Catching Exceptions in Filter

I have a strange problem when trying to catch Exceptions in my filter. I am supposed to run the chain.doFilter() method and catch any exception thereof and handle them in the Filter. But even if the Servlets throw a ServletException they are not caught in the try - catch block i have set up in the filter. However a StringIndexOutOfBoundsException is caught in that block. 

My Filter.doFilter code is below:

public void doFilter(ServletRequest servletRequest,
        ServletResponse response, FilterChain chain)
        throws IOException, ServletException
    {
            String targetPage = null;
            boolean foundError = false;

            try
            {
                logger.info(">>>>>>>>>>>> Let the servlet run ");                
                
                //let the servlet run
                chain.doFilter(request, response);
                
                logger.info(">>>>>>>>>>>> In Filter.....This should not be shown ");

            }
            catch (Exception servex)
            {
                foundError = true;
                    logger.info(">>>>>>>>>>>> Caught exception " + servex);
                
                    //this is unhandled exception. got to system error page
                targetPage = systemErrorPage;

                logger.error(servex.getMessage(), servex);
            }

        }



My servlet doGet() code is simply: 

public void doGet(HttpServletRequest req, HttpServletResponse res)
    throws IOException, ServletException {
        
        String str = "abc";
        //String def = str.substring(5,10);
        
        if(true) 
            throw new ServletException("ErrorMessage");
        
    }

Any ideas? 

Thanks a lot.

Affan

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