Is it possible to redirect in a filter After the doChain?

Question

Hello all,
I have a filter configured...
  <filter>
    <filter-name>Hibernate Session Filter</filter-name>
    <filter-class>com.test.HibernateSessionFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>Hibernate Session Filter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
  </filter-mapping>... in my Web.xml file.
I'm using this filter to implement the Hibernate "Session in View" notion. This is where a suitable interceptor (filter for me!) grabs the request /response on the way in and starts a hibernate session... then your servlets do whatever it is they do (which may/may not involve hibernate)... then on the way out the filter commits the transaction and closes the session.
More can be read about this here...
http://www.hibernate.org/43.html
... but the most interesting bit is really the filter itself:
public class HibernateSessionRequestFilter implements Filter {
    private static Log log = LogFactory.getLog(HibernateSessionRequestFilter.class);
    private SessionFactory sf;
    public void doFilter(ServletRequest request,
                         ServletResponse response,
                         FilterChain chain)
            throws IOException, ServletException {
        try {
            log.debug("Starting a database transaction");
            sf.getCurrentSession().beginTransaction();
            // Call the next filter (continue request processing)
            chain.doFilter(request, response);
            // Commit and cleanup
            log.debug("Committing the database transaction");
            sf.getCurrentSession().getTransaction().commit();
        } catch (StaleObjectStateException staleEx) {
            log.error("This interceptor does not implement optimistic concurrency control!");
            log.error("Your application will not work until you add compensation actions!");
            // Rollback, close everything, possibly compensate for any permanent changes
            // during the conversation, and finally restart business conversation. Maybe
            // give the user of the application a chance to merge some of his work with
            // fresh data... what you do here depends on your applications design.
            throw staleEx;
        } catch (Throwable ex) {
            // Rollback only
            ex.printStackTrace();
            try {
                if (sf.getCurrentSession().getTransaction().isActive()) {
                    log.debug("Trying to rollback database transaction after exception");
                    sf.getCurrentSession().getTransaction().rollback();
            } catch (Throwable rbEx) {
                log.error("Could not rollback transaction after exception!", rbEx);
            // Let others handle it... maybe another interceptor for exceptions?
            throw new ServletException(ex);
    public void init(FilterConfig filterConfig) throws ServletException {
        log.debug("Initializing filter...");
        log.debug("Obtaining SessionFactory from static HibernateUtil singleton");
        sf = HibernateUtil.getSessionFactory();
    public void destroy() {}
}This all works well except when there's a hibernate exception! When there is an exception I'd like to trap the error and redirect the user to an error page showing some text.. or do something informative. The comment in the sample code "// Let others handle it... " ain't very helpful!
When I try to do a redirect/forward I get illegalStateExceptions (?because the response is already committed??) Is there some way I can manage the response being committed? I'm not very clear on how the header states/'response flush' actually happens. I've had a rummage here and see many similar posts, but they all seem to refer to redirection Before the doChain method... whereas I want to redirect after that method call.
Thanks in advance for any advice.

Answer 1

Nope! Not yet. I tried doing a simple ResponseWrapper (just to mess with the response output, inserting text here and there...
import java.io.PrintWriter;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;
class HtmlResponseWrapper
extends HttpServletResponseWrapper
  // A response must provide a PrintWriter and a ServletOutputStream.  So we
  // create new ones here, that uses our HtmlServletOutputStream instead of the
  // default ones...
  private PrintWriter printWriter;
  private HtmlServletOutputStream servletOutputStream;
  public HtmlResponseWrapper(
      ServletResponse servletResponse,
      ServletRequest request,
      String menuInsertMarker,
      String crumbInsertMarker)
  throws java.io.IOException
    // Run the constructor on HttpServletResponseWrapper
    super((HttpServletResponse) servletResponse);
    // Generate the appropriate ServletOutputStream/PrintWriter from the inbound
    // response...
    servletOutputStream = new HtmlServletOutputStream(
        servletResponse.getOutputStream(),
        request,
        menuInsertMarker,
        crumbInsertMarker);
    printWriter = new PrintWriter(servletOutputStream);
  public ServletOutputStream getOutputStream() throws java.io.IOException {
    return servletOutputStream;
  public PrintWriter getWriter() throws java.io.IOException {
    return printWriter;
}... and that worked well. There's lots of examples of this kind of thing on the web. What I need to do now is understand when status changes, buffers get flushed etc. so I can override more than just the getOutputStream and getWriter methods, to keep my request alive until it gets back to the hibernate filter.
Hoped to do it over Christmas... but maybe in the New Year!
Once again - if anyone has any pointers - much appreciated!
Rgds,
T.