Hibernate关联映射 --- 多对多实例分析（双向关联）

http://blog.csdn.net/hanxiaoshuang321123/article/details/7056686

一 概念分析

1．多对多的经典实例（Teacher – Student）

二 代码分析

1．Teacher类

package com.hbsi.domain;
import java.util.Set;
//单向一对多 Teacher 一 对 多 Student
public class Teacher {
private int id;
private String name;
private Set<Student> students;
public Teacher() {
super();
// TODO Auto-generated constructor stub
}
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<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}

}
2．Stduent类

package com.hbsi.domain;
import java.util.Set;
public class Student {
private int id;
private String name;
private Set<Teacher> teachers;
public Student() {
super();
// TODO Auto-generated constructor stub
}
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<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
}
3．Teacher的映射文件 Teacher.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.hbsi.domain">
<class name="Teacher" table="teacher">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" />
<!-- 集合属性的体现 多对多 table属性指定的是中间表的名字 -->
<set name="students" table="teacher_student">
<!--查中间表的外键 和老师的id的关联 老师的id-->
<key column="teacher_id"/>
<!-- 查学生的id和中间表的关系 -->
<many-to-many class="Student" column="student_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
4.Student的映射文件 Student.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.hbsi.domain">
<class name="Student" table="student">
<id name="id" column="id">
<generator class="native" />
</id>
<property name="name" column="name" />
<set name="teachers" table="teacher_student">
<key column="student_id"></key><!-- 查找这个学生被那个老师教 -->
<many-to-many class="Teacher" column="teacher_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
5.测试类

public class Many2Many {
public static void main(String[] args) {
// add();
query(2);
}
//保存
public static void add() {
Session session = null;
Transaction transaction = null;
try {
session = HibernateUtil.getSession();
transaction = session.beginTransaction();

// 添加
Teacher t1 = new Teacher();
t1.setName("王老师");

Teacher t2 = new Teacher();
t2.setName("李老师");

Student s1 = new Student();
s1.setName("王同学");

Student s2 = new Student();
s2.setName("李同学");

Set<Student> ss = new HashSet<Student>();
ss.add(s1);
ss.add(s2);

t1.setStudents(ss);// 告诉Hibernate老师教了那两个学生
t2.setStudents(ss);

session.save(t1);
session.save(t2);
session.save(s1);
session.save(s2);

transaction.commit();

} finally {
if (session != null) {
session.close();
}
}
}
//查询
public static void query(int id) {

Session session = null;
try {
session = HibernateUtil.getSession();
Teacher teacher = (Teacher) session.get(Teacher.class, id);
System.out.println("教室的姓名:" + teacher.getName());
System.out.println("教室的学生列表：");
Iterator<Student> iterator = teacher.getStudents().iterator();
while (iterator.hasNext()) {
Student student = iterator.next();
System.out.println(student.getName() + " ");
}
System.out.println( );

Student student = (Student) session.get(Student.class, id);
System.out.println("学生的姓名:"+student.getName());

System.out.println("学生被教的老师:");
Iterator<Teacher> iterator2 = student.getTeachers().iterator();
while(iterator2.hasNext()){
Teacher teacher2 = iterator2.next();
System.out.println(teacher2.getName()+" ");
}
System.out.println();

} finally {
if (session != null) {
session.close();
}
}
}
}
注：
（1）Set<Teacher> tt = new HashSet<Teacher>();
tt.add(t1);
tt.add(t2);

s1.setTeachers(tt);
s2.setTeachers(tt);
如果在测试保存的时候添加会报错，因为前边已经添加了，已经更新数据库了，再更新会报
org.hibernate.exception.ConstraintViolationException错误，所以要注意

（2）映射文件中的信息一定要写对
<set name="students" table="teacher_student">
<key column="teacher_id"/>
<many-to-many class="Student" column="student_id"></many-to-many>
</set>