This section describes how to establish a datagram communication link. A server application runs a datagram socket in listen mode and a client application connects a client socket to the server socket.
To establish a datagram communication link, one application program must act
as a receiver, create a datagram socket with a given local port number, and
set the datagram socket in the receiving mode waiting for a datagram to
arrive.
With receiver program running and waiting for a datagram to arrive,
the other program can now act as a sender, create a datagram socket with
a given local port number, prepare a datagram with the remote address
and port number in in, and send it over.
J2SDK offers two main classes to support datagram communication:
1. java.net.DatagramSocket with methods:
DatagramSocket(): constructing a datagram socket with a given local address,
and a given local port number.
receive(): waiting for a datagram to arrive, and returning the datagram
to the caller.
send(): sending a datagram through the datagram socket.
2. java.net.DatagramPacket representing the datagram with methods:
getAddress(): getting the remote address from the datagram.
getPort(): getting the remote port number from the datagram.
getData(): getting the data from the datagram.
setAddress(): setting the remote address to the datagram.
setPort(): setting the remote port to the datagram.
setData(): setting the data to the datagram.
The following diagram shows steps involved in sending a datagram
from one program to another program using the methods provided by
DatagramSocket and DatagramPacket classes:
Sender Program Receiver Program
Internet Address #a Internet Address #b
Step Available Port #p Available Port #q
1 ds = new DatagramSocket(#b+#q)
2 dp = new DatagramPacket()
3 ds.receive(dp)
4 ds = new DatagramSocket() (waiting)
5 dp = new DataPacket() (waiting)
6 dp.setAddress() (waiting)
7 dp.setPort() (waiting)
8 dp.setData() (waiting)
9 ds.send(dp) (receiving datagram)
10 (dp is ready)
