Question

So this was working on this project a few months ago. I’m using Google App Engine’s Channel API to push messages to my GWT app. I’m using http://code.google.com/p/gwt-gae-channel/ to interact through GWT.

Lets say I send 1 message to the client: “First Message”

The client will receive the message, “First Message” just fine.

Then let’s say I send another message, “Second Message” to the client.

The client will again receive the message, “First Message”.

This will continue happening. There have been some instances where I’ll receive the second message, and it will be the message that gets stuck repeating.

When I finally close the page, and thus close the channel, I again receive the repeated message without sending something from the server.

Does anyone have any idea what is going on? I don’t think this was happening when I was working on this a few months ago, and I can see no changes to the GAE Channel API.

---

Here is some code:

        String json = AutoBeanHelper.toJson(proxy);
        log.fine("Item's JSON Received: " + json);

        List<ChannelEntity> channels = channelDAO.getByUserId();
        if (channels.size() > 1) {
            log.warning("Multiple channels for single user detected.");
        }

        ChannelService channelService = ChannelServiceFactory.getChannelService();
        for (ChannelEntity channel : channels) {
            channelService.sendMessage(new ChannelMessage(channel.getClientId(), json));
        }

So whenever I store a new item of a specific type (this is in that entities update function):
1. I turn it into JSON.
2. I then log that JSON.
3. I get that users channel.
4. I send it to that users channel.

When I look at my logs, I see that the variable above that I’m logging is showing correctly, meaning I’m logging the correct JSON message but when I display the JSON in an alert on the client-side as soon as it gets to the client, it’s the previous message that seems to be stuck repeating. I really don’t see what I could be doing wrong here.

Let me know if you would like to see another part of the code. For good measure, here is the code on the client:

eventBus.addHandler(ReceivedChannelTokenEvent.TYPE, new ReceivedChannelTokenEventHandler() {
        @Override
        public void onEvent(ReceivedChannelTokenEvent event) {
            ChannelFactory.createChannel(event.getChannelToken(), new ChannelCreatedCallback() {
                @Override
                public void onChannelCreated(Channel channel) {
                    final Socket channelSocket = channel.open(new SocketListener() {
                        @Override
                        public void onOpen() {
                            Window.alert("Channel Opened");
                        }
                        @Override
                        public void onMessage(String json) {
                            Window.alert(json);
                            eventBus.fireEvent(new MessageReceivedEvent(json));

                        }
                        @Override
                        public void onError(SocketError error) {
                            Window.alert("Channel Error: " + error.getDescription());
                            if ( error.getDescription().equals(CHANNEL_ERROR_TOKEN_TIME_OUT) ) {
                                eventBus.fireEvent(new ChannelTimeOutEvent());
                            }
                        }
                        @Override
                        public void onClose() {
                            Window.alert("Channel Closed.");
                        }
                    });

                    Window.addWindowClosingHandler(new Window.ClosingHandler() {
                        @Override
                        public void onWindowClosing(ClosingEvent event) {
                            channelSocket.close();
                        }
                    });
                }
            });
        }
    });

Answer 1

So I was finally able to figure it out. It seems that in the onMessage function within the app when I call

eventBus.fireEvent(new MessageReceivedEvent(json));

it seems that it never returns from this, thus never exiting the onMessage function in the code, causing me to receive the same message repetitively.