I've beeb tasked with writing a servlet that intercepts a call to and JSP in a specific directoy, check that the file exists and if it does just forwarding to that file, if if doesn't I'm to forward to a default JSP.

I've setup the web.xml as follows:

<servlet>
 <description>This is the description of my J2EE component</description>
 <display-name>This is the display name of my J2EE component</display-name>
 <servlet-name>CustomJSPListener</servlet-name>
 <servlet-class> ... CustomJSPListener</servlet-class>
 <load-on-startup>1</load-on-startup>
</servlet>
...
<servlet-mapping>
  <servlet-name>CustomJSPListener</servlet-name>
  <url-pattern>/custom/*</url-pattern>
</servlet-mapping>

And the doGet method of the servlet is as follows:

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        logger.debug(String.format("Intercepted a request for an item in the custom directory [%s]",request.getRequestURL().toString()));
        String requestUri = request.getRequestURI();

            // Check that the file name contains a text string
        if (requestUri.toLowerCase(Locale.UK).contains("someText")){
            logger.debug(String.format("We are interested in this file [%s]",requestUri));
            File file = new File(requestUri);
            boolean fileExists = file.exists();
            logger.debug(String.format("Checking to see if file [%s] exists [%s].",requestUri,fileExists));
                    // if the file exists just forward it to the file
            if (fileExists){
                getServletConfig().getServletContext().getRequestDispatcher(
                requestUri).forward(request,response);
            } else {
                    // Otherwise redirect to default.jsp
                getServletConfig().getServletContext().getRequestDispatcher(
                        "/custom/default.jsp").forward(request,response);
            }
        } else {
                    // We aren't responsible for checking this file exists just pass it on to the requeseted jsp
            getServletConfig().getServletContext().getRequestDispatcher(
                    requestUri).forward(request,response);          
        }
    }

This seems to result in an error 500 from tomcat, I think this is because the servlet is redirecting to the same folder which is then being intercepted again by the servlet, resulting in an infinite loop.

Is there a better way to do this? I'm lead to believe that I could use filters to do this, but I don't know very much about them.