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Tip of the Day
Language: Java
Expertise: Beginner
Apr 5, 1999

UDP Clients

Question:
I would like to create an application in Java that communicates with a UDP connection. How do you specify the connection as UDP rather than TCP?

Answer:
To write UDP clients and servers, you have to use the DatagramSocket class. UDP is a connectionless protocol, so a UDP server doesn't have to perform an accept() operation the way a TCP server using ServerSocket must. Unlike Socket, you do not communicate by writing to an OutputStream and reading from an InputStream. Rather, you send datagrams using the DatagramPacket class. Each packet you receive must have a predefined size and byte buffer. Each packet you send must also have a destination address and port number associated with it. The easiest way to give you the feel for how it all works is to write a sample program. The following program connects to the UDP daytime service on a host and prints the result. The key item to pay attention to is the difference between a send and receive packet. Send packets contain address information, receive packets don't. The raw byte data in a packet can be accessed with getData() and the length of the actual data in the byte array (rather than the length of the array itself), can be obtained with getLength().

import java.io.*;
import java.net.*;

public class Daytime {
  public static final int DAYTIME_PORT = 13;

  public static final String getTime(String hostname) throws IOException {
    InetAddress host;
    DatagramSocket socket = new DatagramSocket();
    byte[] dummyData = new byte[1];
    byte[] timeData  = new byte[256];
    DatagramPacket sendPacket, receivePacket;

    host = InetAddress.getByName(hostname);
    sendPacket = 
      new DatagramPacket(dummyData, dummyData.length, host, DAYTIME_PORT);
    receivePacket = new DatagramPacket(timeData, timeData.length);

    socket.send(sendPacket);
    socket.receive(receivePacket);

    return
      new String(receivePacket.getData(), 0, receivePacket.getLength());

  }

  public static void main(String[] args) {
    String server = "tock.usno.navy.mil";

    if(args.length == 1)
      server = args[0];
    else if(args.length > 1) {
      System.err.println("Usage: Daytime [hostname]");
      return;
    }

    try {
      // Time should include newline, so we don't use println().
      System.out.print(getTime(server));
      System.out.flush(); 
    } catch(IOException e) {
      e.printStackTrace();
      return;
    }
  }                   
}
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