I'm running latest version of concurrent-ruby (1.0.3) and JRuby master.

require 'concurrent'
class Job
  def perform
    puts "ddd"
  end
end

job = Job.new
executor = Concurrent::ImmediateExecutor.new
Concurrent::ScheduledTask.execute(10, args: [job], executor: executor, &:perform)

The last line crashes with java.lang.NoSuchFieldError: SUPPORTS_FENCES (full stacktrace)

The same snippet works fine on jruby-9.1.6.0, so I believe there is a regression in 9.1.7 / master.

@headius