Hibernate-多对多关系映射

多对多关系映射:

多对多和之前的一对多和多对一的区别就是需要再维护一个表，去存储两表的id，然后将两边进行关联起来

我们就以Developer和Project多对多的关系就行说明

首先建立实体类Developer.java，并建立getter和setter方法,属性如下，

private int d_id;private String d_name;// 开发人员，参数的多个项目private Set<Project> projects = new HashSet<Project>();

再建立实体类Project.java，属性如下:

private int prj_id;private String prj_name;// 项目下的多个员工private Set<Developer> developers = new HashSet<Developer>();

然后对应写出主配置文件hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-configuration PUBLIC        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"><hibernate-configuration>      <session-factory>    <!-- jdbc驱动类 -->        <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>        <!-- jdbc的url -->        <property name="hibernate.connection.url">jdbc:oracle:thin:@127.0.0.1:1521:XE</property>        <!-- jdbc的用户名 -->        <property name="hibernate.connection.username">evan</property>        <!-- jdbc的密码 -->        <property name="hibernate.connection.password">123</property><!-- 打印映射信息 -->        <property name="hibernate.show_sql">true</property>        <property name="hibernate.hbm2ddl.auto">create</property>        <!-- 方言 -->        <property name="hibernate.dialect">org.hibernate.dialect.Oracle10gDialect</property>                <!-- 配置c3p0连接池 -->        <!-- 最大连接数 -->        <property name="hibernate.c3p0.max_size">20</property>        <!-- 最小连接数 -->        <property name="hibernate.c3p0.min_size">1</property>        <!-- 获取连接超时时间，若超过这个时间会抛出异常，单位:毫秒 -->        <property name="hibernate.c3p0.timeout">5000</property>        <!-- 创建PreparedStatement对象的最大数量 -->        <property name="hibernate.c3p0.max_statements">100</property>        <!-- 每个指定时间检查连接池中的空闲连接，时间单位:秒 -->        <property name="hibernate.c3p0.idle_test_period">150</property>        <!-- 当连接池使用完毕，c3p0再和数据库建立n个连接放置到连接池中 -->        <property name="hibernate.c3p0.acquire_increment">2</property>        <!-- 每次都验证连接是否可用 -->        <property name="hibernate.c3p0.validate">true</property>                <!-- 将session与当前线程绑定 -->        <property name="current_session_context_class">thread</property>        <!-- 映射路径 -->        <!-- <mapping resource="cn/qblank/hibernate/entity/stu.hbm.xml"/> -->        <!-- <mapping resource="cn/qblank/hibernate/entity/employee.hbm.xml"/>        <mapping resource="cn/qblank/hibernate/entity/dept.hbm.xml"/> -->   </session-factory></hibernate-configuration>

然后再对应地写出实体类的映射文件

project.hbm.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">        <hibernate-mapping package="cn.qblank.hibernate.entity"><class name="Project" table="t_project"><id name="prj_id" type="integer"><generator class="native"></generator></id><property name="prj_name" column="prj_name" type="string"></property><!-- 多对多映射:1.  映射的集合属性： “developers”2.  集合属性，对应的中间表： “t_relation”3. 外键字段:  prjId4. 外键字段，对应的中间表字段:  did5.   集合属性元素的类型 --><set name="developers" table="t_realation" cascade="all"><key column="prjId"></key><many-to-many column="did" class="Developer"></many-to-many></set></class></hibernate-mapping>

developer.hbm.xml

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">        <hibernate-mapping package="cn.qblank.hibernate.entity"><class name="Developer" table="t_developer"><id name="d_id" type="integer"><generator class="native"></generator></id><property name="d_name" column="d_name" type="string"></property><!-- 多对多映射配置： 员工方name  指定映射的集合属性table 集合属性对应的中间表key   指定中间表的外键字段(引用当前表t_developer主键的外键字段)many-to-manycolumn 指定外键字段对应的项目字段class  集合元素的类型 --><set name="projects" table="t_realation"><key column="did"></key><many-to-many column="prjId" class="Project"></many-to-many></set></class></hibernate-mapping>

接下来，我们就来测试一下

package cn.qblank.hibernate.test;import org.hibernate.HibernateException;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;import cn.qblank.hibernate.entity.Developer;import cn.qblank.hibernate.entity.Project;public class ManyToMany {private static SessionFactory factory;private static Transaction trans = null;static {factory = new Configuration().configure().addClass(Project.class)   .addClass(Developer.class)   // 测试时候使用.buildSessionFactory();}public static void main(String[] args) {save();}// 1. 多对多，保存  【只能通过一方维护另外一方，不能重复维护！】public static void save() {Session session = factory.openSession();trans = session.beginTransaction();/* * 模拟数据： 京东商城（老王，evan）租车系统（老丁，evan） */// 创建项目对象Project prj_jd = new Project();Project prj_zc = new Project();prj_jd.setPrj_name("京东商城");prj_zc.setPrj_name("租车系统");// 创建员工对象Developer dev_evan = new Developer();Developer dev_wang = new Developer();Developer dev_ding = new Developer();dev_evan.setD_name("evan");dev_wang.setD_name("老王");dev_ding.setD_name("老丁");// 关系 【项目方】//京东商城prj_jd.getDevelopers().add(dev_evan);prj_jd.getDevelopers().add(dev_wang);//租车系统prj_zc.getDevelopers().add(dev_evan);prj_zc.getDevelopers().add(dev_ding);try {// 保存//插入项目session.save(prj_jd);session.save(prj_zc);   // 必须要设置级联保存 trans.commit();} catch (HibernateException e) {e.printStackTrace();trans.rollback();}finally{session.close();}}}

运行结果如下:

Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: insert into t_project (prj_name, prj_id) values (?, ?)
Hibernate: insert into t_developer (d_name, d_id) values (?, ?)
Hibernate: insert into t_developer (d_name, d_id) values (?, ?)
Hibernate: insert into t_project (prj_name, prj_id) values (?, ?)
Hibernate: insert into t_developer (d_name, d_id) values (?, ?)
Hibernate: insert into t_realation (prjId, did) values (?, ?)
Hibernate: insert into t_realation (prjId, did) values (?, ?)
Hibernate: insert into t_realation (prjId, did) values (?, ?)
Hibernate: insert into t_realation (prjId, did) values (?, ?)

数据库的表的结果如下: