Major Description
The gfsh show dead-locks command ends up depending on this class: org.apache.geode.distributed.internal.deadlock.UnsafeThreadLocal
Most of the time this UnsafeThreadLocal just behaves like a normal jdk ThreadLocal (its super class). But when the gfsh command is executed it causes "get" to be called on UnsafeThreadLocal. It uses a bunch of reflection to prevent "get" from setting an initial value in the case of a miss. This reflection calls setAccessible which will cause get to fail on java 16 and later (see: https://softwaregarden.dev/en/posts/new-java/illegal-access-in-java-16).
To workaround this failure set the JVM command line option: --illegal-access=permit
The current solution adds get(Thread) to UnsafeThreadLocal which is different than ThreadLocal.get(). To fix this we probably need to stop using ThreadLocal and instead keep some kind of collection of the threads waiting for a resource. It might also be possible to ask the resource what threads are waiting for it.