UDP Time Client : UDP Client « Network « Java Tutorial

1. Java Tutorial
2. Network
3. UDP Client

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 UDPTimeClient {

  public static void main(String[] args) throws Exception {

    DatagramChannel channel = DatagramChannel.open();
    // port 0 selects any available port
    SocketAddress address = new InetSocketAddress(0);
    DatagramSocket socket = channel.socket();
    socket.setSoTimeout(5000);
    socket.bind(address);

    SocketAddress server = new InetSocketAddress("time.nist.gov", 37);
    ByteBuffer buffer = ByteBuffer.allocate(8192);
    // time protocol always uses big-endian order
    buffer.order(ByteOrder.BIG_ENDIAN);
    // Must put at least one byte of data in the buffer;
    // it doesn't matter what it is.
    buffer.put((byte) 65);
    buffer.flip();

    channel.send(buffer, server);

    buffer.clear();
    buffer.put((byte) 0).put((byte) 0).put((byte) 0).put((byte) 0);
    channel.receive(buffer);
    buffer.flip();
    long secondsSince1970 = buffer.getLong();

    System.out.println(secondsSince1970);
    channel.close();

  }
}