Where does the "final means final" effort fit in? Can the JVM prevent modification of final fields via JNI, or is --enable-native-access also going to require (or imply) the flag which enables setAccessible() and friends?

Ah, that's a great question, and the answer is in the JEP (https://openjdk.org/jeps/500#Mutating-final-fields-from-nati...).

When running with -Xcheck:jni, you'll get a warning when trying to mutate a final field with JNI.

Now, enabling this check by default without harming JNI performance proved to be too much of an effort. However, mutating final fields with JNI even today can already lead to undefined behaviour, including horrible miscompilation, where different Java methods can read different values of the field, for final fields that the JVM already trusts to be immutable, such as static finals, record components, or a few other cases (indeed, there are non-final fields that the JVM trusts to be assigned only once, and mutating those with JNI is also undefined behaviour). As the compiler starts trusting more final fields after this change, mutating almost all final fields will lead to undefined behaviour. Then again, using JNI can lead to undefined behaviour in many ways.

So to make sure your JNI code isn't mutating finals, test with -Xcheck:jni (as of JDK 26).