菜鸟学习Hibernate——一对多关系映射

来源：互联网发布：sql创建存储过程查询编辑：程序博客网时间：2024/05/29 19:22

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？

下面就为大家讲解一下：

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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