struts+hibernate的问题

为什么我做的时候有 报错?
%%%% Error Creating SessionFactory %%%%
org.hibernate.MappingException: could not instantiate id generator
at org.hibernate.id.IdentifierGeneratorFactory.create(IdentifierGeneratorFactory.java:92)
at org.hibernate.mapping.SimpleValue.createIdentifierGenerator(SimpleValue.java:151)
at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:178)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1005)
at sessionfactory.HibernateSessionFactory.rebuildSessionFactory(HibernateSessionFactory.java:60)
at sessionfactory.HibernateSessionFactory.getSession(HibernateSessionFactory.java:43)
at db.BaseHibernateDAO.getSession(BaseHibernateDAO.java:14)
at db.StudentDAO.findById(StudentDAO.java:49)
at com.yourcompany.struts.action.AddAction.execute(AddAction.java:44)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
Caused by: org.hibernate.MappingException: could not interpret id generator strategy: assign
at org.hibernate.id.IdentifierGeneratorFactory.getIdentifierGeneratorClass(IdentifierGeneratorFactory.java:103)
at org.hibernate.id.IdentifierGeneratorFactory.create(IdentifierGeneratorFactory.java:86)
... 28 more 
我用的是myeclipse里的
HibernateSessionFactory.java文件如下
public class HibernateSessionFactory {    private static String CONFIG_FILE_LOCATION = "/hibernate.cfg.xml";
private static final ThreadLocal<Session> threadLocal = new ThreadLocal<Session>();
    private  static Configuration configuration = new Configuration();
    private static org.hibernate.SessionFactory sessionFactory;
    private static String configFile = CONFIG_FILE_LOCATION;    private HibernateSessionFactory() {
    }
    public static Session getSession() throws HibernateException {
        Session session = (Session) threadLocal.get(); if (session == null || !session.isOpen()) {
if (sessionFactory == null) {
rebuildSessionFactory();
}
session = (sessionFactory != null) ? sessionFactory.openSession()
: null;
threadLocal.set(session);
}        return session;
    }
public static void rebuildSessionFactory() {
try {
configuration.configure(configFile);
sessionFactory = configuration.buildSessionFactory();
} catch (Exception e) {
System.err
.println("%%%% Error Creating SessionFactory %%%%");
e.printStackTrace();
}
}    public static void closeSession() throws HibernateException {
        Session session = (Session) threadLocal.get();
        threadLocal.set(null);        if (session != null) {
            session.close();
        }
    } public static org.hibernate.SessionFactory getSessionFactory() {
return sessionFactory;
} public static void setConfigFile(String configFile) {
HibernateSessionFactory.configFile = configFile;
sessionFactory = null;
} public static Configuration getConfiguration() {
return configuration;
}}action文件如下
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) {
Employee employeeForm = (Employee) form;// TODO Auto-generated method stub
EmployeeDAO employeedao=new EmployeeDAO();
//List list=employeedao.findById(employeeForm.getEmployeeId());
ActionMessages errors=new ActionMessages();
List list=(List) employeedao.findById(employeeForm.getEmployeeId());
if(list.size()>0){
employeedao.save(employeeForm);
return mapping.findForward("sucess");
}else{
return mapping.findForward("false");
} }