Sending a custom message to a server from client using NIO selectors

Question

So as the title suggests, I'm stuck on using selectors properly. The process I have in mind is that first when running this client as a thread, it will connect to the server, wait for a confirmation (CONNACK) and then proceed. So far so good, I am receiving the message SUCCESS! The server is set to read after accepting connection.

while (iterator.hasNext()) {
     final SelectionKey key = (SelectionKey) iterator.next();

     iterator.remove();

     try (SocketChannel keySocketChannel = (SocketChannel) key.channel()) {

     //attempt connection
     if (key.isConnectable()) {
             System.out.print("Connecting");

             if (keySocketChannel.isConnectionPending()) {
                 keySocketChannel.finishConnect();
             }

             while (keySocketChannel.read(buffer) != -1) {
                 buffer.flip();

                 charBuffer = decoder.decode(buffer);
                 System.out.print(".");

                 if (buffer.hasRemaining()) {
                        buffer.compact();
                 } else {
                        buffer.clear();
                 }
                 if (charBuffer.toString().equals(MessageType.CONNACK.getName())) {
                        System.out.println("SUCCESS!");

                        //At this point, it makes sense that the next operation
                        //is a write.
                        key.interestOps(SelectionKey.OP_WRITE);
                        break;
                  }
                  try {
                        Thread.sleep(500);
                  } catch (InterruptedException e) {
                  }
              }
              System.out.println("SUCCESS!");
              key.interestOps(SelectionKey.OP_WRITE);
     } else if (key.isConnectable())  {
              System.out.println("connectable");
     } else if (key.isWritable()) {
              System.out.println("write key");
              write(key, socketChannel);
     } else if (key.isReadable()) {
              System.out.println("read key");
     }
              System.out.println("oops");
     } catch (IOException e) {
              e.printStackTrace();
     }
}

The problem however is when I call the publish method. What I was planning to do was place the request in a synchronized queue. Then in the write method (when key.isWritable()) it checks the queue and if it finds anything, sends it to the server.

PUBLISH

 @Override
        public void publish(String message) {
            //create publish request
            final Request request = new Request(this.topic, message, MessageType.PUBLISH);

            System.out.println("Adding request to queue..");
            synchronized (this.dataQueue) {
                dataQueue.add(request);
            }
            this.selector.wakeup();
        }

The problem is that it never reaches key.isWritable() (hence, print "write key"). What could the problem be?

Answer 1

When you have something to write, just write it, and check the return value. OP_WRITE should only be used in the case where a write() has just returned zero. There are plenty of answers about this here, many of them by me.