public class Server {
    private static boolean shutdownListener;

    public static void main(String[] args) {
        if (args.length != 1) {
            System.err.println("Usage: java Server <port>");
            System.exit(1);
        }

        try {
            int port = Integer.parseInt(args[0]);

            try {
                ServerSocketChannel ssChannel = ServerSocketChannel.open();
                ssChannel.configureBlocking(false);
                ssChannel.socket().bind(new InetSocketAddress(port));
                System.out.println("Listening for connections on port " + port);

                Selector selector = Selector.open();
                ssChannel.register(selector, SelectionKey.OP_ACCEPT);
                while (!shutdownListener) {
                    selector.select();
                    Iterator it = selector.selectedKeys().iterator();
                    while (it.hasNext()) {
                        SelectionKey selKey = (SelectionKey) it.next();
                        it.remove();
                        if (selKey.isAcceptable()) {
                            ServerSocketChannel zzChannel = (ServerSocketChannel) selKey.channel();
                            SocketChannel sChannel = zzChannel.accept();

                            if (sChannel != null) {
                                sChannel.register(selector, sChannel.validOps());
                            } else {
                                // Is this even possible?
                            }
                        }

                        try {
                            if (selKey.isValid() && selKey.isConnectable()) {
                                SocketChannel sChannel = (SocketChannel) selKey.channel();

                                boolean success = sChannel.finishConnect();
                                if (!success) {
                                    // Do some error handling or whatever.
                                    selKey.cancel();
                                }
                            }
                            if (selKey.isValid() && selKey.isReadable()) {
                                SocketChannel zChannel = (SocketChannel) selKey.channel();
                                // Reading
                            }
                            if (selKey.isValid() && selKey.isWritable()) {
                                SocketChannel zChannel = (SocketChannel) selKey.channel();
                                // Writing
                            }
                        } catch (IOException e) {
                            selKey.cancel();
                        }
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
                System.exit(1);
            }
        } catch (NumberFormatException e) {
            System.err.println("Usage: java Server <port>");
            System.exit(1);
        }
    }
}