UDP - Java Network Programming - page 397

Example 12-1. A daytime protocol client
import java.io.* ;
import java.net.* ;
public class DaytimeUDPClient {
private final static int PORT = 13 ;
private static final String HOSTNAME = "time.nist.gov" ;
public static void main ( String [] args ) {
try ( DatagramSocket socket = new DatagramSocket ( 0 )) {
socket . setSoTimeout ( 10000 );
InetAddress host = InetAddress . getByName ( HOSTNAME );
DatagramPacket request = new DatagramPacket ( new byte [ 1 ], 1 , host , PORT );
DatagramPacket response = new DatagramPacket ( new byte [ 1024 ], 1024 );
socket . send ( request );
socket . receive ( response );
String result = new String ( response . getData (), 0 , response . getLength (),
"US-ASCII" );
System . out . println ( result );
} catch ( IOException ex ) {
ex . printStackTrace ();
}
}
}
Typical output is much the same as if you connected with TCP:
$ java DaytimeUDPClient
56375 13-04-11 19:55:22 50 0 0 843.6 UTC(NIST) *
UDP Servers
A UDP server follows almost the same pattern as a UDP client, except that you usually
receive before sending and don't choose an anonymous port to bind to. Unlike TCP,
there's no separate DatagramServerSocket class.
For example, let's implement a daytime server over UDP. Begin by opening a datagram
socket on a well-known port. For daytime, this port is 13:
DatagramSocket socket = new DatagramSocket ( 13 );
As with TCP sockets, on Unix systems (including Linux and Mac OS X) you need to be
running as root in order to bind to a port below 1024. You can either use sudo to run
the program or simply change the port to something 1024 or higher.
Next, create a packet into which to receive a request. You need to supply both a byte
array in which to store incoming data, the offset into the array, and the number of bytes
to store. Here you set up a packet with space for 1,024 bytes starting at 0: