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

 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 }