The Apache Software Foundation's log4j logging library is one of the better logging systems around. It's both easier to use and more flexible than Java's built-in logging system.
Tutorial Details:
This article shows you how to extend log4j with a custom "appender," the part of the system that actually writes the logging messages. This article also provides a simple example of Java's socket APIs and multithreading capabilities.
Logging is the simple process of printing messages of various types to known places. Logging messages can go to a console, to a file, to a remote monitor, or anywhere else that you find convenient. Think of logging as a sophisticated sibling of:
if( debug )
System.out.println("Debugging diagnostic");
Logging has several advantages over simple println() statements, however. The logging system can add contextual information—filename, line number, and date, for example—to the message automatically. You can redirect the messages to different destinations, or change the formatting, without recompiling your program. (In log4j, you just modify a simple properties file.) You can easily turn categories of messages on and off so you can see debug messages when you're debugging, but easily turn them off when you're not, for example.
Logging is central to all of my programs. I use it to monitor my program's progress as it works. I use it to log error messages from library methods that might be used in a server-side context (where there's no console on which to print a stack trace). Most importantly, logging is one of my main debugging tools. Though visual debuggers are handy on occasion, I've noticed that I can find more bugs faster by combining a careful reading of the code with a few well-placed logging messages. (I'm not quite sure why reading/logging seems more effective than visual debugging, but my current theory is that a visual debugger narrows your focus to a single thread of control through the program, so you tend to miss bugs that aren't on that thread.)
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