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());

     }

 }