How to warn the user before session timeout?

public class MyExecListener implements ExecutionInit, UiLifeCycle {

	private String timerId = "timeoutNotifyTimer";

	public void init(Execution exec, Execution parent) throws Exception {
		
		Timer timer = (Timer) exec.getDesktop().getAttribute(timerId);
		if (timer != null) {
			
			if (isSendbyMsgBox(timer, exec))
				return;
			
			timer.stop();
			timer.start();
		}
	}

	private boolean isSendbyMsgBox(Timer timer, Execution exec) {
		HttpServletRequest hreq = (HttpServletRequest)exec.getNativeRequest();
		
		for (int j = 0;; ++j) {
			final String uuid = hreq.getParameter("uuid_"+j);
			if (uuid == null)
				break;
			
			if (uuid.equals(timer.getUuid()))
				return true;
		}
		return false;
	}

	public void afterComponentAttached(Component comp, Page page) {
	}

	public void afterComponentDetached(Component comp, Page prevpage) {
	}

	public void afterComponentMoved(Component parent, Component child,
			Component prevparent) {
	}

	public void afterPageAttached(Page page, Desktop desktop) {
		 Object obj = desktop.getAttribute(timerId);
		
		if (obj == null) {
			
			int tmout = desktop.getWebApp().getConfiguration()
					.getSessionMaxInactiveInterval();
			
			final Timer timer = new Timer((tmout - 2) * 1000);
			
			timer.addEventListener(Events.ON_TIMER, new EventListener() {
				public void onEvent(Event event) throws Exception {
					
					Messagebox.show("Your session are about to be expired",
						"Information", Messagebox.OK,
						Messagebox.INFORMATION, new EventListener() {
							
							@Override
							public void onEvent(Event event) throws Exception {
								if (Messagebox.ON_OK.equals(event.getName())){
									timer.start();
				                }
							}
						});
				}
			});
			timer.setPage(page);
			desktop.setAttribute(timerId, timer);
			timer.start();
		}
	}

	public void afterPageDetached(Page page, Desktop prevdesktop) {
	}

}

<listener>
	<listener-class>test.MyExecListener</listener-class>
</listener>