Request Handlers

Request handlers are useful for catching request parameters or generating dynamic content, such as HTML, images, PDF, or other content. You can provide HTTP content also with stream resources, as described in “Stream Resources”. The stream resources, however, are only usable from within a Vaadin application, such as in an Image component. Request handlers allow responding to HTTP requests made with the application URL, including GET or POST parameters. You could also use a separate servlet to generate dynamic content, but a request handler is associated with the user session and it can easily access data associated with the session and the user.

To handle requests, you need to implement the RequestHandler interface. The handleRequest() method gets the session, request, and response objects as parameters.

If the handler writes a response, it must return true. This stops running other possible request handlers. Otherwise, it should return false so that another handler could return a response. Eventually, if no other handler writes a response, a UI will be created and initialized.

In the following example, we catch requests for a sub-path in the URL for the servlet and write a plain text response. The servlet path consists of the context path and the servlet (sub-)path. Any additional path is passed to the request handler in the pathInfo of the request. For example, if the full path is /myapp/myui/rhexample, the path info will be /rhexample. Also, request parameters are available.

Java

  1. // A request handler for generating some content
  2. VaadinSession.getCurrent().addRequestHandler(
  3.         new RequestHandler() {
  4.     @Override
  5.     public boolean handleRequest(VaadinSession session,
  6.                                  VaadinRequest request,
  7.                                  VaadinResponse response)
  8.             throws IOException {
  9.         if ("/rhexample".equals(request.getPathInfo())) {
  10.             // Generate a plain text document
  11.             response.setContentType("text/plain");
  12.             response.getWriter().append(
  13.                "Here's some dynamically generated content.\n");
  14.             response.getWriter().format(Locale.ENGLISH,
  15.                "Time: %Tc\n", new Date());
  17.             // Use shared session data
  18.             response.getWriter().format("Session data: %s\n",
  19.                 session.getAttribute("mydata"));
  21.             return true; // We wrote a response
  22.         } else
  23.             return false; // No response was written
  24.     }
  25. });

A request handler can be used by embedding it in a page or opening a new page with a link or a button. In the following example, we pass some data to the handler through a session attribute.

Java

  1. // Input some shared data in the session
  2. TextField dataInput = new TextField("Some data");
  3. dataInput.addValueChangeListener(event ->
  4.     VaadinSession.getCurrent().setAttribute("mydata",
  5.         event.getProperty().getValue()));
  6. dataInput.setValue("Here's some");
  8. // Determine the base path for the servlet
  9. String servletPath = VaadinServlet.getCurrent()
  10.         .getServletContext().getContextPath()
  11.         + "/book"; // Servlet
  13. // Display the page in a pop-up window
  14. Link open = new Link("Click to Show the Page",
  15.     new ExternalResource(servletPath + "/rhexample"),
  16.     "_blank", 500, 350, BorderStyle.DEFAULT);
  18. layout.addComponents(dataInput, open);