MyUI.java

package org.test;

import java.util.Arrays;

import javax.servlet.annotation.WebServlet;

import com.vaadin.annotations.Theme;
import com.vaadin.annotations.VaadinServletConfiguration;
import com.vaadin.data.provider.ListDataProvider;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinServlet;
import com.vaadin.ui.Button;
import com.vaadin.ui.ComboBox;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;

/**
 * This UI is the application entry point. A UI may either represent a browser window 
 * (or tab) or some part of an HTML page where a Vaadin application is embedded.
 * <p>
 * The UI is initialized using {@link #init(VaadinRequest)}. This method is intended to be 
 * overridden to add component to the user interface and initialize non-component functionality.
 */
@Theme("mytheme")
public class MyUI extends UI {

    int counter = 0;

    @Override
    protected void init(VaadinRequest vaadinRequest) {
        final VerticalLayout layout = new VerticalLayout();
        setContent(layout);

        ListDataProvider<String> dataProvider = new ListDataProvider<>(Arrays.asList("one", "two", "three"));

        ComboBox<String> comboBox = new ComboBox<>();
        comboBox.setWidth(250, Unit.PIXELS);
        comboBox.setDataProvider(dataProvider);
        comboBox.setItemCaptionGenerator(item -> {
            System.out.println("***: "+counter);
            ++counter;
            return item + " - " + String.valueOf(System.currentTimeMillis()+ " --- "+counter);
        });
        layout.addComponent(comboBox);

        Button button2 = new Button("Refresh");
        button2.addClickListener(event -> dataProvider.refreshItem(comboBox.getValue()));
        layout.addComponent(button2);
    }

    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
    public static class MyUIServlet extends VaadinServlet {
    }
}