Use of System.out and System.err for tracing and logging is a practice highly to be discouraged. Not all appservers send the System outputs to the same places - I've seen specifically differences between where stuff goes in Solaris Tomcat vs. Linux Tomcat. And some appservers may lose the output entirely or scramble System.out and System.err together.
Of course it usually works, and I do it for quick-and-dirty stuff, but I don't do it for production apps, and never for something I expect to be portable.
Tomcat has some very extensive logging abilities attached to its Valve components. I recommend you read up on them and see if you find something you like. If you don't like them, of course, you can write your own Valve to log the way you
do like.
The actual application logging itself is supported as part of
J2EE, JDK 1.4 and later, and various 3d-party add-ons such as log4j. You can start out simple by just using Servlet.log() and add more complex loggers as/if needed.
And, BTW, note that there's a version of Servlet.log that allows you to log Exceptions, so you can capture stack traces!
