EDIT

package eu.etaxonomy.cdm.vaadin.ui;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;

import com.vaadin.annotations.VaadinServletConfiguration;

import com.vaadin.server.VaadinRequest;

import com.vaadin.server.VaadinServlet;

import com.vaadin.ui.Button;

import com.vaadin.ui.Button.ClickEvent;

import com.vaadin.ui.Label;

import com.vaadin.ui.UI;

import com.vaadin.ui.VerticalLayout;

@Theme("mytheme")

@SuppressWarnings("serial")

public class CheckTestUI extends UI {

    @WebServlet(value = {"/app-test/*", "/VAADIN/*"}, asyncSupported = true)

    @VaadinServletConfiguration(productionMode = false, ui = CheckTestUI.class, widgetset = "eu.etaxonomy.cdm.vaadin.AppWidgetSet")

    public static class Servlet extends VaadinServlet {

    }

    @Override

    protected void init(VaadinRequest request) {

        final VerticalLayout layout = new VerticalLayout();

        layout.setMargin(true);

        setContent(layout);

        Button button = new Button("Click Me");

        button.addClickListener(new Button.ClickListener() {

            @Override

            public void buttonClick(ClickEvent event) {

                layout.addComponent(new Label("Thank you for clicking"));

            }

        });

        layout.addComponent(button);

    }

}