图

PS：我上面举例的是一对多双向关联的例子。有双向关联当然就有单向关联。下面举个例子来说明他们的区别

如果Student添加了一条记录当中的classesid在Classes中没有这个id，双向关联的话则先添加学生这条记录然后再班级那里添加这个班级，然后再更新学生的classesid。单向关联的话先添加班级，然后再添加学生。