Our organization is attempting to migrate our Appengine Standard Java 21 application from EE8 to EE10; and we're currently blocked.

I hope there's something simple I'm doing incorrectly, but I've spent more hours than I'd care to admit trying to figure this out (reading documentation, asking ChatGPT, experimenting, etc). I cannot get routing to work as I expect per Jakarta EE10 documentation. To make this completely replicatable, I created a bare-bones Java 21 EE10 Appengine web application that anyone can checkout and deploy to an Appengine Standard project. I'll lay out some of of the issues in this issue, but the README in this repository lists more details with actual examples of some of the behavior I'm experiencing: https://github.com/MuffinTheMan/appengine-standard-ee10-sample/tree/1-original

To summarize, none of the <error-page> mappings in web.xml appear to work when deployed (though they work locally). Additionally, using an HttpServlet to forward a request to a JSP or static file doesn't work when deployed (though this works locally as well). For example, this code in an HttpServlet::doGet (or ::doPost) method never resolves (the browser never receives a response and just hangs eternally):

req.getRequestDispatcher("/WEB-INF/test.jsp").forward(req, resp);

1. There appears to be bugs in the deployed Appengine runtime
2. The local Appengine server behaves differently from the deployed version (which makes troubleshooting a nightmare)