JEP draft: Prepare to make final mean final

A cursory glance at "setAccessible" usage reveals popular libraries such as serializers like gson and jaxb, class manipulation and generation like cglib, aspectj and even jdk.internal.reflect, testing frameworks and libraries including junit, mockito and other mocking libraries, lombok, groovy, spring, and the list goes on and on.

My bet is that this will be yet another "checked exception" or "module system", where many applications now need to add "--add-opens". If you'll use ANY of many of the more popular frameworks or libraries you'll end up giving this assurance away, which will make library developers not able to rely on it and we're back to square one.

We've addressed that in the JEP. Serialization libraries have a special way to circumvent this (until we get Serialization 2.0), and mocking libraries may, indeed, need to set the flag, but they're rarely used in production, so if they don't enjoy some new optimisation -- no big deal.

BTW, this JEP does not apply to setAccessible generally, as that's been restricted since JDK 16, but only to the particular (and more rare) use of setAccessible to mutate instance final fields. As the JEP says, static final fields, records' internal instance fields, and final instance fields of hidden classes cannot be mutated with that approach currently, so it's never been something that's expected to work in all cases.

Would be nice to have a single “--test-mode” flag that is only meant to be set when running tests, and allows for all this leniency, (add opens, etc) in a single flag.

We should separate the problem from the solution. The problem is that running tests may require relatively many integrity-busting flags. That is true.

There are, however, better solutions than a global test-mode flag that, invariably, will be used by some in production out of laziness, leaving no auditable record of what integrity constraints need to be violated and why. When a new team lead is appointed some years later they will have a hard time trying to reduce the entropy.

The better solutions will arrive in due course, but until then, build tools can automatically add most of the necessary flags. They should be encouraged to do that.

So make the flag remove some other feature, which is critical to production, like the ability to run main() or something.

On the other hand, I don’t think the solution to someone holding a shotgun to their foot and threatening to pull the trigger is to make everyone wear armored shoes. They’re already a lost cause, and there are a billion other ways they can shoot their foot off, if they are so inclined.

I agree with the principal of making it hard to screw things up assuming good faith efforts (making it hard to fall in the pit of despair), so overall I like the JEP.