[Java] NIO: Non-Block Socket (ZT)

1. Summary
   Just like function select/poll/epoll on other platforms.
   
   Classes:
   SelectableChannel (SocketChannel, ServerSocketChannel, DatagramChannel, Pipe-Pipe.SinkChannel/Pipe.SourceChannel) + Selector + SelectionKey.
   
2. Client
   // Creates a non-blocking socket channel for the specified host name and port.
   // connect() is called on the new channel before it is returned.
   public static SocketChannel createSocketChannel(String hostName, int port) throws IOException {
       // Create a non-blocking socket channel
       SocketChannel sChannel = SocketChannel.open();
       sChannel.configureBlocking(false);
   
       // Send a connection request to the server; this method is non-blocking
       sChannel.connect(new InetSocketAddress(hostName, port));
       return sChannel;
   }
   
   
   // Create a non-blocking socket and check for connections
   try {
       // Create a non-blocking socket channel on port 80
       SocketChannel sChannel = createSocketChannel("hostname.com", 80);
   
       // Before the socket is usable, the connection must be completed
       // by calling finishConnect(), which is non-blocking
       while (!sChannel.finishConnect()) {
           // Do something else
       }
       // Socket channel is now ready to use
   } catch (IOException e) {
   }
   
   
3. Server
   (Ref: )
   public class MainClass {
     public static void main(String[] args) throws IOException {
       Charset charset = Charset.forName("ISO-8859-1");
       CharsetEncoder encoder = charset.newEncoder();
       CharsetDecoder decoder = charset.newDecoder();
   
       ByteBuffer buffer = ByteBuffer.allocate(512);
   
       Selector selector = Selector.open();
   
       ServerSocketChannel server = ServerSocketChannel.open();
       server.socket().bind(new java.net.InetSocketAddress(8000));
       server.configureBlocking(false);
       SelectionKey serverkey = server.register(selector, SelectionKey.OP_ACCEPT);
   
       for (;;) {
         selector.select();
         Set keys = selector.selectedKeys();
   
         for (Iterator i = keys.iterator(); i.hasNext();) {
           SelectionKey key = (SelectionKey) i.next();
           i.remove();
   
           if (key == serverkey) {
             if (key.isAcceptable()) {
               SocketChannel client = server.accept();
               client.configureBlocking(false);
               SelectionKey clientkey = client.register(selector, SelectionKey.OP_READ);
               clientkey.attach(new Integer(0));
             }
           } else {
             SocketChannel client = (SocketChannel) key.channel();
             if (!key.isReadable())
               continue;
             int bytesread = client.read(buffer);
             if (bytesread == -1) {
               key.cancel();
               client.close();
               continue;
             }
             buffer.flip();
             String request = decoder.decode(buffer).toString();
             buffer.clear();
             if (request.trim().equals("quit")) {
               client.write(encoder.encode(CharBuffer.wrap("Bye.")));
               key.cancel();
               client.close();
             } else {
               int num = ((Integer) key.attachment()).intValue();
               String response = num + ": " + request.toUpperCase();
               client.write(encoder.encode(CharBuffer.wrap(response)));
               key.attach(new Integer(num + 1));
             }
           }
         }
       }
     }
   }
   
4. xxx