ServerSocket

The ServerSocket class is used by a server (or any other program) that wants to sit and wait for a connection request from a client. When you create a ServerSocket object, you specify the port to listen to. To wait for a client rendezvous, call the accept() method. It will block until a client attempts to connect to the port that the ServerSocket object is listening on. When such a request is received, a Socket object is created and returned.

"Note that ServerSocket is not used for communication between the server and the client; it is only used to wait for and establish the connection to the client. Typically, a single ServerSocket object is used over and over again to establish connections to any number of clients." [Flanagan97b, p192] This is demonstrated in the next example.

The choice of port number 8189 was arbitrary. A number of that size is guaranteed not to clash with any standard servers.

The previous example demonstrated how to set-up an input stream (or read) abstraction. This example adds to that an output stream (or write) abstraction.

This simple server:

prompts the client for input by writing to the output stream
accepts input by reading from the input stream
echos the input to standard out, and, back to the client

The user runs "telnet" as the client application. "127.0.0.1" is a special address (called the "local loopback address") that denotes the local machine.

It is not clear how many "streams" need to be closed. Horstmann closes only the socket object [p141], whereas Flanagan closes: the input object, the output object, and the socket object [p193].