Hibernate4 复合主键使用Ehcahe无效呢

Hibernate4 中得实体配置如下，但查询的时候无效果，非复合主键的实体是可以正常进入缓存的。

 */
@Embeddable 
@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE,region="com.kmp.business.entity.StudentClassesPK")
public class StudentClassesPK implements Serializable{
	private static final long serialVersionUID = 8086967036014159916L;
	
	@ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "student_id",referencedColumnName="id",nullable=true)  
	private Student student;
	
	@ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "classes_id",referencedColumnName="id",nullable=true )  
	private Classes classes;
<p class="indent">

@Entity
@Table(name="T_ClassesStudent")
@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE,region="com.kmp.business.entity.StudentClasses")
public class StudentClasses extends BaseEntity{
	private static final long serialVersionUID = -6300584354851171783L;
	
	@EmbeddedId
	StudentClassesPK pk = new StudentClassesPK();
	
<p class="indent">