This section describes how to establish a socket communication link. A server application runs a socket in listen mode and a client application connects a client socket to the server socket.
In the previous section, we only discussed about how socket works with
a communication link that has been established already. Now, let's see
how two application programs can establish a communication link between
the local socket and the remote socket.
To establish a communication link, one application program must act
as a server, create a server socket with a given port number, and
set the server socket in the listen mode waiting for a connection request
from other program.
With one program running as a server listening for a connection request
at a specific port number, the other program can now create a client socket with
a given local port number, the Internet address of the computer system
where the first program is running, and the port number where the server
socket is listening. At this time, a connect request will be send over
to the server socket. The server socket should then accept the connect
request and instantiate a socket object to complete communication link.
J2SDK offers two main classes to support socket communication:
1. java.net.ServerSocket with methods:
bind(): setting the server socket with the local system address,
and a given local port number.
accept(): listening for a connection request, and instantiating
a socket object when the request comes.
2. java.net.Socket with methods:
bind(): setting the socket with the local system address, and
a given local port number.
connect(): sending a connection request to a given remote system
address, and a given remote port number.
The following diagram shows steps involved in establishing a two-way
communication link using the methods provided by ServerSocket and Socket
classes:
Client System Server System
Internet Address #a Internet Address #b
Step Available Port #p Available Port #q
1 ss = new ServerSocket()
2 ss.bind(#b+#q)
3 Socket cs = ss.accept()
4 cs = new Socket() (waiting)
5 cs.bind(#a+#p) (waiting)
6 cs.connect(#b+#q) (receiving request)
7 (estalishing the link) (establishing the link)
8 (cs is ready) (cs is ready)
