code: public void forward(String url, Object... args) {
          E.illegalArgumentIfNot(url.startsWith("/"), "forward URL must starts with single '/'");
          E.illegalArgumentIf(url.startsWith("//"), "forward URL must starts with single `/`");
          E.unexpectedIfNot(H.Method.GET == req().method(), "forward only support on HTTP GET request");
          uploads.clear();
          extraParams.clear();
          bodyParams = null;
          String target = S.fmt(url, args);
          urlPath = UrlPath.of(target);
          UndertowRequest req = $.cast(req());
          state = State.CREATED;
          req.forward(target);
          final RequestHandler requestHandler = router.getInvoker(H.Method.GET, url, this);
          requestHandler.handle(this);
      }
docs: Implement Java Servlet's `RequestDispatcher.forward(String)` semantic.
  
  This method has the following restrictions:
  
  1. the URL must be started with single `/`. i.e. it must be absolute PATH of current service.
  2. the request method must be `GET`.
  
  @param url
  the url template
  @param args
  the url argument