Hiberante 单向多对一和一对多的关联映射

存在如下表，考虑多对一和一对多的配置

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多对一，如何在多的一端配置 从多的一端获取一的一端的数据
student

public class Student {    private int id;    private String name;    private int age;    private Grade grade;//get…set}

Grade

public class Grade {    private int id;    private String name;    //get…set}

Grade.hbm.xml

<hibernate-mapping package="cn.siggy.pojo">    <class name="Grade">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name"/>    </class></hibernate-mapping>

Student.hbm.xml

<hibernate-mapping package="cn.siggy.pojo">    <class name="Student">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name"/>        <property name="age"/>        <!-- 多对一  name 表示属性名  class 指明 属性对应的类          column指 数据库表中的列名 -->            <many-to-one name="grade" class="Grade"         column="grade_id" foreign-key="fk_grade" not-null="true"/>  </class></hibernate-mapping>

测试（session对象的获取封装在HibernateUtil，略）

@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");            session.save(grade);            Student stu = new Student();            stu.setName("张三疯");            stu.setAge(22);            stu.setGrade(grade);            session.save(stu);            tx.commit();        }catch (HibernateException e) {            if(tx!=null)                tx.rollback();            e.printStackTrace();            throw e;        }finally{            HibernateUtil.closeSession();        }    }

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一对多 在一的一端配置 从一的一端获取多的一端的数据 用集合表示
Student

public class Student {    private int id;    private String name;    private int age;//get…set}

Grade

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

Grade.hbm.xml

<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" not-null="true"></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

<hibernate-mapping package="cn.siggy.pojo">    <class name="Student">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name"/>        <property name="age"/></class></hibernate-mapping>

测试

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);            Student stu1 = new Student();            stu1.setName("老王");            stu1.setAge(23);            //关联            grade.getStudents().add(stu);            grade.getStudents().add(stu1);            //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();

session = HibernateUtil.getSession();            tx = session.beginTransaction();            //取数据            Grade grade = (Grade)session.get(Grade.class, 1);            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());            }            tx.commit();