UDP - Java Network Programming - page 422

a datagram with no data at all, bugs in some Java implementations require that you add
at least one byte of data to the datagram. The simple servers we're currently considering
ignore this data.
Once a UDPPoke object has been constructed, clients call its poke() method to send an
empty outgoing datagram to the target and read its response. The response is initially
set to null. When the expected datagram appears, its data is copied into the response
field. This method returns null if the response doesn't come quickly enough or never
comes at all.
The main() method merely reads the host and port to connect to from the command
line, constructs a UDPPoke object, and pokes it. Most of the simple protocols that this
client suits will return ASCII text, so this example attempts to convert the response to
an ASCII string and print it.
Example 12-7. The UDPPoke class
import java.io.* ;
import java.net.* ;
public class UDPPoke {
private int bufferSize ; // in bytes
private int timeout ; // in milliseconds
private InetAddress host ;
private int port ;
public UDPPoke ( InetAddress host , int port , int bufferSize , int timeout ) {
this . bufferSize = bufferSize ;
this . host = host ;
if ( port < 1 || port > 65535 ) {
throw new IllegalArgumentException ( "Port out of range" );
}
this . port = port ;
this . timeout = timeout ;
}
public UDPPoke ( InetAddress host , int port , int bufferSize ) {
this ( host , port , bufferSize , 30000 );
}
public UDPPoke ( InetAddress host , int port ) {
this ( host , port , 8192 , 30000 );
}
public byte [] poke () {
try ( DatagramSocket socket = new DatagramSocket ( 0 )) {
DatagramPacket outgoing = new DatagramPacket ( new byte [ 1 ], 1 , host , port );
socket . connect ( host , port );
socket . setSoTimeout ( timeout );