CDI, Spring, JPA, Mock Testing (Mockito) all break the rules in some manner, like creating instances of objects without calling constructors, changing final class definitions, etc.
CDI, JPA, Spring are all used "in production", while Mockito would be used only in testing.
I have no idea how you balance these two. sun.misc.Unsafe was pretty hard to use and any normal human knew to stay away from it.
Most of the time breaking the rules is a reflection of either a bad practice or lack of functionality in the language. I'm all for experimenting in either of those dimensions, but there's a lot of value to have a default behaviour of not doing that.
Yes actually, especially when it comes to i/o. Let's say I want to simulate how many code handles a REST interaction with a third party service. I want to be able to mock the REST callout without performing any actual i/o. This is why these test frameworks are powerful and useful.
Later if I want I can test to see if the actual callout works by hitting a local endpoint.
Why? Because it's hard for you to mock out actual I/O, which could be solved by a tool that makes it easy to mock out actual I/O.
Testing it as part of your test cases violates the "Test one thing and one thing exactly" principle. The goal is to reduce the surface area of the test as far as possible to distill what the author of the code wrote is correct.
In reality, you're actually testing a bunch of method invocations that won't happen when the code is being used, and to do it, you're also breaking a lot of the contracts that one would otherwise expect to be maintained... maybe that's not the optimal way to do things.