Hibernate4.x之映射关系--多对多-优快云博客

本文介绍Hibernate框架中多对多关联关系的两种映射方式：单向和双向n-n关联。通过示例详细解释了如何配置XML映射文件，并提供测试类实现数据的保存与获取。

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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

 1 <?xml version="1.0"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 4 <!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
 5 <hibernate-mapping package="com.yl.hibernate.n2n">
 6     <class name="Category" table="CATEGORYS">
 7         <id name="id" type="java.lang.Integer">
 8             <column name="ID" />
 9             <generator class="native" />
10         </id>
11         <property name="name" type="java.lang.String">
12             <column name="NAME" />
13         </property>
14         <!-- table:指定中间表 -->
15         <set name="items" table="CATEGORIRIES_ITEMS" inverse="false" lazy="true">
16             <key>
17                 <column name="C_ID" />
18             </key>
19             <!-- 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 -->
20             <many-to-many class="Item" column="I_ID"/>
21         </set>
22     </class>
23 </hibernate-mapping>

Item.hbm.xml

 1 <?xml version="1.0"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 4 <!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
 5 <hibernate-mapping>
 6     <class name="com.yl.hibernate.n2n.Item" table="ITEMs">
 7         <id name="id" type="java.lang.Integer">
 8             <column name="ID" />
 9             <generator class="native" />
10         </id>
11         <property name="name" type="java.lang.String">
12             <column name="NAME" />
13         </property>
14     </class>
15 </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

 1 <?xml version="1.0"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 4 <!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
 5 <hibernate-mapping package="com.yl.hibernate.n2n">
 6     <class name="Category" table="CATEGORYS">
 7         <id name="id" type="java.lang.Integer">
 8             <column name="ID" />
 9             <generator class="native" />
10         </id>
11         <property name="name" type="java.lang.String">
12             <column name="NAME" />
13         </property>
14         <!-- table:指定中间表 -->
15         <set name="items" table="CATEGORIRIES_ITEMS" inverse="false" lazy="true">
16             <key>
17                 <column name="C_ID" />
18             </key>
19             <!-- 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 -->
20             <many-to-many class="Item" column="I_ID"/>
21         </set>
22     </class>
23 </hibernate-mapping>

Item.hbm.xml

 1 <?xml version="1.0"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 4 <!-- Generated 2014-11-29 21:45:04 by Hibernate Tools 3.4.0.CR1 -->
 5 <hibernate-mapping>
 6     <class name="com.yl.hibernate.n2n.Item" table="ITEMs">
 7         <id name="id" type="java.lang.Integer">
 8             <column name="ID" />
 9             <generator class="native" />
10         </id>
11         <property name="name" type="java.lang.String">
12             <column name="NAME" />
13         </property>
14         
15         <set name="categories" table="CATEGORIRIES_ITEMS" inverse="true">
16             <key><column name="I_ID"></column></key>
17             <many-to-many class="com.yl.hibernate.n2n.Category" column="C_ID"></many-to-many>
18         </set>
19         
20     </class>
21 </hibernate-mapping>

测试类：

 1 package com.yl.hibernate.n2n;
 2 
 3 
 4 import java.util.Set;
 5 
 6 import org.hibernate.Session;
 7 import org.hibernate.SessionFactory;
 8 import org.hibernate.Transaction;
 9 import org.hibernate.cfg.Configuration;
10 import org.hibernate.service.ServiceRegistry;
11 import org.hibernate.service.ServiceRegistryBuilder;
12 import org.junit.After;
13 import org.junit.Before;
14 import org.junit.Test;
15 
16 public class HibernateTest {
17 
18     private SessionFactory sessionFactory;
19     private Session session;
20     private Transaction transaction;
21     
22     @Before
23     public void init() {
24         Configuration configuration = new Configuration().configure();
25         ServiceRegistry serviceRegistry = 
26                 new ServiceRegistryBuilder().applySettings(configuration.getProperties())
27                                             .buildServiceRegistry();
28 
29         sessionFactory = configuration.buildSessionFactory(serviceRegistry);
30         
31         session = sessionFactory.openSession();
32 
33         transaction = session.beginTransaction();
34     }
35     @After
36     public void destory() {
37         transaction.commit();
38         
39         session.close();
40         
41         sessionFactory.close();
42     }
43     
44     @Test
45     public void testSave() {
46         Category category1 = new Category();
47         category1.setName("C-AA");
48         
49         Category category2 = new Category();
50         category2.setName("C-BB");
51         
52         
53         Item item1 = new Item();
54         item1.setName("I-AA");
55         
56         Item item2 = new Item();
57         item2.setName("I-BB");
58         
59         //设定关联关系
60         category1.getItems().add(item1);
61         category1.getItems().add(item2);
62         
63         category2.getItems().add(item1);
64         category2.getItems().add(item2);
65         
66         item1.getCategories().add(category1);
67         item1.getCategories().add(category2);
68         
69         item2.getCategories().add(category1);
70         item2.getCategories().add(category2);
71         
72         //执行保存操作
73         session.save(category1);
74         session.save(category2);
75         
76         session.save(item1);
77         session.save(item2);
78     }
79     
80 
81     @Test
82     public void testGet() {
83         Category category = (Category) session.get(Category.class, 1);
84         System.out.println(category.getName());
85         //需要连接中间表
86         Set<Item> items = category.getItems();
87         System.out.println(items.size());
88         
89     }
90         
91 }