更新数据的时候无法更新提示如下信息，不知道有没有人遇到过
使用JSF框架做的网页，在更新数据的时候无法更新提示如下信息，不知道有没有人遇到过，这是什么原因呢？

2011-4-14 18:12:56 javax.faces.event.MethodExpressionActionListener processAction
严重: Received 'javax.persistence.RollbackException' when invoking action listener '#{stuBean.updateStu}' for component 'j_id_jsp_1261413883_23'
2011-4-14 18:12:56 javax.faces.event.MethodExpressionActionListener processAction
严重: javax.persistence.RollbackException: Error while commiting the transaction
at org.hibernate.ejb.TransactionImpl.commit(TransactionImpl.java:71)
at blog.csdn.course.EntityManagerHelper.commit(EntityManagerHelper.java:46)
at blog.csdn.bean.StuBean.updateStu(StuBean.java:78)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.el.parser.AstValue.invoke(AstValue.java:191)
at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:276)
at org.apache.jasper.el.JspMethodExpression.invoke(JspMethodExpression.java:68)
at javax.faces.event.MethodExpressionActionListener.processAction(MethodExpressionActionListener.java:88)
at javax.faces.event.ActionEvent.processListener(ActionEvent.java:77)
at javax.faces.component.UIComponentBase.broadcast(UIComponentBase.java:743)
at javax.faces.component.UICommand.broadcast(UICommand.java:368)
at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:447)
at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:752)
at com.sun.faces.lifecycle.InvokeApplicationPhase.execute(InvokeApplicationPhase.java:97)
at com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:251)
at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:117)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:244)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:619)
Caused by: org.hibernate.HibernateException: identifier of an instance of blog.csdn.student.Major was altered from 01 to 02
at org.hibernate.event.def.DefaultFlushEntityEventListener.checkId(DefaultFlushEntityEventListener.java:81)
at org.hibernate.event.def.DefaultFlushEntityEventListener.getValues(DefaultFlushEntityEventListener.java:187)