public class Student {
    private Integer studentId;
    private String studentName;
    private Classes classes;
}

  public class Classes{
    private Integer classId;
    private String className;
    private Set<Student> students = new HashSet<Student>();
}

<hibernate-mapping package="wxm.beans">
    <class name="Student" table="student">
        <id name="studentId" type="integer" column="student_id">
            <generator class="identity" />
        </id>
        <property name="studentName" length="20" not-null="true"
            column="student_name" />
        <property name="perform" />

        <many-to-one name="classes" column="class_id"
             outer-join="true" class="wxm.beans.Classes" />

    </class>
</hibernate-mapping>

<hibernate-mapping package="wxm.beans">
    <class name="Classes" table="t_class">
        <id name="classId" type="integer" column="class_id">
            <generator class="identity" />
        </id>
        <property name="className" length="20" not-null="true"
            column="class_name" />

        <set name="students" outer-join="true" inverse="true">
            <key column="class_id" />
            <one-to-many class="wxm.beans.Student" />
        </set>

    </class>
</hibernate-mapping>

@Test
public void save() {
    Classes claz = new Classes("英语班");
    classService.addClass(claz);

    Student student = new Student("王小二");
    student.setClasses(classService.getClass(1));
    student.setPerform(55.5f);
    studentService.addStudent(student);
        
    }

@Test
    public void delClass() {
        classService.getClass(1).getStudents().remove(
                studentService.getStudent(1));
        classService.delClass(1);
    }