一天没有解决的小问题，请教各位


jsp代码  
<script  language="javascript">  
var  nodes  =  new  Array();  
nodes[0]  =  new  Node(1,'zzzzz','','',0,'true');  
 
<%  
           List  nodelist  =  (List)request.getAttribute("nodes");  
           //session.removeAttribute("tree_folder");  
           int  index  =  1;  
           Iterator  roomIt  =  nodelist.iterator();  
           while(roomIt.hasNext()){  
                       Node  room  =  (Node)roomIt.next();  
                       int  parent  =  index;  
                       out.println("nodes["+index+"]  =  new  Node("+(index  +  1)  +",'"+room.getName()+"','"+room.getPath()+"','',0,'true');");  
                       index++;  
                       java.util.Iterator  casesIt  =  room.getChildren().iterator();  
                       while(casesIt.hasNext()){  
                                   Node  cases  =  (Node)casesIt.next();  
                                   out.println("nodes["+index+"]  =  new  Node("+(index  +  1)+",'"+cases.getName()+"','"+cases.getPath()+"','',"+parent+",'false');");  
                                   index++;  
                       }  
           }    
%>  
 
错误提示：  
HTTP  Status  500  -  
type  Exception  report  
message  
description  The  server  encountered  an  internal  error  ()  that  prevented  it  from  fulfilling  this  request.  
exception  
org.apache.jasper.JasperException:  Exception  in  JSP:  /mainframe/menulist.jsp:25  
 
22:              List  nodelist  =  (List)request.getAttribute("nodes");  
23:              //session.removeAttribute("tree_folder");  
24:              int  index  =  1;  
25:              Iterator  roomIt  =  nodelist.iterator();  
26:              while(roomIt.hasNext()){  
27:                          Node  room  =  (Node)roomIt.next();  
28:                          int  parent  =  index;  
 
java.lang.NullPointerException  
           org.apache.jsp.mainframe.menulist_jsp._jspService(menulist_jsp.java:76)  
           org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)  
           javax.servlet.http.HttpServlet.service(HttpServlet.java:802)  
           org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:334)  
           org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)  
           org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)  
           javax.servlet.http.HttpServlet.service(HttpServlet.java:802)  
 
note  The  full  stack  trace  of  the  root  cause  is  available  in  the  Apache  Tomcat/5.5.20  logs.  
Apache  Tomcat/5.5.20