Hibernate的多对多映射关系

example：
　　　　老师(teacher)和学生(Student)就是一个多对多的关系吧？老师可以有多个学生，学生也可以由多个老师，那在Hibernate中多对多是怎样实现的呢？？

在Hibernate中多对多关系分为两种：1单向的多对多，2双向的多对多

下面详细说明一些两种有什么不同和实现步骤

　　I单向的多对多实现步骤：

　　1新建teacher、student表，可以任意选择在其中一个表添加另一个表的集合，（例如在teacher中添加student的set<Student>集合，例如private set<student> students）

　　代码如下：

　　teacher：

package com.atguigu.hibernate.n2n;

import java.util.Set;

public class Teacher {
    private Integer ID;
    private String  Name;
    private Set<Student> students;
    public Integer getID() {
        return ID;
    }
    public void setID(Integer iD) {
        ID = iD;
    }
    public String getTea_Name() {
        return Name;
    }
    public void setTea_Name(String Name) {
        this.Name =Name;
    }
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }

}

Student：
　　

package com.atguigu.hibernate.n2n;

public class Student {
    private Integer S_ID;
    private String  s_Name;
    public Integer getS_ID() {
        return S_ID;
    }
    public void setS_ID(Integer s_ID) {
        S_ID = s_ID;
    }
    public String getS_Name() {
        return s_Name;
    }
    public void setS_Name(String s_Name) {
        this.s_Name = s_Name;
    }

}

2在有set集合属性的持久化类的.hbm.xml文件里添加集合信息和many-to-many信息，另外一个表的.hbm.xml文件跟平常一样即可

teacher：

　　

<?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.atguigu.hibernate.n2n.Teacher" table="TEACHER">

        <id name="ID" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>

        <property name="Name" type="java.lang.String" access="field">
            <column name="NAME" />
        </property>
          <!--
                name:set集合的名字
                Table：中间表的名字
                key--column:中间表的外键列的名字，这个类的值就是属于<class name="">这个类的ID值
                many-to-many的column：也是指定中间表的外键列的名字，这列的值就属于many-to-many的class类的ID值
         -->
        <!---注释部分可能会引起XML格式错误，--->
        <set name="students" table="t_s_Connection">
            <key>
                <column name="T_ID" />
            </key>
            <many-to-many class="com.atguigu.hibernate.n2n.Student"  column="S_ID"/>
        </set>

    </class>
</hibernate-mapping>
    

student：

　　

<?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.atguigu.hibernate.n2n.Student" table="STUDENT">

        <id name="S_ID" type="java.lang.Integer">
            <column name="S_ID" />
            <generator class="native" />
        </id>

        <property name="s_Name" type="java.lang.String">
            <column name="S_NAME" />
       </property>

    </class>
</hibernate-mapping>

然后配置一下Hibernate.cfg.xml文件即可

单向的多对多关系，就是这样了，但是其的增删改查又有什么特点？？？这个稍后再探究

II双向的多对多关系如何配置
1在双方持久化类（就是teacher和student）中互相添加对方的对象集合，并且当中有一个需要实例化

例如：在teacher中添加private set<student>

　　　 在student中添加private set<teacher>=new hashSet<teacher>();

代码：teacher:

package com.atguigu.hibernate.n2n;

import java.util.Set;

public class Teacher {
    private Integer ID;
    private String  Name;
    private Set<Student> students;
    public Integer getID() {
        return ID;
    }
    public void setID(Integer iD) {
        ID = iD;
    }
    public String getTea_Name() {
        return Name;
    }
    public void setTea_Name(String Name) {
        this.Name =Name;
    }
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }

}

student:

package com.atguigu.hibernate.n2n;

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

public class Student {
    private Integer S_ID;
    private String  s_Name;
    private Set<Teacher> teachers=new HashSet<Teacher>();

    public Set<Teacher> getTeachers() {
        return teachers;
    }
    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }
    public Integer getS_ID() {
        return S_ID;
    }
    public void setS_ID(Integer s_ID) {
        S_ID = s_ID;
    }
    public String getS_Name() {
        return s_Name;
    }
    public void setS_Name(String s_Name) {
        this.s_Name = s_Name;
    }

}

2在双方的持久化类中添加set和many-to-many设置

teacher.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.atguigu.hibernate.n2n.Teacher" table="TEACHER">

        <id name="ID" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>

        <property name="Name" type="java.lang.String" access="field">
            <column name="NAME" />
        </property>
          <!--
                name:set集合的名字
                Table：中间表的名字
                key--column:中间表的外键列的名字（可以放任意名字），就是指明中间表的外键列是当前持久化类的的ID属性
             many-to-many的column：也是指定中间表的外键列的名字，这列的值就属于many-to-many的class类的ID值
         -->
        <set name="students" table="t_s_Connection">
            <key>
                <column name="T_ID" />
            </key>
            <many-to-many class="com.atguigu.hibernate.n2n.Student"  column="S_ID"/>
        </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.atguigu.hibernate.n2n.Student" table="STUDENT">

        <id name="S_ID" type="java.lang.Integer">
            <column name="S_ID" />
            <generator class="native" />
        </id>

        <property name="s_Name" type="java.lang.String">
            <column name="S_NAME" />
       </property>
       <set name="teachers" table="t_s_Connection">
               <key>
                   <column name="S_ID"></column>

               </key>
               <many-to-many class="com.atguigu.hibernate.n2n.Teacher" column="T_ID"></many-to-many>
       </set>
    </class>
</hibernate-mapping>

然后在Hibernate.cfg.xml文件配置一下mapping-resource即可

但是双向的多对多关系的增删改查又是怎样的呢？？

而且单向的多对多关系和多向的多对多关系两者有什么不同，有什么有缺点？？