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 }
时间： 2024-08-23 05:54:49