Question

I’m trying to build a GWT (2.4.0) application that can communicate (full-duplex) with a server using a text-based protocol. To accomplish this I’m using Kaazing Websocket Gateway Version 3.3.2 to act as a proxy.

GWT App <—–[websocket]—–> Kaazing <—–[tcp]—–> Backend Server

This is my Kaazing configuration:

<service>
  <accept>ws://127.0.0.1:9444/foobar</accept>
  <connect>tcp://localhost:50189</connect>
  <type>proxy</type>
  <cross-site-constraint>
    <allow-origin>http://127.0.0.1:8888</allow-origin>
  </cross-site-constraint>  
</service>

This setup works with the Java implementation of the Kaazing client. Now I want to use this service in a GWT application.

This is my Code for the GWT client implementation:

    WebSocket ws;
    try {
        ws = new WebSocket("ws://127.0.0.1:9444/foobar");
        ws.addCloseHandler(new CloseHandler() {
            @Override
            public void onClose(CloseEvent ev) {
            }
        });
        ws.addMessageHandler(new MessageHandler() {
            @Override
            public void onMessage(MessageEvent ev) {
                System.out.println(ev.getData());
            }
        });
        ws.addOpenHandler(new OpenHandler() {
            @Override
            public void onOpen(OpenEvent ev) {
            }
        });
    } catch (WebSocketException e) {
        e.printStackTrace();
    }

The GWT client can successfully send text to the backend server. It just can’t process any response from the server. A JavaScript Exception is thrown as soon as ev.getData() is called.

Uncaught JavaScript exception [Uncaught java.lang.ClassCastException: com.google.gwt.core.client.JavaScriptObject$ cannot be cast to java.lang.String]

If I change the WebSocket address in the code above to ws://127.0.0.1:9444/echo to use the Kaazing echo service, any text I send is successfully received by the GWT application. If I change the service back to proxy (or broadcast) the above JavaScript exception is thrown every time a message from the server is received.

The same code in a Java program (using the Kaazing java client) works just fine.

As I am no GWT expert, is there any way I can further debug the problem or is there a simple solution I just fail to see?

Answer 1

I believe I’ve seen that issue before, do you mind sharing your gwtclient.gwt.xml file content? Or, maybe, try adding ” or ” depending on whether you are using ByteSocket or WebSocket in the ‘gwtclient.gwt.xml’ of the GWT client.

Hope this helps,

-Marcelo