root cause

org.hibernate.MappingException: Unknown entity: com.zb.struts.db.User
 org.hibernate.impl.SessionFactoryImpl.getEntityPersister(SessionFactoryImpl.java:514)
 org.hibernate.impl.SessionImpl.getEntityPersister(SessionImpl.java:1302)
 org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:89)
 org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:186)
 org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
 org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:175)
 org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
 org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
 org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:530)
 org.hibernate.impl.SessionImpl.save(SessionImpl.java:518)
 org.hibernate.impl.SessionImpl.save(SessionImpl.java:514)
 com.zb.struts.action.User_hibernateAction.execute(User_hibernateAction.java:72)
 org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
 org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:226)
 org.apache.struts.action.ActionServlet.process(ActionServlet.java:1164)
 org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:415)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:802)