RequestDispatcher semantics (JRun bug?)

[Oliver Burn]

I am currently doing an evaluation of JRun 2.3 (build 152) for use on our
project. Our architecture requires the use of the
javax.servlet.RequestDispatcher.forward() method. I have been prototyping using
the forward() method with JRun and have come across the problem that I cannot
forward a request back to the same Servlet (with different parameters). I am not
sure if this is a bug with JRun or my misunderstanding of the Servlet 2.1
specification. Reading the Servlet 2.1 API it states about the forward() method:

  You cannot use this method if a ServletOutputStream object or PrintWriter
  object has been obtained from the response. In that case, the method throws an
  IllegalStateException.

It does not state that a request cannot be forwarded back to the same Servlet.
However JRun is throwing an exception stating that you cannot forward to the
same URI. My questions are:
o Is this a JRun bug or am I completely hosed?
o Does anybody have a work around?
o Why would JRun want to have this restriction? It prevents (valid) recursive
  programs.

I have attached the test Servlet I wrote. Interestingly, if you call it with the
URL 
"http://localhost/servlet/jruntest.ForwardTest/different?forward=/servlet/jruntest.ForwardTest"
it complains about the same URI even though they are different!

Any insights appreciated,
Oliver

===================== ForwardTest.java ===================================
package jruntest;

import java.io.*;
import java.util.Enumeration;
import javax.servlet.*;
import javax.servlet.http.*;

public class ForwardTest
   extends javax.servlet.http.HttpServlet
{
   public void doPost (HttpServletRequest req, HttpServletResponse res)
      throws ServletException, IOException
   {
      doGet(req, res);
   }

   public void doGet (HttpServletRequest req, HttpServletResponse res)
      throws ServletException, IOException
   {
      // Check if forward
      String forward = req.getParameter("forward");
      if (forward != null) {
         RequestDispatcher rd = getServletConfig().getServletContext()
            .getRequestDispatcher(forward);
         if (rd == null) {
            PrintWriter out = startPage(res);
            out.println("Unable to get dispatcher for <B>" + forward + "</B>");
            endPage(out);
         }
         else {
            System.err.println("Forwarding to " + forward);
            try {
               rd.forward(req, res);
            }
            catch (Exception ex) {
               PrintWriter out = startPage(res);
               out.println("Failed in forward to <B>" + forward + "</B>");
               out.println("when my current URI is " + req.getRequestURI());
               out.println("<pre>");
               ex.printStackTrace(out);
               out.println("</pre>");
               endPage(out);
            }
            System.err.println("Back from forward to " + forward);
         }
      }
      else {
         PrintWriter out = startPage(res);

         out.println("Request information:");
         out.println("<pre>");
         print(out, "Request URL", HttpUtils.getRequestURL(req).toString());
         print(out, "Request URI", req.getRequestURI());
         print(out, "Servlet path", req.getServletPath());
         print(out, "Query string", req.getQueryString());
         out.println("</pre>");

         Enumeration e = req.getParameterNames();
         if (e.hasMoreElements()) {
            out.println("<h1>Servlet parameters (Single Value style):</h1>");
            out.println("<pre>");
            while (e.hasMoreElements()) {
               String name = (String)e.nextElement();
               out.println(" " + name + " = " + req.getParameter(name));
            }
            out.println("</pre>");
         }

         endPage(out);
      }
   }

   private void print(PrintWriter out, String name, String value)
      throws IOException
   {
      out.print(" " + name + ": ");
      out.println(value == null ? "&lt;none&gt;" : value);
   }

   private void print(PrintWriter out, String name, int value)
      throws IOException
   {
      out.print(" " + name + ": ");
      if (value == -1) {
         out.println("&lt;none&gt;");
      }
      else {
         out.println(value);
      }
   }

   private PrintWriter startPage(HttpServletResponse res)
      throws IOException
   {
      res.setContentType("text/html");
      final PrintWriter out = res.getWriter();
      out.println("<html><head>");
      out.println("<title>ForwardTest Output</title>");
      out.println("</head>");
      out.println("<body>");
      out.println("<h1>ForwardTest Output</h1>");
      return out;
   }

   private void endPage(PrintWriter out)
   {
      out.println("</body></html>");
   }

   public String getServletInfo()
   {
      return "A servlet that tests request forwarding";
   }
}

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