code snippet for java.nio.channels.Selector. http://tutorials.jenkov.com/java-nio/selectors.html

selector-snippet.java

// the remote server
SocketAddress remote = new InetSocketAddress(Inet4Address.getByName(hostName), port);
// open a selector
Selector selector = Selector.open();
// open a channel
SocketChannel channel = SocketChannel.open();
// non-blocking
channel.configureBlocking(false);
// pending connect
channel.connect(remote);
// initiative register to the selector, and interest the connection-op
channel.register(selector, SelectionKey.OP_CONNECT);
ByteBuffer packBuffer = ByteBuffer.allocate(0xffff);
while (true) {
    // blocking select "ready" channels
    int readyChannels = selector.select();
    if (readyChannels == 0) {
        System.out.println("wake up, but no interest ops...");
        continue;
    }
    Set<SelectionKey> selectedKeys = selector.selectedKeys();
    Iterator<SelectionKey> keyIterator = selectedKeys.iterator();
    while (keyIterator.hasNext()) {
        SelectionKey key = keyIterator.next();
        if (key.isAcceptable()) {
            // a connection was accepted by a ServerSocketChannel.
            // in this demo we will never be here
            System.out.println("acceptable!");
            SocketChannel socketChannel = ((ServerSocketChannel) key.channel()).accept();
            System.out.println("accept a channel:" + socketChannel);
        } else if (key.isConnectable()) {
            // a connection was established with a remote server.
            System.out.println("connectable!");
            boolean connectionPending = ((SocketChannel) key.channel()).isConnectionPending();
            System.out.println("connectionPending:" + connectionPending);
            if (connectionPending) {
                // really connect to remote
                boolean finishConnect = ((SocketChannel) key.channel()).finishConnect();
                System.out.println("finishConnect:" + finishConnect);
            }
            // we must write "hello world" to server first
            // after connected
            channel.register(selector, SelectionKey.OP_WRITE);
        } else if (key.isReadable()) {
            // a channel is ready for reading
            System.out.println("readable!");
            if (init) {
                packBuffer.clear();
                int read = ((SocketChannel) key.channel()).read(packBuffer);
                System.out.println("read bytes:" + read);
                handleData(packBuffer);
                // is need to send heartbeat after read something ?
                if (isNeedHeartBeat() || isNeedSendSomething())
                    channel.register(selector, SelectionKey.OP_WRITE);
            } else {
                System.out.println("dosen't init");
            }
        } else if (key.isWritable()) {
            // a channel is ready for writing
            System.out.println("writable");
            if (!init) {
                // send "hello" to server
                sayHello(key);
                init = true;
            }
            if (isNeedHeartBeat())
                sendHeartBeat(key);
            // or send others to server here
            if (isNeedSendSomething())
                ((SocketChannel) key.channel()).write(something);
            // and we interest what server response immediately
            channel.register(selector, SelectionKey.OP_READ);
        }
        // this channel is done processing
        keyIterator.remove();
        // next time the channel becomes "ready" 
        // the selector will add it to the selected key set again.
    }