【hibernate框架】多对多双向关联(XML实现)

多对多的双向关联：(非常少用)
通过老师可以知道他教了多少学生，这是单项多对多。而如果同时通过学生知道有多少个老师教他，那么就是多对多双向的关联。


XML实现：
Student.java:

package cn.edu.hpu.many2many;


import java.util.HashSet; 
import java.util.Set;


public class Student {
  private int id;
  private String name;  
  private Set<Teacher> teachers=new HashSet<Teacher>();
  
  public Set<Teacher> getTeachers() {
    return teachers;
  }
  public void setTeachers(Set<Teacher> teachers) {
    this.teachers = teachers;
  }


  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;
  }
  
}

Teacher.java:

package cn.edu.hpu.many2many;


import java.util.HashSet;
import java.util.Set;


import cn.edu.hpu.many2many.Student;


public class Teacher {
  private int id;
  private String name;
  private Set<Student> students=new HashSet<Student>();
  //选择set的原因是因为，set互相之间不会有重复的
  //跟数据库模型比较匹配


  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;
  }
  
}

配置文件：

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="cn.edu.hpu.many2many">
    <class name="Student" table="xm_student" dynamic-update="true">
        <!-- 主键 这里的主键名字刚好也是'id'-->
            <id name="id" column="id">
                <generator class="native"></generator>
            </id>  
        
        <property name="name"></property> 
        <set name="teachers" table="t_s">
           <key column="student_id"></key><!-- 指定关联字段的名字叫什么 -->
           <many-to-many class="cn.edu.hpu.many2many.Teacher" column="teacher_id"/>
        </set>
    </class>
</hibernate-mapping>

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="cn.edu.hpu.many2many">
    <class name="Teacher" table="xm_teacher" dynamic-update="true">
        <!-- 主键 这里的主键名字刚好也是'id'-->
            <id name="id" column="id">
                <generator class="native"></generator>
            </id>  
        
        <property name="name"></property> 
        <set name="students" table="t_s">
           <key column="teacher_id"></key><!-- 指定关联字段的名字叫什么 -->
           <many-to-many class="cn.edu.hpu.many2many.Student" column="student_id"/>
        </set>
    </class>
</hibernate-mapping>

在hibernate.cfg.xml中配置：

<mapping resource="cn/edu/hpu/many2many/Student.hbm.xml"/> 
<mapping resource="cn/edu/hpu/many2many/Teacher.hbm.xml"/>

生成的sql语句：

 

create table t_s (

         student_id integer not null,

         teacher_id integer not null,

         primary key (teacher_id, student_id)

     )



     create table xm_student (

         id integer not null auto_increment,

         name varchar(255),

         primary key (id)

     )



     create table xm_teacher (

         id integer not null auto_increment,

         name varchar(255),

         primary key (id)

     )



     alter table t_s 

         add index FK1BF687B5893C2 (teacher_id), 

         add constraint FK1BF687B5893C2 

         foreign key (teacher_id) 

         references xm_teacher (id)



     alter table t_s 

         add index FK1BF686ABD4922 (student_id), 

         add constraint FK1BF686ABD4922 

         foreign key (student_id) 

        references xm_student (id)