关于 struts的初级问题（修正）

报错：  
type  Exception  report  
 
message    
 
description  The  server  encountered  an  internal  error  ()  that  prevented  it  from  fulfilling  this  request.  
 
exception    
 
java.lang.NullPointerException  
           org.apache.struts.tiles.xmlDefinition.FactorySet.getDefinition(FactorySet.java:156)  
           org.apache.struts.tiles.definition.ComponentDefinitionsFactoryWrapper.getDefinition(ComponentDefinitionsFactoryWrapper.java:124)  
           org.apache.struts.tiles.TilesRequestProcessor.processTilesDefinition(TilesRequestProcessor.java:180)  
           org.apache.struts.tiles.TilesRequestProcessor.processForwardConfig(TilesRequestProcessor.java:309)  
           org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:279)  
           org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)  
           org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)  
           javax.servlet.http.HttpServlet.service(HttpServlet.java:709)  
           javax.servlet.http.HttpServlet.service(HttpServlet.java:802)  
 
 
note  The  full  stack  trace  of  the  root  cause  is  available  in  the  Apache  Tomcat/5.0.28  logs.  
 
源文件：  
LoginAction.java---------------------------------------  
/*  
 *  创建日期  2007-9-13  
 *  
 *  TODO  要更改此生成的文件的模板，请转至  
 *  窗口  －  首选项  －  Java  －  代码样式  －  代码模板  
 */  
package  nwpu.edu.eclipse.struts.action;  
 
import  org.apache.struts.action.ActionForm;  
 
import  javax.servlet.http.HttpServletRequest;  
import  javax.servlet.http.HttpServletResponse;  
import  org.apache.struts.action.Action;  
import  org.apache.struts.action.ActionForm;  
import  org.apache.struts.action.ActionForward;  
import  org.apache.struts.action.ActionMapping;  
 
import  nwpu.edu.eclipse.struts.form.LoginForm;  
/**  
 *  @author  pk  
 *  
 *  TODO  要更改此生成的类型注释的模板，请转至  
 *  窗口  －  首选项  －  Java  －  代码样式  －  代码模板  
 */  
public  class  LoginAction  extends  Action  {  
 
           private  static  final  long  serialVersionUID  =  1L;  
 
             
           public  ActionForward  execute(ActionMapping  mapping,    
                                                                             ActionForm  form,    
                                                                                                 HttpServletRequest  request,  
                                                                                                 HttpServletResponse  response)  
           throws  Exception{  
                         
                       String  encoding  =  request.getCharacterEncoding();  
                         
                       LoginForm  actionForm  =(LoginForm)form;  
                       String  userid  =  actionForm.getUsername();  
                       String  password  =  actionForm.getPassword();  
                         
                       if(userid.equals("user")&&password.equals("123456"))  
                       {  
                             return(mapping.findForward("success"));              
                       }  
                       else  
                       {  
                                   return  (mapping.findForward("fail"));              
                                     
                       }  
 
           }  
}  
------------------------  
struts-config.xml  
<?xml  version="1.0"?>  
<!DOCTYPE  struts-config  PUBLIC  "-//Apache  Software  Foundation//DTD  Struts  Configuration  1.1//EN"  "http://jakarta.apache.org/struts/dtds/struts-config_1_1.dtd">  
<struts-config>  
   <data-sources>  
   </data-sources>  
   <form-beans>  
       <form-bean  name="loginForm"  type="nwpu.edu.eclipse.struts.form.LoginForm"/>  
   </form-beans>  
   <global-exceptions>  
   </global-exceptions>  
   <global-forwards>  
   </global-forwards>  
   <action-mappings>  
<!--<action  forward="/eclipse_Struts.jsp"  path="/index">  
       </action>  
       <action  forward="/welcome.jsp"  path="/welcome">  
       </action>-->  
         
       <action  path="/logincheck"    
                       name="loginForm"    
                       type="nwpu.edu.eclipse.struts.action.LoginAction"    
                       scope="request"    
                       validate="true">  
               <forward  name="success"  path="/eclipse_Struts.jsp"></forward>    
               <forward  name="fail"  path="/error.jsp"></forward>        
       </action>  
         
   </action-mappings>  
   <controller  processorClass="org.apache.struts.tiles.TilesRequestProcessor"/>  
   <message-resources  parameter="MessageResources"/>  
   <plug-in  className="org.apache.struts.tiles.TilesPlugin">  
       <set-property  property="moduleAware"  value="true"/>  
   </plug-in>  
   <plug-in  className="org.apache.struts.validator.ValidatorPlugIn">  
       <set-property  property="pathnames"  value="/WEB-INF/validator-rules.xml,  /WEB-INF/validation.xml"/>  
   </plug-in>  
</struts-config>  
---------  
请大家看看什么原因？谢谢