ThreadGroup#list patch for JRuby 1.7.4 (-1.7.18)

thread_group_list.rb

if JRUBY_VERSION < '1.7.19'
  require 'thread'; require 'java'
  org.jruby.RubyThreadGroup.field_reader :rubyThreadList
  ThreadGroup.class_eval do
    def list
      threads = []
      list = to_java.rubyThreadList
      list.synchronized do # iterator must be sync-ed
        list.each { |t| threads << t if t }
      end
      threads
    end
  end
end