Sockets for Servers - Java Network Programming - page 284

a specified port, methods that configure the various server socket options, and the usual
miscellaneous methods such as toString() .
In Java, the basic life cycle of a server program is this:
1. A new ServerSocket is created on a particular port using a ServerSocket() con‐
structor.
2. The ServerSocket listens for incoming connection attempts on that port using its
accept() method. accept() blocks until a client attempts to make a connection,
at which point accept() returns a Socket object connecting the client and the
server.
3. Depending on the type of server, either the Socket 's getInputStream() method,
getOutputStream() method, or both are called to get input and output streams
that communicate with the client.
4. The server and the client interact according to an agreed-upon protocol until it is
time to close the connection.
5. The server, the client, or both close the connection.
6. The server returns to step 2 and waits for the next connection.
Let's demonstrate with one of the simpler protocols, daytime. Recall from the Chap‐
ter 8 that a daytime server listens on port 13. When a client connects, the server sends
the time in a human-readable format and closes the connection. For example, here's a
connection to the daytime server at time-a.nist.gov :
$ telnet time - a . nist . gov 13
Trying 129.6 . 15.28 ...
Connected to time - a . nist . gov .
Escape character is ' ^] ' .
56375 13 - 03 - 24 13 : 37 : 50 50 0 0 888.8 UTC ( NIST ) *
Connection closed by foreign host .
Implementing your own daytime server is easy. First, create a server socket that listens
on port 13:
ServerSocket server = new ServerSocket ( 13 );
Next, accept a connection:
Socket connection = server . accept ();
The accept() call blocks . That is, the program stops here and waits, possibly for hours
or days, until a client connects on port 13. When a client does connect, the accept()
method returns a Socket object.