Francois,
there was some discussion recently:
https://coderanch.com/t/355374/Servlets/java/would-best-tool-debug-servlet There is a french eclipse plugin, but they are still working on it to make it more usable. WSAD has a Tomcat debugger. I only have used the websphere debugger, and its slow.
I aggree with Wiliam Brogden in the mentioned discussion. Best debugger is a programmers brain. If you have your business logic in the JavaBeans you can write them.
Test them. When they work, you can write the web-presentation layer. I am following this strategy and I do not miss a debugger very much. You can also use log4u package from jarkarta to get debug messages out of your app (much better than System.out.println(). You can also use System.out.println() and a textpad pointed to the current tomcat log file. Log4u is better and its quite easy to set up.
I think in the netbeans book at O'Reilly they describe tomcat debugging with netbeans.
But as I said. I think one does not need very much a debugger if you have a clean separation of business logic and presentation logic.
Problem arise when your beans needs quite complex object graphs from the users http-session.
regards Axel