[[generating-dynamic-resources-based-on-uri-or-parameters]]
Generating dynamic resources based on URI or parameters
-------------------------------------------------------

You can dynamically generate responses based on e.g. query parameters by
creating your own `RequestHandler` and registering it with the session.

In this way, you can for instance create an image that draws a text,
given as a parameter to the image. This has been done in the example
below:

[source,java]
....
public class DynamicImageUI extends UI {
  public static final String IMAGE_URL = "myimage.png";

  private final RequestHandler requestHandler = new RequestHandler() {
    @Override
    public boolean handleRequest(VaadinSession session,
          VaadinRequest request, VaadinResponse response)
          throws IOException {
      if (("/" + IMAGE_URL).equals(request.getPathInfo())) {
        // Create an image, draw the "text" parameter to it and output
        // it to the browser.
        String text = request.getParameter("text");
        BufferedImage bi = new BufferedImage(100, 30,
                BufferedImage.TYPE_3BYTE_BGR);
        bi.getGraphics().drawChars(text.toCharArray(), 0,
                text.length(), 10, 20);
        response.setContentType("image/png");
        ImageIO.write(bi, "png", response.getOutputStream());

        return true;
      }
      // If the URL did not match our image URL, let the other request
      // handlers handle it
      return false;
    }
  };

  @Override
  public void init(VaadinRequest request) {
    Resource resource = new ExternalResource(IMAGE_URL + "?text=Hello!");

    getSession().addRequestHandler(requestHandler);

    // Add an image using the resource
    Image image = new Image("A dynamically generated image", resource);

    setContent(image);
  }

  @Override
  public void detach() {
    super.detach();

    // Clean up
    getSession().removeRequestHandler(requestHandler);
  }
}
....