[johnb]

I don't think that's true. There are plenty of threadsafe gems with C extensions, and plenty of non-threadsafe pure ruby gems that mutate class/class-instance variable willy-nilly.

[boundlessdreamz]

gems with C extensions are not supported https://github.com/jruby/jruby/wiki/C-Extension-Alternatives

I forgot about thread safety in pure ruby gems but that is not a huge problem - https://github.com/jruby/jruby/wiki/Gems-known-not-to-be-thr...