vaadin-framework/documentation/articles/CustomizingTheStartupPageInAnApplication.asciidoc

---
title: Customizing The Startup Page In An Application
order: 25
layout: page
---

[[customizing-the-startup-page-in-an-application]]
= Customizing the startup page in an application

In Vaadin 6, the startup page - used to bootstrap a new Vaadin UI
instance in the browser - was generated as a monolithic chunk of HTML
and was not easily customizable. In Vaadin 7, we added a new facility
for registering special _bootstrap listeners_ that are invoked before
the bootstrap response is sent. In addition, instead of bare HTML in a
string, the response is now built as a DOM tree that is easy to
manipulate programmatically.

Here's an example of a simple bootstrap listener:

[source,java]
....
import org.jsoup.nodes.Comment;
import org.jsoup.nodes.Element;
import org.jsoup.nodes.Node;
import org.jsoup.parser.Tag;

// ...

new BootstrapListener() {
  @Override
  public void modifyBootstrapPage(BootstrapPageResponse response) {
    response.getDocument().body().appendChild(new Comment("Powered by Vaadin!", ""));
  }

  @Override
  public void modifyBootstrapFragment(BootstrapFragmentResponse response) {
    // Wrap the fragment in a custom div element
    Element myDiv = new Element(Tag.valueOf("div"), "");
    List<Node> nodes = response.getFragmentNodes();
    for(Node node : nodes) {
      myDiv.appendChild(node);
    }
    nodes.clear();
    nodes.add(myDiv);
  }
}
....

The HTML library we use is http://jsoup.org/[jsoup]. It provides a very
convenient API for traversing, manipulating and extracting data from a
DOM, and is HTML5 compliant.

The `BootstrapListener` interface contains two methods, one of which is
usually left empty. This is because a Vaadin application can be either
stand-alone, in which case it "owns" the whole page its UI resides in,
or embedded, such as a portlet, in which case it does not control the
content of the page it is embedded in.

The `modifyBootstrapFragment` method is called in both cases. It
receives a `BootstrapFragmentResponse` that represents the HTML fragment
that is inserted in the host page, whether the page is controlled by
Vaadin or not. Hence, you only need to implement this method if you do
not care about the host page, whether your application is embedded or
standalone.

The `modifyBootstrapPage` method is called with a
`BootstrapPageResponse` argument that represents the whole bootstrap
page, including the fragment mentioned above. Thus, it is only invoked
when the application is standalone and actually responsible for
generating the page. This method allows you to, for instance, add things
to the `head` element. The `BootstrapPageResponse` class also allows
setting arbitrary HTTP response headers:

[source,java]
....
public void modifyBootstrapPage(BootstrapPageResponse response) {
  response.setHeader("X-Powered-By", "Vaadin 7");
}
....

But how and where should the bootstrap listeners be registered? It
should be only once per session, and right in the beginning, so that
they are already added when the first response is sent.

To do that you should write a custom servlet that extends
`VaadinServlet`, or a custom portlet extending `VaadinPortlet`, and a
session init listener that adds the bootstrap listener to the new
session.

[source,java]
....
class MyVaadinServlet extends VaadinServlet {
  @Override
  protected void servletInitialized() throws ServletException {
    super.servletInitialized();
    getService().addSessionInitListener(new SessionInitListener() {
      @Override
      public void sessionInit(SessionInitEvent event) {
        event.getSession().addBootstrapListener(listener);
      }
    });
  }
}

// Or...

class MyVaadinPortlet extends VaadinPortlet {
  @Override
  protected void portletInitialized() throws PortletException {
    super.portletInitialized();
    getService().addSessionInitListener(new SessionInitListener() {
      @Override
      public void sessionInit(SessionInitEvent event) {
        event.getSession().addBootstrapListener(listener);
      }
    });
  }
}
....