As it stands JRuby 2.2 mode (on 9.0.3.0) is inconsistent with MRI 2.1+ when it comes to module_function and define_method behavior. Take the following sample:

module MyModule module_function
  methods = { :hello => :world }
  methods.each do |key, val|
    define_method key do
      val
    end
  end
end

In Ruby 2.0 module_function is not inherited, but in 2.1+ it is inherited, this leads to inconsistent behavior when moving between JRuby and MRI.