hibernate笔记--双向一对多映射方法

　　前两节写了两个例子,分别是单向多对一的映射和单向一对多的映射,这一节继续以这个例子讲一下双向一对多的映射方法,如下图所示:

　　很多时候,我们既想从一对端获取多对端的信息,又想从多对端获取一对端的数据,这就是需要双向一对多的映射关系,这也是最常见的表与表的映射关系,在hibernate中应当这样配置:

新建Studnet实体类:

public class Student {

    private int id;
    private String name;
    private String sex;
    private Grade grade;
//get/set方法省略
}

新建Grade实体类:

public class Grade {

    private int id;
    private String name;
    private String teacher;
    private Set<Student> students=new HashSet<Student>(0);
      //get/set方法省略
}

当前包下新建Student的映射文件Student.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">

<hibernate-mapping package="com.wang.pojo">
    <class name="Student" >
    <id    name="id">
        <generator class="native"></generator>
    </id>
    
    <property name="name"></property>
    <property name="sex"></property>
    <many-to-one name="grade" class="Grade" column="gradeId" not-null="true"></many-to-one>
    </class>
</hibernate-mapping>

当前包下新建Grade的映射文件Grade.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">

<hibernate-mapping package="com.wang.pojo">
    <class name="Grade" >
    <id    name="id">
        <generator class="native"></generator>
    </id>
    
    <property name="name"></property>
    <property name="teacher"></property>
    <!-- set是一种集合属性,可以配置set,list等,这里是Grade的集合属性  name是属性名称 -->
    <set name="students">
    <!-- key表示外键 column表示外键列名  注意在双向一对多的关系中,一对端即这里的Grade不需要设置notnull="true"-->
        <key column="gradeId" ></key>
        <one-to-many class="Student"/>
    </set>
    </class>
</hibernate-mapping>

将两个映射文件添加到hibernate.cfg.xml中去,新建测试类,测试1:自动生成表结构2:存取数据3:读取数据(分别测试从Student一端读取,以及从grade一段读取):

@Test
    public void testCreateDB() {
        Configuration cfg = new Configuration().configure();
        SchemaExport se = new SchemaExport(cfg);
        // 第一个参数是否生成ddl脚本 第二个参数是否执行到数据库
        se.create(true, true);
    }

    @Test
    public void testSave() {
        Session session = HibernateUtil.getSession();
        Transaction tx = session.beginTransaction();
        // Score s=new Score(2,12,97);
        // session.save(s);
        Grade g=new Grade(1, "终极一班", "王大炮");
        
        Student s1=new Student(12, "李云龙", "男", g);
        Student s2=new Student(13, "赵刚", "男", g);
        
        session.save(g);
        session.save(s1);
        session.save(s2);
        tx.commit();
        session.close();
    }
    @Test
    public void testGet() {
        Session session = HibernateUtil.getSession();
        Transaction tx = session.beginTransaction();
        //从Grade一端获取数据
        Grade g=(Grade)session.get(Grade.class, 1);
        System.out.println("id="+g.getId()+"name="+g.getName()+"teacher="+g.getTeacher());
        Iterator<Student> it = g.getStudents().iterator();
        while(it.hasNext()){
            Student s=it.next();
            System.out.println(s);
        }
        //从Student一端获取数据
        Student s1=(Student)session.get(Student.class, 1);
        System.out.println(s1.getName()+"====="+s1.getGrade().getName());
        tx.commit();
        session.close();
    }

　　总结:在双向一对多的映射关系中,注意一的一端(Grade)配置:<key>标签中不需要指定not-null="true";如果想要在一的一端(Grade)来维护关系,那么在多的一端(Student)不用指定关联列非空.