From: Charles Oliver Nutter <headius@...>
Date: 2010-03-16T03:53:13+09:00
Subject: Singleton methods without the singleton class

Hi all!

JRuby currently allows you to add singleton methods to normal Java
objects like this:

foo = java.lang.String.new('blah')
def foo.bar
  # do something
end

Unfortunately, this feature requires us to keep a weak table of all
the Java objects that enter Ruby space, since we can't attache this
singleton method directly to the object itself. I would like to
eliminate this feature at some point, but I recognize that since
people are using it we need an alternative.

So I've hacked together a short script that provides a "Singletonizer"
module that can simulate singleton methods without actually creating a
new singleton class:

http://gist.github.com/333174

The basic idea is to go ahead and add the method to the actual class,
but add it via a hashed lookup on a per-instance basis. Objects which
have had the method added will have a corresponding entry in their
attached_methods table. Objects that don't will raise NoMethodError as
usual.

Thoughts? Comments? This version is obviously not threadsafe, but it
gets you pretty close to singleton methods without requiring singleton
classes.

- Charlie