null id in cn.csdn.domain.UserBook entry (don't flush the Session after an excep

这个错误怎么解决啊？
--------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException: org.hibernate.AssertionFailure: null id in cn.csdn.domain.UserBook entry (don't flush the Session after an exception occurs)
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:535)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:433)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
cn.csdn.util.EncodingFilter.doFilter(EncodingFilter.java:20)
root cause org.hibernate.AssertionFailure: null id in cn.csdn.domain.UserBook entry (don't flush the Session after an exception occurs)
org.hibernate.event.def.DefaultFlushEntityEventListener.checkId(DefaultFlushEntityEventListener.java:78)
org.hibernate.event.def.DefaultFlushEntityEventListener.getValues(DefaultFlushEntityEventListener.java:187)
org.hibernate.event.def.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:143)
org.hibernate.event.def.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:219)
org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:99)
org.hibernate.event.def.DefaultAutoFlushEventListener.onAutoFlush(DefaultAutoFlushEventListener.java:58)
org.hibernate.impl.SessionImpl.autoFlushIfRequired(SessionImpl.java:996)
org.hibernate.impl.SessionImpl.listCustomQuery(SessionImpl.java:1717)
org.hibernate.impl.AbstractSessionImpl.list(AbstractSessionImpl.java:165)
org.hibernate.impl.SQLQueryImpl.list(SQLQueryImpl.java:175)
cn.csdn.dao.BookDaoImpl.doSelects(BookDaoImpl.java:226)
cn.csdn.action.PageAction.up(PageAction.java:55)
sun.reflect.GeneratedMethodAccessor158.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
org.apache.struts.actions.DispatchAction.dispatchMethod(DispatchAction.java:270)
org.apache.struts.actions.DispatchAction.execute(DispatchAction.java:187)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
cn.csdn.util.EncodingFilter.doFilter(EncodingFilter.java:20)
note The full stack trace of the root cause is available in the Apache Tomcat/7.0.0 logs.
--------------------------------------------------------------------------------Apache Tomcat/7.0.0