From: Charles Oliver Nutter <headius@...>
Date: 2010-04-25T16:41:07+09:00
Subject: Re: DrX, an object inspector

On Sat, Apr 24, 2010 at 10:38 AM, David Masover <ninja@slaphack.com> wrote:
>> For getting the data, you don't even need to write an extension to
>> JRuby. The "jruby" library gives you introspective access to all
>> objects at a very direct level:
> It seems like this should be possible to do from bare Ruby, in a portable way.
> Clumsy, but possible. For example, as long as people don't get smart and use
> Ruby 1.9's BasicObject, you can do this:

No, I'm pretty sure this isn't possible on regular Ruby. What's
happening in JRuby is that you're getting direct access to the Java
object representing a Ruby class or object, and you then can do
anything to that object you'd normally have to write Java code for.

Now that I'm on my machine, here's a more extensive example:

require 'jruby'

class MyObject
  def foo
    'hello'
  end
end
obj = MyObject.new
cls = obj.class

obj_ref = JRuby.reference obj
cls_ref = JRuby.reference cls

# accessing the raw method table on a class
methods = cls_ref.getMethods # this was the ".methods" call previously
p methods.class # => Java::JavaUtilConcurrent::ConcurrentHashMap
foo_method = methods['foo']
# foo_method is an instance of our internal method object
p foo_method.call(JRuby.runtime.current_context, obj, cls_ref, 'foo')
# => 'hello'

# changing the object's class(!?)
class Foo; end
p obj.class # => 'Object'
obj_ref.setMetaClass(JRuby.reference(Foo))
p obj.class # => 'Foo'

# modifying the current scope
scope = JRuby.runtime.current_context.current_scope
p scope.static_scope.variables.to_a
# => ["obj", "cls", "obj_ref", "cls_ref", "methods", "foo_method", "scope"]
index = scope.static_scope.variables.to_a.index('scope')
scope.set_value_depth_zero('hello', index)
p scope # => 'hello'

...and so on. Essentially anything you can call from Java, you can
call from Ruby once you have the appropriate reference, so you should
be able to programmatically walk all of the JRuby runtime structures
and any Ruby or Java objects they reference without writing any Java
code.

- Charlie