[pron]

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.

[LadyCailin]

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.

[pron]

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.

[LadyCailin]

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.

[pron]

> 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.

I don't think so, either, it's just that I think there are better solutions than a test-mode flag at the level of the `java` launcher. If the mechanism that runs the tests can automatically configure the appropriate capabilities without requiring the user running the tests to do manual configuration then the problem is solved for those who just want to easily run tests just as well as a test-mode configuration.

The idea of a test-mode flag has been floated before and considered; we're not ruling it out, but if such a mode is ever added, I can't tell you now what it would mean exactly. In any event, it's better to carefully study the nature of the problem and its origins before suggesting a particular solution. As Brian Goetz likes to say, today's solutions may well become tomorrow's problems.

> They’re already a lost cause, and there are a billion other ways they can shoot their foot off, if they are so inclined.

True, but our experience shows that it's not a good idea to make the bad choice the easiest one, or people may pick it out of laziness. Let those who want to shoot themselves in the foot work for it. If nothing else, it increases the chance that they learn what their (not-entirely-trivial) configuration means, and maybe they'll realise they don't want it after all.

Someone might point out that there are still ways to do the wrong thing out of laziness by blindingly copying a configuration from StackOverflow etc., but we're not done yet.