PushWithoutThreads.java

@Push(transport = Transport.LONG_POLLING)
public class PushWithoutThreads extends DelayedUI {

	@WebServlet(value = "/*", asyncSupported = true)
	@VaadinServletConfiguration(productionMode = false, ui = Vaadin7helloworldUI.class)
	public static class Servlet extends VaadinServlet {
	}

	private VerticalLayout layout;

	@Override
	protected void init(VaadinRequest request) {
		layout = new VerticalLayout();
		layout.addComponent(new Label("Please wait while doing heavy lifting on the server..."));
		setContent(layout);
	}

	@Override
	protected void delayedInit() {
		sleep(5000);
		layout.removeAllComponents();
		layout.addComponent(new Label("Thank you for waiting, we are now ready to rock"));
		Button button = new Button("Do heavy lifting");
		button.addClickListener(new Button.ClickListener() {
			@Override
			public void buttonClick(ClickEvent event) {
				ProgressBar progressBar = new ProgressBar();
				progressBar.setValue(0f);
				layout.addComponent(progressBar);
				push(); // Force changes to be sent to the client
				sleep(3000); // Heavy lifting part 1
				progressBar.setValue(0.5f);
				push();
				sleep(2000); // Heavy lifting part 2
				layout.removeComponent(progressBar);
				layout.addComponent(new Label("Heavy lifting done, thanks for waiting patiently"));
			}
		});
		layout.addComponent(button);
	}

	protected void sleep(int i) {
		try {
			Thread.sleep(i);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

	}