import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.DatagramChannel;
import java.nio.charset.Charset;

public class DatagramServer {
	public static void main(String[] args) throws Exception {
		DatagramChannel channel = DatagramChannel.open();
		channel.socket().bind(new InetSocketAddress(1234));

		ByteBuffer bf = ByteBuffer.allocate(1024);

		while (true) {
			System.out.println("listening  ... ");
			SocketAddress remoteAddress = channel.receive(bf);

			new DealClientThread(channel, remoteAddress, bf);
		}
	}
}

class DealClientThread implements Runnable {

	private DatagramChannel channel;
	private SocketAddress remoteAddress;
	private ByteBuffer bf;

	public DealClientThread(DatagramChannel channel,
			SocketAddress remoteAddress, ByteBuffer bf) {
		this.channel = channel;
		this.remoteAddress = remoteAddress;
		this.bf = bf;

		new Thread(this).start();
	}

	public void run() {
		try {
			// reveive
			bf.flip();
			String receivedString = Charset.forName("UTF-8").newDecoder()
					.decode(bf).toString();

			System.out.println("Receivet client " + remoteAddress.toString()
					+ " message: " + receivedString);

			// send
			String sendString = "Hi, client(" + remoteAddress.toString()
					+ ") I had receive your message.";
			ByteBuffer bfSend = ByteBuffer.wrap(sendString.getBytes("UTF-8"));
			channel.send(bfSend, remoteAddress);
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
}

import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.DatagramChannel;
import java.nio.charset.Charset;
import java.util.Date;

public class DatagramClient {
	public static void main(String[] args) throws Exception {
		// send
		DatagramChannel channel = DatagramChannel.open();
		String sendString = "Hi, server. @" + new Date().toString();
		ByteBuffer sendBuffer = ByteBuffer.wrap(sendString.getBytes("UTF-8"));
		channel.send(sendBuffer, new InetSocketAddress("127.0.0.1", 1234));
		System.out.println("Send message to server end");

		// receive
		ByteBuffer receiveBuffer = ByteBuffer.allocate(80);
		SocketAddress remoteAddress = channel.receive(receiveBuffer);
		receiveBuffer.flip();

		String receivedString = Charset.forName("UTF-8").newDecoder()
				.decode(receiveBuffer).toString();
		System.out.println("Receive message from " + remoteAddress.toString()
				+ " server: " + receivedString);
	}
}