THIS is the real problem. For _most_ projects upgrading from 8 to 9 meant just adding a few packages that were removed from default JDK with Jigsaw. That part is generally fine, even with OSGi hell.
The Javax -> Jakarta changes for validation, EE, XML, etc. are a damn nightmare!
And if you're using microservices it's either a total migration (a no go for anyone minimizing bugs and outages), or a slow one-by-one trudge, if you use some commons between them that commons will need to support both javax and jakarta together, superbly fun
The Javax -> Jakarta changes for validation, EE, XML, etc. are a damn nightmare!