Olvasnivaló:

• http://tutorials.jenkov.com/java-nio/nio-vs-io.html
• http://www.javaworld.com/article/2073344/core-java/use-select-for-high-speed-networking.html

   ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
 
   serverSocketChannel.socket().bind(new InetSocketAddress(9999));
   serverSocketChannel.configureBlocking(false);  // ez a sor jelzi a blokkolásmentes működést
 
   while(true){
    SocketChannel socketChannel = serverSocketChannel.accept();
 
    if(socketChannel != null){
        // az összeköttetés létrejött
    }
   }

  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.net.Socket;
  import java.util.Random;
 
  public class Client implements Runnable {
	private String host;
	private int port;
 
	// Bounds on how much we write per cycle
	private static final int minWriteSize = 1024;
	private static final int maxWriteSize = 65536;
 
	// Bounds on how long we wait between cycles
	private static final int minPause = (int) (0.05 * 1000);
	private static final int maxPause = (int) (0.5 * 1000);
 
	// Random number generator
	Random rand = new Random();
 
	public Client(String host, int port, int numThreads) {
		this.host = host;
		this.port = port;
 
		for (int i = 0; i < numThreads; ++i) {
			new Thread(this).start();
		}
	}
 
	public void run() {
		byte buffer[] = new byte[maxWriteSize];
		try {
			Socket s = new Socket(host, port);
			InputStream in = s.getInputStream();
			OutputStream out = s.getOutputStream();
 
			while (true) {
				int numToWrite = minWriteSize
						+ (int) (rand.nextDouble() * (maxWriteSize - minWriteSize));
				for (int i = 0; i < numToWrite; ++i) {
					buffer[i] = (byte) rand.nextInt(256);
				}
				out.write(buffer, 0, numToWrite);
				int sofar = 0;
				while (sofar < numToWrite) {
					sofar += in.read(buffer, sofar, numToWrite - sofar);
				}
				System.out.println(Thread.currentThread() + " wrote " + numToWrite);
 
				int pause = minPause + (int) (rand.nextDouble() * (maxPause - minPause));
				try {
					Thread.sleep(pause);
				} catch (InterruptedException ie) {
				}
			}
		} catch (IOException ie) {
			ie.printStackTrace();
		}
	}
 
	static public void main(String args[]) throws Exception {
		new Client("localhost", 4444, 3);
	}
  }

  public class Server implements Runnable {
	// The port we will listen on
	private int port;
	// A pre-allocated buffer for encrypting data
	private final ByteBuffer buffer = ByteBuffer.allocate(16384);
	public Server(int port) {
		this.port = port;
		new Thread(this).start();
	}
	public void run() {
		try {
			// Instead of creating a ServerSocket,
			// create a ServerSocketChannel
			ServerSocketChannel ssc = ServerSocketChannel.open();
			// Set it to non-blocking, so we can use select
			ssc.configureBlocking(false);
			// Get the Socket connected to this channel, and bind it
			// to the listening port
			ServerSocket ss = ssc.socket();
			InetSocketAddress isa = new InetSocketAddress(port);
			ss.bind(isa);
			// Create a new Selector for selecting
			Selector selector = Selector.open();
			// Register the ServerSocketChannel, so we can
			// listen for incoming connections
			ssc.register(selector, SelectionKey.OP_ACCEPT);
			System.out.println("Listening on port " + port);
			while (true) {
				// See if we've had any activity -- either
				// an incoming connection, or incoming data on an
				// existing connection
				int num = selector.select();
				// If we don't have any activity, loop around and wait
				// again
				if (num == 0) {
					continue;
				}
				// Get the keys corresponding to the activity
				// that has been detected, and process them
				// one by one
				Set keys = selector.selectedKeys();
				Iterator it = keys.iterator();
				while (it.hasNext()) {
					// Get a key representing one of bits of I/O
					// activity
					SelectionKey key = (SelectionKey) it.next();
					// What kind of activity is it?
					if ((key.readyOps() & SelectionKey.OP_ACCEPT) == SelectionKey.OP_ACCEPT) {
						System.out.println("acc");
						// It's an incoming connection.
						// Register this socket with the Selector
						// so we can listen for input on it
						Socket s = ss.accept();
						System.out.println("Got connection from " + s);
						// Make sure to make it non-blocking, so we can
						// use a selector on it.
						SocketChannel sc = s.getChannel();
						sc.configureBlocking(false);
						// Register it with the selector, for reading
						sc.register(selector, SelectionKey.OP_READ);
					} else if ((key.readyOps() & SelectionKey.OP_READ) == SelectionKey.OP_READ) {
						SocketChannel sc = null;
						try {
							// It's incoming data on a connection, so
							// process it
							sc = (SocketChannel) key.channel();
							boolean ok = processInput(sc);
							// If the connection is dead, then remove it
							// from the selector and close it
							if (!ok) {
								key.cancel();
								Socket s = null;
								try {
									s = sc.socket();
									s.close();
								} catch (IOException ie) {
									System.err.println("Error closing socket "
											+ s + ": " + ie);
								}
							}
						} catch (IOException ie) {
							// On exception, remove this channel from the
							// selector
							key.cancel();
							try {
								sc.close();
							} catch (IOException ie2) {
								System.out.println(ie2);
							}
							System.out.println("Closed " + sc);
						}
					}
				}
				// We remove the selected keys, because we've dealt
				// with them.
				keys.clear();
			}
		} catch (IOException ie) {
			System.err.println(ie);
		}
	}
	// Do some cheesy encryption on the incoming data,
	// and send it back out
	private boolean processInput(SocketChannel sc) throws IOException {
		buffer.clear();
		sc.read(buffer);
		buffer.flip();
		// If no data, close the connection
		if (buffer.limit() == 0) {
			return false;
		}
		// Simple rot-13 encryption
		for (int i = 0; i < buffer.limit(); ++i) {
			byte b = buffer.get(i);
			if ((b >= 'a' && b <= 'm') || (b >= 'A' && b <= 'M')) {
				b += 13;
			} else if ((b >= 'n' && b <= 'z') || (b >= 'N' && b <= 'Z')) {
				b -= 13;
			}
			buffer.put(i, b);
		}
		sc.write(buffer);
		System.out.println("Processed " + buffer.limit() + " from " + sc);
		return true;
	}
	static public void main(String args[]) throws Exception {
		new Server(4444);
	}
  }