Hibernate4.x之映射关系--多对多

n-n的关联关系分为以下两类：

1.单向n-n关联关系：

• n-n的关联必须使用连接表
• 与1-n映射类似，必须为set集合元素添加key子元素，指定CATEGORIES_ITEMS表中参照CATEGORIES表的外键为CATEGORIY_ID。与1-n关联映射不同的是，建立n-n关联时，集合中的元素使用many-to-many。many-to-many子元素的class属性指定items接种存放的是Item对象，column属性指定CATEGORIES_ITEMS表中参照ITEMS表的外键为ITEM_ID

　　

--------------------------------代码--------------------------------------------------------

Category.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">
<!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping package="com.yl.hibernate.n2n">
    <class name="Category" table="CATEGORYS">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        <!-- table:指定中间表 -->
        <set name="items" table="CATEGORIRIES_ITEMS" inverse="false" lazy="true">
            <key>
                <column name="C_ID" />
            </key>
            <!-- 使用many-to-many 指定多对多的关联关系， column指定set集合中的持久化类在中间表的外键列的名称 -->
            <many-to-many class="Item" column="I_ID"/>
        </set>
    </class>
</hibernate-mapping>

Item.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">
<!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
    <class name="com.yl.hibernate.n2n.Item" table="ITEMs">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
    </class>
</hibernate-mapping>

2.双向n-n关联关系

• 双向n-n关联需要两端都使用集合属性
• 双向n-n关联必须使用连接表
• 集合属性应增加key子元素用以映射外键列，集合元素里还应增加many-to-many子元素关联实体类
• 在双向n-n关联的两边都需指定连接表的表名及外键列的列名。两个集合元素set的table元素的值必须指定，且必须相同。set元素的两个子元素：key和many-to-many多必须指定column属性，其中，key和many-to-many分别制定本持久化类和关联类在连接表中的外键列名，因此两边的key与many-to-many的column属性交叉相同。也就是说，一边的set元素的key的column值为a，many-to-many的column为b，则另一边的set元素的key的column值为b，many-to-many的column值为a
• 对于双向n-n关联，必须把其中一端的inverse设置为true，否则两端都维护关联关系可能会造成逐渐冲突

Category.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">
<!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping package="com.yl.hibernate.n2n">
    <class name="Category" table="CATEGORYS">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        <!-- table:指定中间表 -->
        <set name="items" table="CATEGORIRIES_ITEMS" inverse="false" lazy="true">
            <key>
                <column name="C_ID" />
            </key>
            <!-- 使用many-to-many 指定多对多的关联关系， column指定set集合中的持久化类在中间表的外键列的名称 -->
            <many-to-many class="Item" column="I_ID"/>
        </set>
    </class>
</hibernate-mapping>

Item.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">
<!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
    <class name="com.yl.hibernate.n2n.Item" table="ITEMs">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        
        <set name="categories" table="CATEGORIRIES_ITEMS" inverse="true">
            <key><column name="I_ID"></column></key>
            <many-to-many class="com.yl.hibernate.n2n.Category" column="C_ID"></many-to-many>
        </set>
        
    </class>
</hibernate-mapping>

测试类：

package com.yl.hibernate.n2n;


import java.util.Set;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

public class HibernateTest {

    private SessionFactory sessionFactory;
    private Session session;
    private Transaction transaction;
    
    @Before
    public void init() {
        Configuration configuration = new Configuration().configure();
        ServiceRegistry serviceRegistry = 
                new ServiceRegistryBuilder().applySettings(configuration.getProperties())
                                            .buildServiceRegistry();

        sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        
        session = sessionFactory.openSession();

        transaction = session.beginTransaction();
    }
    @After
    public void destory() {
        transaction.commit();
        
        session.close();
        
        sessionFactory.close();
    }
    
    @Test
    public void testSave() {
        Category category1 = new Category();
        category1.setName("C-AA");
        
        Category category2 = new Category();
        category2.setName("C-BB");
        
        
        Item item1 = new Item();
        item1.setName("I-AA");
        
        Item item2 = new Item();
        item2.setName("I-BB");
        
        //设定关联关系
        category1.getItems().add(item1);
        category1.getItems().add(item2);
        
        category2.getItems().add(item1);
        category2.getItems().add(item2);
        
        item1.getCategories().add(category1);
        item1.getCategories().add(category2);
        
        item2.getCategories().add(category1);
        item2.getCategories().add(category2);
        
        //执行保存操作
        session.save(category1);
        session.save(category2);
        
        session.save(item1);
        session.save(item2);
    }
    

    @Test
    public void testGet() {
        Category category = (Category) session.get(Category.class, 1);
        System.out.println(category.getName());
        //需要连接中间表
        Set<Item> items = category.getItems();
        System.out.println(items.size());
        
    }
        
}