please pay attention to your web.xml when config mapping  to it
you should specify complete address in mapping not /user.*
try it.

In here you can find  one sample :

web.xml
  <servlet>
    <servlet-name>FrontServlet</servlet-name>
    <servlet-class>ir.nsy.dlib.com.frontcontroller.FrontServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>FrontServlet</servlet-name>
    <url-pattern>/frontServlet</url-pattern>
  </servlet-mapping>

the code of servlet
public class FrontServlet extends HttpServlet  {
  //private static final String CONTENT_TYPE = "text/html; charset=windows-1252";

  public void init(ServletConfig config) throws ServletException {
    super.init(config);
  }

  public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    processRequest(request , response);
    try  {
      RequestContext ctx = new RequestContextImp(request, response);
      ctx.initialize(request);
      ctx.setRequest(request);
      ctx.setResponse(response);
      ctx.setServletContext(getServletContext());
      WebCommandFactory.getApplicationControllerCommand(ctx.getCommandName()).handleRequest(ctx);
    } catch (Exception ex)  {
      ex.printStackTrace();
    } 
  }

  public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    processRequest(request , response);
    try  {
      RequestContext ctx = new RequestContextImp();
      ctx.initialize(request);
      ctx.setRequest(request);
      ctx.setResponse(response);
      ctx.setServletContext(getServletContext());
      WebCommandFactory.getApplicationControllerCommand(ctx.getCommandName()).handleRequest(ctx);
    } catch (Exception ex)  {
      ex.printStackTrace();
    } 
  }
  private void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("UTF-8");
  }
for navigate to specific jsp  do like down:
  static class DispatcherViewImpl implements DispatcherView {
    public void dispatch(HttpServletRequest request, HttpServletResponse response, final String page, ServletContext ctx)
              throws javax.servlet.ServletException, java.io.IOException {
                try  {
                  RequestDispatcher dispatcher = ctx.getRequestDispatcher(page);
                  dispatcher.forward(request, response);
                } catch (Exception ex)  {
                  ex.printStackTrace();
                }
              }
  }

Thanks U for your help,and favour, but these are all the things that I've done before.
I've also tried thr filtering and j_security_check for my login form.
I believe that this might be a problem of my web  server, or sth else(I really do not know!).
But at least you're the one who helped a lot.
Thanks you more and more, and waiting to hear -READ I BELIEVE!- from U more and soon.