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;
  @Before
  public 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();
  }
  @After
  public void destroy(){
    transaction.commit();//提交事务
    session.close();//关闭会话
    sessionFactory.close();//关闭会话工厂
  }
  @org.junit.Test
  public 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.Test
  public 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.Test
  public 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.Test
  public 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：