destroyable_test.rb

module Destroyable
  def call_destructor(id)
    @destructors[id].each do |o|
      o.__destroy(id) if o.respond_to? :__destroy
    end
    @destructors.delete(id)
  end
  
  def is_destroyable
    @destructors = {}
    include InstanceMethods
  end
  
  def set_resources(id, res)
    @destructors[id] = res
  end
  
  module InstanceMethods
    def initialize
      ObjectSpace.define_finalizer(self, self.class.method(:call_destructor).to_proc)  
    end

    def declare_resources(res)
      self.class.set_resources(object_id, res)
    end
  end
end

class MyBaseClass
  extend Destroyable
end

class MyClass < MyBaseClass
  is_destroyable

  def initialize
    super
    puts "#{object_id}: MyClass created"
    @db = DBConnection.new("jdbc://foo@bar/")
    declare_resources([@db])
  end
  
  def access_db
    @db.do_stuff
  end
end

class DBConnection
  def initialize(conn_string)
    @conn_string = conn_string
    @connected = false
  end
  
  def connect
    puts "Connecting to #{@conn_string}"
    @db = "Foo" + rand.to_s
    @connected = true
  end
  
  def do_stuff
    connect unless @connected
    puts "Doing stuff with #{@conn_string}"
  end
  
  def query_active?
    return rand < 0.5
  end
  
  def __destroy(id)
    puts " #{id}: Closing #{@conn_string}."
    if query_active?
      puts " #{id}: Just kidding, can't close while query is running."
    elsif !@connected
      puts " #{id}: Just kidding, DB connection already closed."
    else
      puts " #{id}: Connection closed."
      @connected = false
    end
  end
end

# test code
3.times {
    MyClass.new.access_db
}
ObjectSpace.garbage_collect

puts "done"

jruby1.4.0_output

6: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
8: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
10: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
done
 10: Closing jdbc://foo@bar/.
 10: Just kidding, can't close while query is running.
 8: Closing jdbc://foo@bar/.
 8: Connection closed.
 6: Closing jdbc://foo@bar/.
 6: Connection closed.

ruby1.8_output

-608142678: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
-608142818: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
-608143018: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
 -608142678: Closing jdbc://foo@bar/.
 -608142678: Connection closed.
 -608142818: Closing jdbc://foo@bar/.
 -608142818: Connection closed.
 -608143018: Closing jdbc://foo@bar/.
 -608143018: Just kidding, can't close while query is running.
done

ruby1.9_output

73998480: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
73998290: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
73998100: MyClass created
Connecting to jdbc://foo@bar/
Doing stuff with jdbc://foo@bar/
 73998480: Closing jdbc://foo@bar/.
 73998480: Just kidding, can't close while query is running.
 73998290: Closing jdbc://foo@bar/.
 73998290: Connection closed.
done
 73998100: Closing jdbc://foo@bar/.
 73998100: Just kidding, can't close while query is running.