Forum OpenACS Q&A: Response to A Technical Paper on Java
The architecture for ACS Java 4.5 and beyond is dramatically different from ACS Java 4. It doesn't generate servlets for you from AJP's or anything like that. You actually write the servlets and can use Java IDE's to step through your own code and debug (or just println if you wish). You can also automate testing with JUnit. This is a big win for developing the ACS. At aD, we have now implemented a release process that involves weekly formal builds, automated regression testing, and scalability testing and tuning. Doing these things greatly helps ensure that changes we make to one place in the ACS don't break functionality somewhere else.
Dan's comments are relevant to ACS Java 4, but they don't apply to ACS Java 4.5+. And, they certainly don't apply to Java-based solutions in general. I encourage you to take a look at http://developer.arsdigita.com/acs-java if you haven't already.