ChatServlet.java

@WebServlet(urlPatterns = { "/chat/*" }, asyncSupported=true, loadOnStartup = 1)
public class ChatServlet extends HttpServlet {

	private Queue<AsyncContext> clients = new ConcurrentLinkedQueue<AsyncContext>();
	private BlockingQueue<String> messages = new LinkedBlockingQueue<String>();
	private int contador;

	static {
		System.out.println("tomcat carregou?");
	}

	@Override
	public void init() throws ServletException {
		final ExecutorService executors = Executors.newCachedThreadPool();
		Executors.newSingleThreadExecutor().execute(new Runnable() {
			@Override
			public void run() {
				while (true) {
					try {
						final String message = messages.take();

						for (final AsyncContext ctx : clients) {
							
							executors.execute(new Runnable() {
								@Override
								public void run() {
									try {
										if (ctx.getResponse().isCommitted()) {
											System.out.println("headers enviados ja");
										}
										System.out.println(ctx);
										
										PrintWriter writer = ctx.getResponse().getWriter();
										writer
												.println(message);
										writer.flush();
									} catch (IOException e) {
										e.printStackTrace();
									}
								}

							});
						}
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}

		});
	}

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse arg1)
			throws ServletException, IOException {
		System.out.println(Thread.currentThread());
		System.out.println(req.isAsyncStarted());
		AsyncContext ctx = req.startAsync();
		ctx.setTimeout(3000000);
		clients.add(ctx);
		messages.add(String.format("cliente %d chegou<br/>%n", contador++));
	}
}