Hibernate关联映射 --- 多对多实例分析（双向关联）

 

Hibernate关联映射 --- 多对多实例分析（双向关联）

一、多对多关联分析：

   多对多关联的实现是通过中间表实现的，通过中间表，将这两个表之间的多对多关联关系转换为它们分别和中间表直接的一对多的关联关系。对于Hibernate来说，并不需要建立专门的中间对象来实现这种转换。但是中间表还是需要建立的。

Hibernate是通过<many-to-many>元素来描述对象之间多对多的关系的。

二、多对多关联的实例分析：

分析图：

步骤分析：

（1）首先要在Domain包下新建Teacher类和Student类。

（2）编写这两个类所对应的映射文件

源代码：

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("老师1");

 

           Teacher t2 = new Teacher();

           t2.setName("老师2");

 

           Student s1 = new Student();

           s1.setName("学生1");

 

           Student s2 = new Student();

           s2.setName("学生2");

 

           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>