I realize that I am probably a long way behind the curve here but I have just (3rd Oct 2006) discovered jconsole - a fantastically easy way to find out what your Java Virtual Machine (JVM) is doing. Essentially jconsole provides you with information about the threads currently running, memory usage, CPU usage and all manner of other bit of information that are useful when developing software and large software in particular.
Using jconsole with Eclipse and Tomcat
By default when Eclipse starts Tomcat running it doesn't start the VM with the management agent enabled. This means that jconsole can't attach to the VM. To remedy this right click on the web application you would like to monitor and select Run As > Run.... In the run dialog select the server you will run the application in and the argument tab. Enter the argument:
The name, it would seem, is arbitrary but needs to be specified.