Hibernate的多对多映射

一、创建Java工程，新建Lib文件夹，加入Hibernate和数据库（如MySql、Oracle、SqlServer等）的Jar包，创建 hibernate.cfg.xml 文件，并配置，配置项如下：

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE hibernate-configuration PUBLIC
         "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
         "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
 <hibernate-configuration>
     <session-factory>

         <!-- 配置连接数据库的基本信息 -->
         <property name="connection.username">root</property>
         <property name="connection.password"></property>
         <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
         <!-- <property name="connection.url">jdbc:mysql:///mis</property> -->
         <property name="connection.url">
             <![CDATA[jdbc:mysql://localhost:3306/mis?useUnicode=true&characterEncoding=utf8]]>
         </property>

         <!-- 配置 hibernate 的基本信息 -->
         <!-- hibernate 所使用的数据库方言 -->
         <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>

         <!-- 执行操作时是否在控制台打印 SQL -->
         <property name="show_sql">true</property>

         <!-- 是否对 SQL 进行格式化 -->
         <property name="format_sql">true</property>

         <!-- 指定自动生成数据表的策略 -->
         <property name="hbm2ddl.auto">update</property>

         <!-- 设置 Hibernate 的事务隔离级别 :读已提交的记录-->
         <property name="connection.isolation"></property>

         <!-- 删除对象后, 使其 OID 置为 null -->
         <property name="use_identifier_rollback">true</property>

         <!-- 配置 C3P0 数据源 -->
         <property name="hibernate.c3p0.max_size"></property>
         <property name="hibernate.c3p0.min_size"></property>
         <property name="c3p0.acquire_increment"></property>

         <property name="c3p0.idle_test_period"></property>
         <property name="c3p0.timeout"></property>

         <property name="c3p0.max_statements"></property>

         <!-- 设定 JDBC 的 Statement 读取数据的时候每次从数据库中取出的记录条数 -->
         <property name="hibernate.jdbc.fetch_size"></property>

         <!-- 设定对数据库进行批量删除，批量更新和批量插入的时候的批次大小 -->
         <property name="jdbc.batch_size"></property>

         <!-- 指定关联的 .hbm.xml 文件 -->
         <!--
         <mapping resource="com/mcs/hibernate/entities/onetoone/foreign/Manager.hbm.xml" />
         <mapping resource="com/mcs/hibernate/entities/onetoone/foreign/Department.hbm.xml" />
           -->
         <!--
         <mapping resource="com/mcs/hibernate/entities/onetoone/primary/Manager.hbm.xml" />
         <mapping resource="com/mcs/hibernate/entities/onetoone/primary/Department.hbm.xml" />
          -->
         <!--
         <mapping resource="com/mcs/hibernate/entities/manytomany/Category.hbm.xml" />
         <mapping resource="com/mcs/hibernate/entities/manytomany/Product.hbm.xml" />
          -->
         <!--
         <mapping resource="com/mcs/hibernate/entities/subclass/Person.hbm.xml" />
          -->
         <!--
         <mapping resource="com/mcs/hibernate/entities/joined/subclass/Person.hbm.xml" />
          -->

         <mapping resource="com/mcs/hibernate/entities/union/subclass/Person.hbm.xml" />

     </session-factory>

 </hibernate-configuration>

二、建立多对多的映射

1、创建Java实体类

 package com.mcs.hibernate.entities.manytomany;

 import java.util.HashSet;
 import java.util.Set;

 public class Category {

     private Integer id;
     private String name;

     private Set<Product> products = new HashSet<>();

     public Integer getId() {
         return id;
     }

     public void setId(Integer id) {
         this.id = id;
     }

     public String getName() {
         return name;
     }

     public void setName(String name) {
         this.name = name;
     }

     public Set<Product> getProducts() {
         return products;
     }

     public void setProducts(Set<Product> products) {
         this.products = products;
     }

 }

 package com.mcs.hibernate.entities.manytomany;

 import java.util.HashSet;
 import java.util.Set;

 public class Product {

     private Integer id;
     private String name;

     private Set<Category> categories = new HashSet<>();

     public Integer getId() {
         return id;
     }
     public void setId(Integer id) {
         this.id = id;
     }
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public Set<Category> getCategories() {
         return categories;
     }
     public void setCategories(Set<Category> categories) {
         this.categories = categories;
     }

 }

2、根据实体类创建对应的 hbm.xml文件

 <?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 -- :: by Hibernate Tools 3.5..Final -->
 <hibernate-mapping>
     <class name="com.mcs.hibernate.entities.manytomany.Category" table="CATEGORIES">
         <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="products" table="CATEGORIES_PRODUCTS" inverse="true">
             <key>
                 <column name="CATEGORY_ID" />
             </key>
             <!-- 使用 many-to-many 指定多对多的关联关系. column 执行 Set 集合中的持久化类在中间表的外键列的名称  -->
             <many-to-many class="com.mcs.hibernate.entities.manytomany.Product" column="PRODUCT_ID"/>
         </set>
     </class>
 </hibernate-mapping>

 <?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 -- :: by Hibernate Tools 3.5..Final -->
 <hibernate-mapping>
     <class name="com.mcs.hibernate.entities.manytomany.Product" table="PRODUCTS">
         <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="categories" table="CATEGORIES_PRODUCTS">
             <key>
                 <column name="PRODUCT_ID" />
             </key>
             <!-- 使用 many-to-many 指定多对多的关联关系. column 执行 Set 集合中的持久化类在中间表的外键列的名称  -->
             <many-to-many class="com.mcs.hibernate.entities.manytomany.Category" column="CATEGORY_ID"/>
         </set>
     </class>
 </hibernate-mapping>

3、备注：

　　1、使用 many-to-many 指定多对多的关联关系.

　　　　table: 指定中间表

　　　　Key：当前持久化类在中间表的外键列的名称

　　　　column：执行 Set 集合中的持久化类在中间表的外键列的名称

　　2、为了不重复更新，在其中的一端设置 inverse="true"的属性

　　3、在查询时需要连接中间表