a_hacked_up_script_to_make_gems_happy.rb

require 'rubygems'

module Gem
  class << self

    def hacked_set_paths(gpaths)
      @gem_path = gpaths
      @gem_path << Gem.dir
    end

  end
end


Gem.hacked_set_paths(["file:/a"])

p Gem.path

irb.jruby.1.5.5.txt

jruby-1.5.5 > require 'rubygems'
 => false 
jruby-1.5.5 > Gem.path
 => ["/Users/ericanderson/.rvm/gems/jruby-1.5.5@warbler", "/Users/ericanderson/.rvm/gems/jruby-1.5.5@global"] 
jruby-1.5.5 > ENV['GEM_PATH'] = 'a'
 => "a" 
jruby-1.5.5 > Gem.clear_paths
 => nil 
jruby-1.5.5 > Gem.path
 => ["/Users/ericanderson/.rvm/gems/jruby-1.5.5@warbler"] 
jruby-1.5.5 > ENV['GEM_PATH']
 => "a" 

irb.ruby.1.8.7.txt

~ (1.8.7) $ irb
ruby-1.8.7-p302 > require 'rubygems'
 => false
ruby-1.8.7-p302 > Gem.path
 => ["/Users/ericanderson/.rvm/gems/ruby-1.8.7-p302", "/Users/ericanderson/.rvm/gems/ruby-1.8.7-p302@global"]
ruby-1.8.7-p302 > ENV['GEM_PATH'] = "a"
 => "a"
ruby-1.8.7-p302 > Gem.clear_paths
 => nil
ruby-1.8.7-p302 > Gem.path
 => ["a", "/Users/ericanderson/.rvm/gems/ruby-1.8.7-p302"]
ruby-1.8.7-p302 > ENV['GEM_PATH']
 => "a"

jruby_partial.rb

module Gem

  ConfigFile::PLATFORM_DEFAULTS['install'] = '--env-shebang'
  ConfigFile::PLATFORM_DEFAULTS['update']  = '--env-shebang'

  @jar_paths = []

  class << self
    alias_method :original_set_paths, :set_paths
    def set_paths(gpaths)
      original_set_paths(gpaths)
      @gem_path.reject! {|p| !readable_path? p }
      @jar_paths.each {|p| @gem_path << p unless @gem_path.include?(p) } if @jar_paths
    end

    def readable_path?(p)
      p =~ /^file:/ || File.exists?(p)
    end

  end

So, jruby_partial.rb has the code that jruby ships. There are 2 big duhs for my problems.

1. In my IRB tests, the file obviously doesn't exist
2. The place I was using this in code was trying to set file:/some.jar!afile.txt, but since gems will split on ':' it never matter!