hibernate初探之单向一对多映射

什么叫做单向一对多的映射呢？

我们首先要知道映射是有方向的，分为单向和双向。

比如班级和学生。站在班级的角度，一个班级可以有多个学生，  在这里班级就是“一”方 学生就是“多”方

这种关系就是一对多的关系，如果只站在班级的角度，那就是单向的一对多的关系。

一对多的关系我们应该怎么实现呢？

在数据库中我们会通过主外键的方式实现，在多学生表中添加一个外键指向班级

在hibernate中我们会通过在“一”的一方使用<set>元素表示持有“多”的一方的对象，即在“一”方中使用集合 表示持有“多”方

使用工具

• Junint单元测试工具
• mysql-connector-java-5.1.7-bin.jar
  
• hibernate 4.2.4.jar

hibernate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"><!-- Generated by MyEclipse Hibernate Tools.                   --><hibernate-configuration><session-factory><property name="dialect">org.hibernate.dialect.MySQLDialect</property><property name="connection.url">jdbc:mysql://localhost:3306/susu</property><property name="connection.username">root</property><property name="connection.password">123</property><property name="connection.driver_class">com.mysql.jdbc.Driver</property><property name="myeclipse.connection.profile">MySQLDriver</property><property name="show_sql">true</property><property name="format_sql">true</property><property name="hbm2ddl.auto">create</property><mapping resource="com/scx/entity/Grade.hbm.xml" /><mapping resource="com/scx/entity/Students.hbm.xml" /></session-factory></hibernate-configuration>

因为我们还没有在数据库中创建表 所以要在hibernate配置文件设置

<property name="hbm2ddl.auto">create</property>

持久化类

students   学生类

package com.scx.entity;public class Students {//学生主键private int sId;//学生姓名private String sName;//学生性别private String sex;//学生年龄private int age;public Students(int sId, String sName, String sex, int age) {this.sId = sId;this.sName = sName;this.sex = sex;this.age = age;}public Students() {}public int getsId() {return sId;}public void setsId(int sId) {this.sId = sId;}public String getsName() {return sName;}public void setsName(String sName) {this.sName = sName;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}}

grade  班级类

package com.scx.entity;import java.util.HashSet;import java.util.Set;public class Grade {//班级主键private int gId;//班级名称private String gName;//学生集合private Set<Students> students = new HashSet<Students>();public Grade(int gId, String gName, Set<Students> students) {this.gId = gId;this.gName = gName;this.students = students;}public Grade() {}public int getgId() {return gId;}public void setgId(int gId) {this.gId = gId;}public String getgName() {return gName;}public void setgName(String gName) {this.gName = gName;}public Set<Students> getStudents() {return students;}public void setStudents(Set<Students> students) {this.students = students;}//向班级中集合中添加学生public void addStudents(Students stu) {students.add(stu);}}

对象关系映射文件

Students.hbm.xml

<?xml version="1.0" encoding="utf-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!--     Mapping file autogenerated by MyEclipse Persistence Tools--><hibernate-mapping>    <class name="com.scx.entity.Students" table="students" catalog="hibernate">        <id name="sId" type="java.lang.Integer">            <column name="SID" />            <generator class="native" />        </id>        <property name="sName" type="java.lang.String">            <column name="SNAME" length="20" />        </property>        <property name="sex" type="java.lang.String">            <column name="SEX" length="2" />        </property>        <property name="age" type="java.lang.Integer">            <column name="AGE"  />        </property>    </class></hibernate-mapping>

Grade.hbm.xml

<?xml version="1.0" encoding="utf-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!--     Mapping file autogenerated by MyEclipse Persistence Tools--><hibernate-mapping>    <class name="com.scx.entity.Grade" table="grade" catalog="hibernate">        <id name="gId" type="java.lang.Integer">            <column name="GID" />            <generator class="native" />        </id>        <property name="gName" type="java.lang.String">            <column name="GNAME" length="20" />        </property>        <!-- 配置单向的一对多关联关系 -->       <set name="students" table="students">       <!-- 制定关联的外键列 -->       <key column="GID"></key>       <!-- 持久化对象所对应的实体类 -->       <one-to-many class="com.scx.entity.Students"/>       </set>    </class></hibernate-mapping>

注意看 在Grade.hbm.xml中我们需要配置单向的一对多关联关系

<set name="students" table="students">       <!-- 制定关联的外键列 -->       <key column="GID"></key>       <!-- 持久化对象所对应的实体类 -->       <one-to-many class="com.scx.entity.Students"/>       </set>

使用junit通过Hibernate API编写访问数据库的代码

import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;import org.hibernate.service.ServiceRegistry;import org.hibernate.service.ServiceRegistryBuilder;import org.junit.After;import org.junit.Before;import com.scx.entity.Grade;import com.scx.entity.Students;public class Test {private Configuration config;private ServiceRegistry serviceRegistry;private SessionFactory sessionFactory;private Session session;private Transaction transaction;@Beforepublic void init() {//创建配置对象config = new Configuration().configure();//创建服务注册对象serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();//创建会话工厂对象sessionFactory=config.buildSessionFactory(serviceRegistry);//会话对象session=sessionFactory.openSession();//开启事务transaction=session.beginTransaction();}@Afterpublic void destroy(){transaction.commit();//提交事务session.close();//关闭会话sessionFactory.close();//关闭会话工厂}@org.junit.Testpublic void testSave(){Students stu1=new Students(1, "小明", "男", 15);Students stu2=new Students(2, "小红", "女", 15);Grade g1=new Grade();Grade g2=new Grade();g1.setgName("软件一班");g2.setgName("软件二班");g1.addStudents(stu1);g2.addStudents(stu2);session.save(g1);session.save(g2);session.save(stu1);session.save(stu2);}}

实例化了两个学生 两个班级 分别属于一个班级 

运行结果如下：

根据班级查找班级所有学生

首先修改hibernate配置文件中的生成表策略为update

<property name="hbm2ddl.auto">update</property>

然后新添加一个测试

@org.junit.Testpublic void findStudentsByGrade(){Grade g=(Grade) session.get(Grade.class, 1);System.out.println(g.getgName());Set<Students> students=g.getStudents();for (Students stu : students) {System.out.println(stu.getsName()+" "+stu.getSex()+" "+stu.getAge());}}

运行结果：

更新学生信息

@org.junit.Testpublic void update(){Grade g=new Grade();g.setgName("软件三班");Students stu=(Students) session.get(Students.class, 1);g.addStudents(stu);session.save(g);}

新建一个班级为软件三班 并将编号为1的学生添加到软件三班

运行结果：

删除学生

@org.junit.Testpublic void delete(){Students stu=(Students) session.get(Students.class, 2);session.delete(stu);}

查找编号为2的学生 并删除

运行结果如下：