hibernate中两个级联表，一个student表和一个book表，student表中有主键id,是book表中的外键；在用Student的类无法得到set集
查询的代码
Session session=bo.HibernateSessionFactory.getSession();
Session session=bo.HibernateSessionFactory.getSession();

//查询每个同学所拥有的所有书x；
Query query=session.createQuery("from Student");
List list=query.list();
for(int i=0;i<list.size();i++)
{
Student student=(Student)list.get(i);
System.out.println(student.getId());//到这里可以
Set listbooks=student.getBooks();//出错
System.out.println(listbooks);
/*Iterator it=listbooks.iterator();
while(it.hasNext())
{
Book book=(Book)it.next();
System.out.println("姓名  "+student.getName()+" 书名    "+book.getName());
}*/
}


Book.hbm.xml文件
<?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="po.Book" table="BOOK" schema="DB2ADMIN">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <many-to-one name="student" class="po.Student" fetch="select">
            <column name="SID" />
        </many-to-one>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="30" />
        </property>
    </class>
</hibernate-mapping>

Stuent.hbm.xml文件
<?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="po.Student" table="STUDENT" schema="DB2ADMIN">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="10" />
        </property>
        <property n