菜鸟学习Hibernate——一对多关系映射

     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。

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