marking for 9.2 ... its not a hard requirement, more of a discussion - we could post-pone for 9.3
but it would be nice to have "less" loading - be clear what is loaded (we officially only require 'java')
... and as noted above reflecting JRuby's classes (with JI) ends up with some invalid class state atm.
see #4528

@enebo @kares Where do we stand on this? I'm not opposed, but I have no idea whether these features are in use elsewhere.

I believe we should constrain require 'jruby' to only the places we know it will be used. Putting it near the top of JI makes no sense to me. So I think we should apply this PR but I also wonder why it is here at all? Do we use #reference or something somewhere? If so the second part of this PR would be to put the include wherever we do that.

think it has been there historically to have JRuby.runtime available - mostly due ext loading.
but we know have a better way of loading exts using JRuby::Util.load_ext(name) which avoids initializing JI for JRuby's internals ...
on places JRuby still needs JRuby.runtime an explicit require 'jruby' should do.
the only down side this has is that it might break some extensions
but I believe most of them should require 'jruby' if they depend on JRuby.runtime

9.3.0.0 seems like a good time...significant # and if this does break some extensions we have some time to get them to add a single line to fix it.

okay - will update PR as this implicit require 'jruby' now lives in Java

PR is now ready ... the require 'jruby' removal is pretty much just two lines
needed to adjust tests and one place we assumed the library to be loaded, also avoided the requirement (to require 'jruby') from io/nonblock (would cause self-reflection on JRuby's Java classes so its all for the best)

I'm approving this but it does feel a little weird now to have JRuby defined by default without any methods in it. Perhaps it was a bad idea originally to use the namespace directly for all these methods that would be better in their own namespaces (compilation utilities etc).

in any case, I'm fine going forward with this since most people don't even know about these utilities, and they're very much JRuby-specific anyway.