~/projects/jruby ➔ jruby -J-Djruby.ji.newStyleExtension=true impl.rb
class Foo_2071503186
class org.jruby.RubyObject
class Foo2_2071503278
class java.lang.Object

~/projects/jruby ➔ cat impl.rb
require 'java'
require 'jruby'

class Foo
  include java.lang.Runnable
end

f = Foo.new

# inspect the object from the Java side of the world
ref = JRuby.reference(f)
puts ref.getClass
puts ref.getClass.getSuperclass

class Foo2 < java.lang.Object
  include java.lang.Runnable
end

f2 = Foo2.new

# inspect again
ref = JRuby.reference(f2)
puts ref.getClass
puts ref.getClass.getSuperclass