Hibernate使用其他实体类的多个字段作为联合主键，该怎么配置？

Question

1.有实体类Student

public class Student {
    //id为主键
    private String id;
    private String passwd;
    private String name;
    private String sex;
    private String school;
    private String major;
    private String stuClass;

    //省略了无参构造，getter/setter等方法

2.有实体类Subject

public class Subject {
    //id为主键
    private Integer id;
    private Exam exam;
    private String question;
    private String answer;
    private int score;

    //省略了无参构造，getter/setter等方法

3.有实体类AnswerSubject

public class AnswerExam {
    //使用Sstudent的id和Subject的id作为联合主键
    private Student student;
    private Subject subject;
    private String stuAnswer;

    ///省略了无参构造，getter/setter等方法

4.请问，在AnswerSubject.hbm.xml该怎么配置联合主键？已知知识如下：

    <!-- 配置联合主键 -->
    <composite-id name="id">
        <key-property name="student"
            type="com.abc.entities.Student">
            <column name="STUDENT" not-null="true">
            </column>
        </key-property>
        <key-property name="subject"
            type="com.abc.entities.Subject">
            <column name="SUBJECT" not-null="true">
            </column>
        </key-property>
    </composite-id>

    <property name="stuAnswer" type="java.lang.String">
        <column name="STUANSWER" />
    </property>

</class>