Question

I’m implementing simple netty server for a multiplayer game. I’m just trying to figure out Netty.

I test the server via telnet. What i done is broadcast the messages to all channels. It’s working smoothly. Also I remove channels from map on close event which is fine.

But the problem is if one of the clients disconnect unexpectedly, before closed callback, messageReceived callback called which the sender is disconnected channel.

How can i properly ignore the message comes from disconnected client?
I use StringBuffer in messagedReceived but for the case StringBuffer.toString is also not a proper string. At the end disconnected channel broadcast pointless message to other channels and itself, when receiver channel is itself throws an exception Connection reset by peer
which it’s normal because the channel itself is not available at the moment.

Here is the code ;

     @Override
     public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {

            System.out.println();
            System.out.println("------------------");


            Channel current = e.getChannel();
            System.out.println("SenderChannel:"+current.getId());

            if(!current.isOpen())
                System.out.println("Not Open");

            ChannelBuffer buf = (ChannelBuffer) e.getMessage();

            StringBuffer sbs = new StringBuffer();

            while(buf.readable()) {
                sbs.append((char) buf.readByte());
            }

            String s = sbs.toString();


            System.out.println(s);

            String you = "You:" + s;
            String other = "Other:" + s;



            byte [] uResponse = you.getBytes();
            byte [] otherResponse = other.getBytes();


            Iterator iterator = channelList.entrySet().iterator();
            while(iterator.hasNext()){
                  Map.Entry pairs = (Map.Entry)iterator.next();

                  Integer key = (Integer)pairs.getKey();
                  Channel c = (Channel)pairs.getValue();

                  System.out.println("ReceiverChannel:"+c.getId());

                  if(key != current.getId())
                      c.write(ChannelBuffers.wrappedBuffer(otherResponse));
                  else
                      c.write(ChannelBuffers.wrappedBuffer(uResponse));

            }

     }


    @Override
    public void channelDisconnected(ChannelHandlerContext ctx,
            ChannelStateEvent e){

        Channel ch = e.getChannel();


            channelList.remove(ch.getId());
            System.out.println();
            System.out.println("*****************");
            System.out.println("DisconnectEvent:"+ch.getId());
            System.out.println("*****************");
            System.out.println();
            ch.close();

    }

Answer 1

You can’t solve the problem in the manner that you would like. If there’s a network problem then technically the sender could disconnect at any time, for example

• as soon as the thread enters messageReceived
• while you’re iterating through channelList
• while you’re iterating through channelList but after you’ve echoed back to the sender
• after you’ve broadcast the message

Netty can’t raise the disconnected event while messageReceived is processing because you’re running in the thread that will raise the event (unless you have a non-ordered execution handler in your pipeline). The correct solution really depends on your application. If the broadcast results in all the other receivers responding it’s probably better / easier to have the server suppress any messages destined for a client that’s no longer connected.

Also, if you’re really going to use strings then take a look at StringEncoder / StringDecoder. There’s no guarantee in your code that the message event buffer contains a complete string.