Java Network Programming, Second Edition

Writing a Protocol Handler

To demonstrate a complete protocol handler, let’s write one for the finger protocol, which is defined in RFC 1288 and was introduced in Chapter 10. Finger is a relatively simple protocol compared to JDK-supported protocols such as HTTP and FTP. The client connects to port 79 on the server and sends a list of usernames followed by a carriage return/linefeed pair. The server responds with ASCII text containing information about each of the named users or, if no names were listed, a list of the currently logged in users. For example:

% telnet rama.poly.edu 79
Trying 128.238.10.212...
Connected to rama.poly.edu.
Escape character is '^]'.

Login       Name            TTY    Idle    When    Where
jacola   Jane Colaginae    *pts/7       Tue 08:01  208.34.37.104
marcus   Marcus Tullius     pts/15  13d Tue 17:33  farm-dialup11.poly.e
matewan  Sepin Matewan     *pts/17  17: Thu 15:32  128.238.10.177
hengpi   Heng Pin          *pts/10      Tue 10:36  128.238.18.119
nadats   Nabeel Datsun      pts/12   56 Mon 10:38  128.238.213.227
matewan  Sepin Matewan     *pts/8     4 Sun 18:39  128.238.10.177
Connection closed by foreign host.

Or, requesting information about a specific user:

% telnet rama.poly.edu 79
Trying 128.238.10.212...
Connected to rama.poly.edu.
Escape character is '^]'.
marcus
Login       Name            TTY    Idle    When    Where
marcus   Marcus Tullius     pts/15  13d Tue 17:33  farm-dialup11.poly.e

Since there’s no standard for the format of a finger URL, we will start by creating one. Ideally, this should look as much like an http URL as possible. Therefore, we will implement ...