To extend the VaadinService class you need to override VaadinServlet.createServletService() method which is done by introducing your own VaadinServlet into the app. Your new servlet will automatically take precedence over the old VaadinServlet.

Just add this class to your project:

@WebServlet(urlPatterns = "/*", asyncSupported = true)
public class MyServlet extends VaadinServlet {

  @Override
  protected VaadinServletService createServletService(DeploymentConfiguration deploymentConfiguration) throws ServiceException {
    VaadinServletService service = new VaadinServletService(this,
            deploymentConfiguration) {
      // your own stuff
    };
    service.init();
    return service;
  }
}

You can for example override VaadinServletService.createInstantiator() to provide your own Instantiator as documented here: Cached Vaadin Routes. However, for this particular case it’s better to use InstantiatorFactory as documented at Vaadin: Custom Instantiator.