Network Clients - Java - page 520

IAN'S BASIC STEPS: UDP CLIENT

UDP is a bit more involved, so I'll list the basic steps for generating a UDP client:

1. Create a DatagramSocket with no arguments (the form that takes two arguments is used
on the server).

2. Optionally connect() the socket to an InetAddress (see Finding and Reporting Network
Addresses ) and port number.

3. Create one or more DatagramPacket objects; these are wrappers around a byte array that
contains data you want to send and is filled in with data you receive.

4. If you did not connect() the socket, provide the InetAddress and port when construct-
ing the DatagramPacket .

5. Set the packet's length and use sock.send(packet) to send data to the server.

6. Use sock.receive() to retrieve data.

So why would we even use UDP? UDP has a lot less overhead than TCP, which can be par-
ticularly valuable when sending huge amounts of data over a reliable local network or a few
hops on the Internet. Over long-haul networks, TCP is probably preferred because TCP
handles retransmission of lost packets for you. And obviously, if preserving record boundar-
ies makes your life easier, that may be a reason for considering UDP.

Example 13-8 is a short program that connects via UDP to the Daytime date and time server
used in Reading and Writing Textual Data . Because UDP has no real notion of “connection,”
the client typically initiates the “conversation,” which sometimes means sending an empty
packet; the UDP server uses the address information it gets from that to return its response.

Example 13-8. DaytimeUDP.java
public public class DaytimeUDP {

/** The UDP port number */
public

class DaytimeUDP

public final final static static int int DAYTIME_PORT = 13 ;

/** A buffer plenty big enough for the date string */
protected protected final final static static int int PACKET_SIZE = 100 ;

/** The main program that drives this network client.
* @param argv[0] hostname, running daytime/udp server