problem with error mapping

Dear sir

I am using sun microsystem application server in which i am trying to deployee a
file in which i am using three file
1. first for user input as Calculate.html
2. second for calculate as Calculate.class
3. Third for customise ErrorServlet page as ErrorServlet.class

in html file i am writting like
<form action="http://localhost:8080//err_demo/servlet1">

and i am adding remaning 2 and 3 file in same context root as err_demo with alias
servlet1,servlet2 respectivly. Now what should i write in error mapping below option
Resource to be called
i am writting there
/servlet2/ErrorServlet

but when error is occuring calculate class is not calling this Error page

by the way calculate class thrwoing NumberFormatException

coding of these file as below
--------------------------------------------------------------------------------------

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import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
 
public class ErrorServlet extends HttpServlet{
   final String EXC="javax.servlet.error.exception";
   final String MSG="javax.servlet.error.message";
   final String ST="javax.servlet.error.status_code";
  
   public void doGet(HttpServletRequest req,HttpServletResponse res) throws ServletException,IOException{
     ServletContext sc=getServletContext();
     PrintWriter pw=res.getWriter();
     Exception exc=(Exception)req.getAttribute("EXC");
     Integer st_cd=(Integer)req.getAttribute("ST");
     String msg=(String)req.getAttribute("MSG");
     
     pw.println("<html>");
     pw.println("<body>");
     pw.println("<br>");
     pw.println("<h1>Sorry, an error has occured that has prevented the server from servicing your request</h1>");
     pw.println("<font size=5>");
     pw.println("<table align=center>");
     pw.println("<tr bgcolor=lightgrey>");
     pw.println("<td><b>Status code :</b></td><td>" + st_cd+"</td>");
     
     pw.println("</tr>");
     pw.println("<tr>");
     pw.println("<td><b>Type of Exception </b></td><td>" + exc.getClass() + "</td>");
     pw.println("</tr>");
     pw.println("<tr bgcolor=lightgray>");
     pw.println("<td><b>Message Description : </b></td><td" + msg + "</td>");
     
     String str=exc.toString()+st_cd.toString()+msg;
     sc.log("Exception occured ",exc);
 
     pw.println("</tr>");
     pw.println("</table>");
     pw.println("</font>");
 
     pw.println("<hr>");
     pw.println("<hr>");
     pw.println("<center><h1>Please try again....</h1></center>");
     pw.println("</body>");
     pw.println("</html>");
     }
 
  }

* When i am entering the number 5,Y in HTML page it should call that of error page
but unfortunately not i think that problem is with mapping process but what problem i
am unable to recognised that please tell me that what should i have to write in error mapping

with regard

Arun kumar maalik

Can you also post the error-page mapping in your web.xml ?