Evil JRuby

After my last post I got several comments about evil.rb. Of course I had evil.rb in mind when doing some of it, but I also forgot to describe the two most evil methods of the JRuby module: runtime and reference. The runtime method will return the currently executing JRuby runtime as a Java Integration, meaning you can get access to almost anything you want with it. For example, if you want to take a look at the global CacheMap (used to cache method instances):

require 'jruby'
JRuby::runtime.cache_map

Whoops. And that's just the beginning. Are you interested in investigating the current call frame or activation frame (DynamicScope in JRuby):

require 'jruby'
p JRuby::runtime.current_context.current_frame
a = 1
p JRuby::runtime.current_context.current_scope

Of course, you can call all accessible (and some inaccessible) methods on these objects, just like if you were working with it from Java. Use the API's and take a look. You can change things without problem.

And that also brings us to one of the easiest examples of evil.rb, changing the frozen flag on a Ruby object. Well, with the reference method, that's easy:

require 'jruby'

str = "foo"
str.freeze

puts str.frozen?
JRuby::reference(str).setFrozen(false)
puts str.frozen?

JRuby::reference will return the same object sent in, wrapped in a Java Integration layer, meaning that you can inspect and modify it to your hearts like. In this way, you can get at the internals of JRuby in the same way you can using evil.rb for MRI. And I guess these features should mainly be used for looking and learning about the internals of JRuby.

So, have fun and don't be evil (overtly).