hibernate的映射之三(多对多单向关联)

Many-to-Many 多对多的映射可以使用一组Java集合不包含任何重复的元素来实现。我们已经看到了Hibernate如何设置映射集合。

集(SET)被映射到与映射表中<set>元素，并以java.util.HashSet初始化。您可以使用Set集合在类中时，集合不需要重复的元素。

在现实生活中多对多的例子挺多的。

例如：学生与老师，订单与商品等

下面我就以员工和项目来做例子，一个员工可以做多个项目，一个项目可以被多个员工做。

1.准备JavaBean

项目·实体·类

public class Project {
 ???private Integer proid;
 ???private String proname;
 ?
 ???public Integer getProid() {
 ???????return proid;
 ???}

 ???public void setProid(Integer proid) {
 ???????this.proid = proid;
 ???}

 ???public String getProname() {
 ???????return proname;
 ???}

 ???public void setProname(String proname) {
 ???????this.proname = proname;
 ???}
}

员工实体类（员工实体类中需要植入一个项目集合。）

public class Employee {
 ???private Integer empid;
 ???private String empname;
 ???//植入项目集合 ??一个员工可以参与多个项目
 ???private Set<Project> projects=new HashSet<Project>();

 ???public Integer getEmpid() {
 ???????return empid;
 ???}

 ???public void setEmpid(Integer empid) {
 ???????this.empid = empid;
 ???}

 ???public String getEmpname() {
 ???????return empname;
 ???}

 ???public void setEmpname(String empname) {
 ???????this.empname = empname;
 ???}

 ???public Set<Project> getProjects() {
 ???????return projects;
 ???}

 ???public void setProjects(Set<Project> projects) {
 ???????this.projects = projects;
 ???}
}

2.持久化配置文件
Employee.hbm.xml配置文件

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
 ???????"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 ???????"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<!-- 映射文件开始 -->
<hibernate-mapping package="cn.day04mapping.manytomany.entity">
 ???<!--表名称-->
 ???<class name="Employee" table="EMPLOYEE" schema="root">
 ???????<!--列名-->
 ??????<id name="empid" column="EMPID">
 ??????????<!--主键生成的策略 ?native：自动生成主键字段-->
 ??????????<generator class="native"></generator>
 ??????</id>
 ????<property name="empname" column="EMPNAME"></property>
 ???????<!--单相关联-->
 ???????<!--植入set 标签 ?table 中间表的表名 ?-->
 ???????<set name="projects" ?table="PROEMP">
 ???????????<!--column 多的一方表的外键列-->
 ???????????<key column="REMPID"></key>

 ???????????<!--column ?中间表的另外一个外键列-->
 ???????????<many-to-many column="RPROID" class="Project"></many-to-many>
 ???????</set>
 ???</class>
</hibernate-mapping>

Project.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
 ???????"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
 ???????"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<!-- 映射文件开始 -->
<hibernate-mapping package="cn.day04mapping.manytomany.entity">
 ???<!--表名称-->
 ???<class name="Project" table="PROJECT" schema="root">
 ???????<!--列名-->
 ??????<id name="proid" column="PROID">
 ??????????<!--主键生成的策略 ?native：自动生成主键字段-->
 ??????????<generator class="native"></generator>
 ??????</id>
 ????<property name="proname" column="PRONAME"></property>
 ???</class>
</hibernate-mapping>


3.hibernate.cfg.xml主配置文件

<?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>
 ???????<!-- 指定数据库所用到的驱动 -->
 ???????<property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
 ???????<!-- 指定数据库链接的url，hibernate链接的数据库名 -->
 ???????<property name="connection.url">jdbc:oracle:thin:@localhost:1521:orcl</property>
 ???????<!-- 指定连接数据库的用户名 -->
 ???????<property name="connection.username">root</property>
 ???????<!-- 指定连接数据库的用户口令 -->
 ???????<property name="connection.password">root</property>
 ???????<!-- Enable Hibernate‘s automatic session context management -->
 ???????<property name="current_session_context_class">thread</property>

 ???????<!--格式化sql -->
 ???????<property name="format_sql ">true</property>
 ???????<!-- 打印sql 控制台-->
 ???????<property name="show_sql">true</property>
 ???????<!-- 指定数据库方言 -->
 ???????<property name="dialect">org.hibernate.dialect.Oracle10gDialect</property>
 ???????<!-- 根据需要自动创建数据库表 -->
 ???????<property name="hbm2ddl.auto">update</property>

 ???????<!--多对多-->
 ???????<mapping resource="cn/day04mapping/manytomany/entity/Project.hbm.xml"></mapping>
 ???????<mapping resource="cn/day04mapping/manytomany/entity/Employee.hbm.xml"></mapping>


 ???????<!--延迟加载-->
 ???????<mapping resource="cn/day05lazy/entity/Student.hbm.xml"></mapping>

 ???</session-factory>
</hibernate-configuration>


4.书写测试类和方法

public class HibernateUtil {
 ?????static Configuration cfg;
 ?????static SessionFactory factory;
 ?????static ThreadLocal<Session> td;
 ???//静态代码块
 ???static {
 ???????cfg=new Configuration().configure();
 ???????factory=cfg.buildSessionFactory();
 ???????td=new ThreadLocal<Session>();
 ???}
 ???public static Session getSession(){
 ????????//看看有无线程变量
 ???????Session session = td.get();
 ???????if (session==null){
 ???????????//线程中没有session对象 创建一个
 ????????????session = factory.openSession();
 ????????????td.set(session);
 ???????}
 ???????//没有和当前线程绑定
 ???????return session;
 ???}
 ???//关闭session
 ???public static void closeSession(){
 ???????Session session = td.get();
 ???????td.set(null);
 ???????session.close();
 ???}
}

//多对多单向关联的查询
@Test
public void test01(){
 ???Session session = HibernateUtil.getSession(); ???//得到session
 ???Employee employee = session.get(Employee.class, 1);
 ???System.out.println(employee.getEmpname());
 ???System.out.println("=============");
 ???for (Project project:employee.getProjects()){
 ???????System.out.println(project.getProname());
 ???}
}

//多对多单向关联的添加
@Test
public void test03(){
 ???Session session = HibernateUtil.getSession();
 ???//开启事务
 ???Transaction transaction = session.beginTransaction();
 ???Employee employee=new Employee();
 ???employee.setEmpname("微冷的雨");

 ???Project project=new Project();
 ???project.setProname("开发部");
 ???Project project1=new Project();
 ???project1.setProname("研发部");

 ???employee.getProjects().add(project);
 ???employee.getProjects().add(project1);

 ???//保存数据
 ???session.save(employee);
 ???session.save(project);
 ???session.save(project1);
 ???//提交事务
 ???transaction.commit();
 ???session.close();
}