vaadin-framework/documentation/advanced/advanced-requesthandler.asciidoc

---
title: Request Handlers
order: 4
layout: page
---

[[advanced.requesthandler]]
= 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
<<../application/application-resources#application.resources.stream,"Stream
Resources">>. The stream resources, however, are only usable from within a
Vaadin application, such as in an [classname]#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 [interfacename]#RequestHandler#
interface. The [methodname]#handleRequest()# method gets the session, request,
and response objects as parameters.

If the handler writes a response, it must return [literal]#++true++#. This stops
running other possible request handlers. Otherwise, it should return
[literal]#++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 [parameter]#pathInfo# of the request. For example, if the
full path is [filename]#/myapp/myui/rhexample#, the path info will be
[filename]#/rhexample#. Also, request parameters are available.


[source, java]
----
// 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.


[source, java]
----
// 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);
----