1. 学生表student 和年级表 grade

2. 类Student 和Grade类

  • Student
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public class Student {
    private int id;
    private String name;
    private int age;
    private Grade grade;
    //get…set
}
  • Grande
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public class Grade {
    private int id;
    private String name;
    private Set<Student> students = new HashSet<Student>(0);    
    //get…set
}

3. 映射文件

  • Grade.hbm.xml
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<hibernate-mapping package="cn.siggy.pojo">
    <class name="Grade">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="name"/>
        <!-- set是 Grade中的集合属性  name属性名称 -->
        <set name="students">
            <!-- key表示外键  column外键列名-->
            <key column="grade_id"></key>
            <!-- one-to-many 一对多  类Grade 中  students 所表示类型 -->
            <one-to-many class="Student"/>
        </set>
    </class>
</hibernate-mapping>
  • Student.hbm.xml
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<hibernate-mapping package="cn.siggy.pojo">
    <class name="Student">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="name"/>
        <property name="age"/>
        <many-to-one name="grade" class="Grade" column="grade_id" not-null="true"></many-to-one>
    </class>
</hibernate-mapping>

4. 测试代码

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@Test
public void testSave() throws HibernateException, SerialException, SQLException{
    Session session = null;
    Transaction tx = null;
    try{
        session = HibernateUtil.getSession();
        tx = session.beginTransaction();
        Grade grade = new Grade();
        grade.setName("基础");

        Student stu = new Student();
        stu.setName("张三疯");
        stu.setAge(22);
        stu.setGrade(grade);
        Student stu1 = new Student();
        stu1.setName("老王");
        stu1.setAge(23);
        stu1.setGrade(grade);
        //关联
        //保存数据的顺序 是根据外键的配置来决定的
        //如果外键不能为null,那么先保存一的一端
        //如果外键可以为null,则可以随意保存
        session.save(grade);
        session.save(stu);
        session.save(stu1);

        tx.commit();

    }catch (HibernateException e) {
        if(tx!=null)
            tx.rollback();
        e.printStackTrace();
        throw e;
    }finally{
        HibernateUtil.closeSession();
    }
}
@Test
public void testGet(){
    Session session = null;
    Transaction tx = null;
    try{
        session = HibernateUtil.getSession();
        tx = session.beginTransaction();
        //取数据
        Grade grade = (Grade)session.get(Grade.class, 2);
        System.out.println("gradeName="+grade.getName());
        System.out.println("grade所对应的多的一端的数据");
        Iterator<Student> iter = grade.getStudents().iterator();
        for(;iter.hasNext();){
            Student temp = iter.next();
            System.out.println("name="+temp.getName()+"\tage="+temp.getAge());
        }
        System.out.println("========================");
        Student stu = (Student)session.get(Student.class, 1);
        System.out.println("studentname="+stu.getName()+"gradeName="+
            stu.getGrade().getName());
        tx.commit();
    }catch (HibernateException e) {
        if(tx!=null)
            tx.rollback();
        e.printStackTrace();
        throw e;
    }finally{
        HibernateUtil.closeSession();
    }
}

总结：

  在双向的一对多关联映射中，注意一的一端的配置：<key>中不用指定非空not-null;如果想要一的一端来维护关系，那么在多的一端不用指定关联列非空。