We are migrating Java 8 -> 17 now and it’s been a right royal pain. I’m glad we didn’t jump to 11 LTS, and instead make one big leap. It might be a very long time before we do another LTS upgrade if they keep making poor migration choices.
Which likely means touching a pretty significant portion of your codebase for the upgrade, and then who knows what 3rd party libraries you depend on are expecting...
So it may be as simple as Find/Replace for some folks, for others, it might be a deep dark rabbit hole.
Not breaking things used to be Java's MO. Yes, that means a ton of legacy cruft built up over the years... but we used to be able to depend on Java to "Just Work".
Perhaps some of this is necessary. After all, the C# folks seem to have no problem breaking everything to add new features... but I'd assert the Java community as a whole is much less tolerant of breaking changes and rapid deployment of features.
There’s plenty of dependencies that provide javax implementations as well as jakarta.
The real difficulty for us was upgrading the Play framework since the Scala community likes to make breaking changes with every release.
And yet, the overwhelming majority of the Java community are still using JRE8 with none of these new features.
There are some that love this new pace of language development, but very few are actually using new features in production code.