Amazing discovery of the day: JNI on Android lets you access object fields that you're not supposed to, according to Java rules.
Is this capability to bypass access restrictions documented anywhere? Is this an official JNI behavior or specific to the Android flavor of JNI? Is this an undefined behavior? Will the OOP police come for me any moment now?
I understand that relying on unpublished object fields is inherently dangerous and may break anytime; that's not the question here.
UPDATE: looks like applications that target API28 no longer have this capability.
Amazing discovery of the day: JNI on Android lets you access object fields that you're not supposed to, according to Java rules.
The abbreviation JNI does not appear anywhere in the question and answers that you linked to, except as a dynamically-generated link to this very question.
Is this documented anywhere?
Any decent book on Java development should cover public, private, etc.
Is this an official JNI behavior or Android-specific?
Neither.
What is Android-specific is compile-time steps to make it difficult for you to add code in some android, java, and javax packages.
Is this an undefined behavior?
That depends on what underlying noun or concept you are tying to tying to the pronoun "this".
If "this" is "accessing private, etc. stuff", then the behavior is not undefined.
If "this" is "accessing something specific private, etc. in the Android framework", that is undefined. There are many, many versions of Android around, and many, many versions of framework classes. Their internal implementations are not identical. Anything not exposed via the Android SDK is eligible for change by Google, device manufacturers, ROM mod maintainers, etc.