Since ruby does not have checked exceptions the behavior here: https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/ext/thread/Mutex.java#L91 is very surprising. If users really need this behavior they should use a ReentrantLock directly.

The only question I have now is if its removal might be even more surprising to existing users. Thoughts?