Java Servlet Programming

Executing Programs

Sometimes a servlet needs to execute an external program. This is generally important in situations where an external program offers functionality that isn’t easily available from within Java. For example, a servlet could call an external program to perform an image manipulation or to check the status of the server. Launching an external program raises a number of security concerns. For this reason, it’s an action that can be taken only by servlets running with a fairly lenient security manager—specifically, a security manager that grants permission for the servlet to call the exec() method of java.lang.Runtime.

Finger

The finger program queries a (possibly remote) computer for a list of currently logged in users. It’s available on virtually all Unix systems and some Windows NT machines with networking capabilities. The finger program works by connecting to a finger daemon (usually named fingerd) that listens on port 79. finger makes its request of fingerd using a custom “finger” protocol, and fingerd replies with the appropriate information. Most Unix systems run fingerd, though many security-conscious administrators turn it off to limit information that could be used for break-in attempts. It’s still fairly rare to find fingerd on Windows systems. Run without any arguments, finger reports all users of the local machine. The local machine must be running fingerd. Here’s an example:

% finger Login Name TTY Idle When Office jhunter Jason Hunter q0 3:13 Thu ...