Comment by creatonez

It will be a while before JNI itself is dead, because far too much stuff still relies on it. The unsafe helpers in the Java standard library will die first because they are fundamentally incompatible with Valhalla, but it's likely the simple cases will last a while as there are vast swaths of glue code that needs to be rewritten.

IIRC Minecraft is still using entirely JNI and no FFM yet. That will probably start to change in the near future, though. Some modders have already been replacing their own natives with FFM versions.