ChatServer.java

import io.undertow.Handlers;
import io.undertow.Undertow;
import io.undertow.websockets.WebSocketConnectionCallback;
import io.undertow.websockets.core.AbstractReceiveListener;
import io.undertow.websockets.core.BufferedTextMessage;
import io.undertow.websockets.core.WebSocketChannel;
import io.undertow.websockets.core.WebSockets;
import io.undertow.websockets.spi.WebSocketHttpExchange;

public class ChatServer {

    public static void main(final String[] args) {

        System.out.println("To see chat in action is to open two different browsers and point them at http://localhost:8080");

        Undertow server = Undertow.builder()
                .addHttpListener(8080, "localhost")
                .setBufferSize(128)
                .setHandler(Handlers.path()
                        .addPrefixPath("/myapp", Handlers.websocket(new WebSocketConnectionCallback() {

                            @Override
                            public void onConnect(WebSocketHttpExchange exchange, WebSocketChannel channel) {
                                channel.getReceiveSetter().set(new AbstractReceiveListener() {

                                    @Override
                                    protected void onFullTextMessage(WebSocketChannel channel, BufferedTextMessage message) {
                                        final String messageData = message.getData();
                                        for (WebSocketChannel session : channel.getPeerConnections()) {
                                            WebSockets.sendText(messageData, session, null);
                                        }
                                    }
                                });
                                channel.resumeReceives();
                            }

                        })))
                .build();

        server.start();
    }

Client.java

import java.util.Random;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

import org.asynchttpclient.DefaultAsyncHttpClient;
import org.asynchttpclient.DefaultAsyncHttpClientConfig;
import org.asynchttpclient.ws.WebSocket;
import org.asynchttpclient.ws.WebSocketTextListener;
import org.asynchttpclient.ws.WebSocketUpgradeHandler;

public class Client
{
   public static final int WEB_SOCKET_MAX_FRAME_SIZE = 256;
   public static final Random random = new Random();
   public static final DefaultAsyncHttpClientConfig CONFIG =
      new DefaultAsyncHttpClientConfig.Builder().setWebSocketMaxFrameSize(WEB_SOCKET_MAX_FRAME_SIZE).build();
   public static final DefaultAsyncHttpClient CLIENT = new DefaultAsyncHttpClient(CONFIG);
   public static final String CHARACTERS =
      "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZäöüÄÖÜ!\"§$%&/()=?*'_:;1234567890";

   public static void main(String[] args) throws InterruptedException, ExecutionException, TimeoutException
   {
      final WebSocketUpgradeHandler handler =
         new WebSocketUpgradeHandler.Builder().addWebSocketListener(new MyWebSocketListener()).build();
      CLIENT.prepareGet("ws://localhost:8080/myapp").execute(handler).get(20, TimeUnit.SECONDS);
   }

   private static class MyWebSocketListener implements WebSocketTextListener
   {
      @Override
      public void onMessage(final String message)
      {
         System.out.println("MESSAGE: " + message);
      }

      @Override
      public void onOpen(final WebSocket websocket)
      {
         System.out.println("OPEN");
         websocket.sendMessage(randomString(2 * WEB_SOCKET_MAX_FRAME_SIZE));
      }

      @Override
      public void onClose(final WebSocket websocket)
      {
         System.out.println("CLOSE");
      }

      @Override
      public void onError(final Throwable t)
      {
         System.out.println("ERROR: " + t.getMessage());
      }
   }

   public static String randomString(final int length)
   {
      final StringBuilder buffer = new StringBuilder();
      for (int i = 0; i < length; i++)
      {
         buffer.append(CHARACTERS.charAt(random.nextInt(CHARACTERS.length())));
      }
      return buffer.toString();
   }
}