Hibernate ORM Many to Many

　　探讨下Hibernate中对象间多对多的关联关系。以同学借阅图书为例，一个同学可以借阅多本图书，而一本图书可以被多个同学借阅。

　　建议对于多对多的关系处理借助中间类，从数据库实现上就是说借助第三张表。当然具体需求具体分析，你也可以采用其他的策略。不过这里重点介绍这种方式。来看看实现吧。

　　１.Student.java

 

package com.template.model;

import java.util.List;

/**
 * Created by IntelliJ IDEA.
 * User: ZhongGang
 * Date: 11-7-20
 * Time: 下午9:31
 * To change this template use File | Settings | File Templates.
 */
public class Student {
    private Integer id;
    private String name;

    private List<Borrowed> borroweds;

    public Student() {
    }
}

 

 

　　２.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 default-access="field">

    <class name="com.template.model.Student" table="student">
        <id name="id" column="id" type="java.lang.Integer">
            <generator class="native"/>
        </id>
        <property name="name" column="name" type="java.lang.String"/>

        <bag name="borroweds" table="borrowed">
            <key column="studentid"/>
            <one-to-many class="com.template.model.Borrowed"/>
        </bag>
    </class>

</hibernate-mapping>

 

　　３.Book.java

 

package com.template.model;

import java.util.List;

/**
 * Created by IntelliJ IDEA.
 * User: ZhongGang
 * Date: 11-7-20
 * Time: 下午9:31
 * To change this template use File | Settings | File Templates.
 */
public class Book {
    private Integer id;
    private String name;

    private List<Borrowed> borroweds;

    public Book() {
    }
}

 

　　４.Book.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 default-access="field">

    <class name="com.template.model.Book" table="book">
        <id name="id" column="id" type="java.lang.Integer">
            <generator class="native"/>
        </id>
        <property name="name" column="name" type="java.lang.String"/>

        <bag name="borroweds" table="borrowed">
            <key column="bookid"/>
            <one-to-many class="com.template.model.Borrowed"/>
        </bag>
    </class>

</hibernate-mapping>

 

　　５.Borrowed.java

 

package com.template.model;

/**
 * Created by IntelliJ IDEA.
 * User: ZhongGang
 * Date: 11-7-20
 * Time: 下午9:31
 * To change this template use File | Settings | File Templates.
 */
public class Borrowed {
    private Integer id;
    private Student student;
    private Book book;

    public Borrowed() {
    }
}

 

　　６.Borrowed.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 default-access="field">

    <class name="com.template.model.Borrowed" table="borrowed">
        <id name="id" column="id" type="java.lang.Integer">
            <generator class="native"/>
        </id>

        <many-to-one name="book" class="com.template.model.Book" column="bookid"/>
        <many-to-one name="student" class="com.template.model.Student" column="studentid"/>
    </class>

</hibernate-mapping>

 

 

　　Borrwed就是辅助中间类了，用于维护学生和书籍之间的多对多关系。借助中间类通过两个一对多的关联关系来实现多对多的关系，这样的好处是有助于以后的扩展。如果一个多对多的关联上又有多个属性的话，就有必要将关系抽象出来作为一个对象来处理了，使得程序结构清晰，也有助于编程和理解。