Warning: "singleton on non-persistent Java type" with JRuby 1.7.4

[dvgica]

Hi,

I'm using mocha (and thus, this gem) on a JRuby 1.7.4 project which uses some Java classes.

It appears that line 4 of object_methods.rb triggers the following warning:

~/.rvm/gems/jruby-1.7.4@project/gems/metaclass-0.0.1/lib/metaclass/object_methods.rb:4 warning: singleton on non-persistent Java type Java::User (http://wiki.jruby.org/Persistence)

I did read the JRuby wiki about this: https://github.com/jruby/jruby/wiki/Persistence. But I'm not terribly sure what can be done in metaclass, if anything.

Any ideas? I don't think it's super-critical, but something to be aware of, perhaps.

Thanks!

/cc @kopchickm @karlj-spindance

[floehopper]

I can't seem to reproduce the warning. Here's what I've tried:

# jruby-warning.rb
require 'bundler/setup'
require 'metaclass'

p java.lang.String.__metaclass__

$ ruby -v
jruby 1.7.4 (1.9.3p392) 2013-05-16 2390d3b on Java HotSpot(TM) 64-Bit Server VM 1.6.0_31-b04-413-12C60 [darwin-x86_64]

$ ruby -w jruby-warning.rb
#<Class:Java::JavaLang::String>

If this really is a problem, we could (a) detect JRuby within the #__metaclass__ method and set __persistent__ to true as per the wiki; or (b) try to supress this specific warning somehow.

[floehopper]

To be honest. Given that Object#singleton_class has been added to Ruby since v1.9.2 [1], I suspect Mocha can start using that and this whole gem can go away soon!

[1] http://ruby-doc.org/core-1.9.2/Object.html#method-i-singleton_class

[dvgica]

Thanks for looking into this. I was able to reproduce the issue by slightly modifying your test script:

require 'rubygems' # didn't use Bundler
require 'metaclass'

p java.lang.String.new("test").__metaclass__

Results in:

$ ruby -w jruby-warning.rb 
/home/davidv/.rvm/gems/jruby-1.7.4/gems/metaclass-0.0.1/lib/metaclass/object_methods.rb:4 warning: singleton on non-persistent Java type Java::JavaLang::String (http://wiki.jruby.org/Persistence)
#<Class:#<Java::JavaLang::String:0x3bd54235>>

Ruby version:

$ ruby -v
jruby 1.7.4 (1.9.3p392) 2013-05-16 2390d3b on Java HotSpot(TM) 64-Bit Server VM 1.7.0_40-b43 +indy [linux-amd64]

[floehopper]

Hmm. This seems to be more fundamental than I first thought. Setting __persistent__ to true on the first line of #__metaclass__ doesn't solve the problem. And even calling Object#singleton_class (i.e. nothing to do with the metaclass gem) without setting __persistent__ to true generates a warning:

p java.lang.String.new("test").singleton_class

$ ruby -w jruby-warning.rb
jruby-warning.rb:5 warning: singleton on non-persistent Java type Java::JavaLang::String (http://wiki.jruby.org/Persistence)
#<Class:#<Java::JavaLang::String:0x2904b5ae>>

I'm not sure I can see a way around this other than explicitly setting __persistent__ to true for the relevant Java class yourself.

java.lang.String.__persistent__ = true
p java.lang.String.new("test").singleton_class

$ ruby -w jruby-warning.rb
#<Class:#<Java::JavaLang::String:0x6153e0c0>>

Personally I think I'd be tempted to try to avoid stubbing the Java class directly - perhaps by wrapping it in some kind of Ruby adaptor class. Sorry - I hope that helps.

[dvgica]

That does help, thanks.

I did manage to set __persistent__ in #__metaclass__. I'm not sure how my way is different than yours, but it did seem to work (I just monkeypatched Metaclass::ObjectMethods right in jruby-warning.rb:

module Metaclass::ObjectMethods
  def __metaclass__
    self.class.__persistent__ = true
    class << self
      self
    end
  end
end

Digging deeper, it looks like we have this issue for another gem besides metaclass (https://github.com/apotonick/representable).

So it seems that setting __persistent__ is really the only option... I'm just not sure whether it's more appropriate to do it in the gem (as above), or in my application. Maybe @headius could weigh in here?

[floehopper]

Oh. Cool. I'm not sure what I was doing (have deleted it), but whatever it was it was wrong!

In order to come up with a better solution, I think we'd need to look in detail at how Mocha is using the singleton class and see whether we could store state in a different data structure as per the alternatives section on the JRuby wiki page.

[floehopper]

I think we ought to be able to use RUBY_PLATFORM == 'java' to decide whether or not to set __persistent__ to true.