RequestHandler.handleRequest() before UI.init()?

RequestHandler.handleRequest() before UI.init()?

I have Vaadin 7 application. I define vaadin servlet as a session
listener, register it on servletInitialized(), and in sessionInit() I add
request handler using session.addRequestHandler(). Request handler defines
some resources to be used to build components in UI. Components are built
in UI.init().
My problem is the fact that UI.init() is called before
requestHandler.handleRequest(). How can I add request listener which will
be triggered before initializing UI?