Java Tutorial/Network/DatagramChannel

Set up DatagramChannel

import java.io.IOException;
import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.DatagramChannel;
public class UDPTimeServer {
  public final static int DEFAULT_PORT = 37;
  public static void main(String[] args) throws IOException {
    int port = 37;
    ByteBuffer in = ByteBuffer.allocate(8192);
    ByteBuffer out = ByteBuffer.allocate(8);
    out.order(ByteOrder.BIG_ENDIAN);
    SocketAddress address = new InetSocketAddress(port);
    DatagramChannel channel = DatagramChannel.open();
    DatagramSocket socket = channel.socket();
    socket.bind(address);
    System.err.println("bound to " + address);
    while (true) {
      try {
        in.clear();
        SocketAddress client = channel.receive(in);
        System.err.println(client);
        long secondsSince1970 = System.currentTimeMillis();
        out.clear();
        out.putLong(secondsSince1970);
        out.flip();
        out.position(4);
        channel.send(out, client);
      } catch (Exception ex) {
        System.err.println(ex);
      }
    }
  }
}

Socket based on new IO and DatagramChannel

import java.io.IOException;
import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.DatagramChannel;
import java.util.Date;
public class MainClass {
  public static void main(String[] args) throws IOException {
    DatagramChannel channel = DatagramChannel.open();
    SocketAddress address = new InetSocketAddress(0);
    DatagramSocket socket = channel.socket();
    socket.bind(address);
    SocketAddress server = new InetSocketAddress("time-a.nist.gov", 37);
    channel.connect(server);
    ByteBuffer buffer = ByteBuffer.allocate(8);
    buffer.order(ByteOrder.BIG_ENDIAN);
    // send a byte of data to the server
    buffer.put((byte) 0);
    buffer.flip();
    channel.write(buffer);
    // get the buffer ready to receive data
    buffer.clear();
    // fill the first four bytes with zeros
    buffer.putInt(0);
    channel.read(buffer);
    buffer.flip();
    // convert seconds since 1900 to a java.util.Date
    long secondsSince1900 = buffer.getLong();
    long differenceBetweenEpochs = 2208988800L;
    long secondsSince1970 = secondsSince1900 - differenceBetweenEpochs;
    long msSince1970 = secondsSince1970 * 1000;
    Date time = new Date(msSince1970);
    System.out.println(time);
  }
}