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.

// A request handler for generating some content
VaadinSession.getCurrent().addRequestHandler(
        new RequestHandler() {
    @Override
    public boolean handleRequest(VaadinSession session,
                                 VaadinRequest request,
                                 VaadinResponse response)
            throws IOException {
        if ("/rhexample".equals(request.getPathInfo())) {
            // Generate a plain text document
            response.setContentType("text/plain");
            response.getWriter().append(
               "Here's some dynamically generated content.\n");
            response.getWriter().format(Locale.ENGLISH,
               "Time: %Tc\n", new Date());

            // Use shared session data
            response.getWriter().format("Session data: %s\n",
                session.getAttribute("mydata"));

            return true; // We wrote a response
        } else
            return false; // No response was written
    }
});

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.

// Input some shared data in the session
TextField dataInput = new TextField("Some data");
dataInput.addValueChangeListener(event ->
    VaadinSession.getCurrent().setAttribute("mydata",
        event.getProperty().getValue()));
dataInput.setValue("Here's some");

// Determine the base path for the servlet
String servletPath = VaadinServlet.getCurrent()
        .getServletContext().getContextPath()
        + "/book"; // Servlet

// Display the page in a pop-up window
Link open = new Link("Click to Show the Page",
    new ExternalResource(servletPath + "/rhexample"),
    "_blank", 500, 350, BorderStyle.DEFAULT);

layout.addComponents(dataInput, open);