There are obviously many pieces of this library that don't play well with JRuby (forking in particular), but most of the lower level pieces do work well. However, there's a problem with use of the AWS::Flow::Core::Future class -- it's referenced without namespace in several places in the code, and in JRuby, in at least some load orders, Future is resolved to java.util.concurrent.Future, resulting in errors such as:

NoMethodError undefined method `new' for Java::JavaUtilConcurrent::Future:Module

I know this library is effectively unmaintained right now, but I'm sharing this in case others run into the same problem.