add platform = 'java' to tell rubygems it is a jruby only gem and use
jruby itself when installing and using the gem.

Could you be more specific, please? Also, where should I add platform = 'java' exactly?

I see.

The commands I use to build and install the gem are as follows:

gem build gem_name.gemspec
install ./gem_name-0.0.0

How can these be any different from jruby?

BTW I did try to only add s.platform = 'java' and it still gave me the same error after building and installing the gem (using the previous commands)

I never saw the install command ! installing gems usually works: gem install -l gem_name-0.0.0-java.gem (please ensure to use the gem file produced by the gem build command.

I ran the command:
gem build gem_name.gemspecs
which resulted in the file
gem_name-0.0.0-java.gem
so I ran the command:
gem install -l gem_name-0.0.0-java.gem
and I STILL got the error:
LoadError: cannot load such file -- java

the error says you are not using jruby. what is gem env telling you ?

$ gem env
RubyGems Environment:
- RUBYGEMS VERSION: 2.4.5
- RUBY VERSION: 2.2.2 (2015-04-13 patchlevel 95) [x64-mingw32]

I see now that Ruby gems are being used and not Jruby. How do I specify that Jruby Gems are being used?

I mean, thanks for linking me to that page but I already have jruby installed and my question is, how do I make sure Gem is using the jruby gems and not the Ruby gems?

since both Jruby and Ruby use the same following command:

gem build gem_name.gemspec

ah - I see.

better use https://github.com/sstephenson/rbenv to install jruby or RVM to
install JRuby (link on the getting started page) or maybe jruby -S gem install gem_name-0.0.0-java.gem if you installed it and set the PATH
right. or use the bin/jruby executable directly.

Using jruby -S gem build... and jruby -S gem install... worked for me, finally. thank you. :)

I have a Gem wholly written in Ruby, supporting Rails. However, I want to bring it to JRuby to be usable on my Java code, and I am curious if there are any resources to make this happen?

This is all I have done to bring Java to it that Ruby Gem.

# ...
Gem::Specification.new do |spec|
  # ...
  if RUBY_PLATFORM =~ /java/
    spec.platform    = 'java'
end

These are all the dependencies the Gem supports:

Gem::Specification.new do |s|
  s.platform     = Gem::Platform::RUBY

  s.required_ruby_version = '>= 2.5'

  s.files = Dir['CHANGELOG', 'README.md', 'MIT-LICENSE', 'CONTRIBUTORS', 'lib/**/*', 'vendor/**/*']
  s.require_path = 'lib'

  s.metadata['allowed_push_host'] = 'https://rubygems.org'

  s.has_rdoc = true if Gem::VERSION < '1.7.0'

  s.add_dependency('activesupport', '>= 4.2')
  s.add_dependency('builder', '>= 2.1.2', '< 4.0.0')
  s.add_dependency('i18n', '>= 0.6.9')
  s.add_dependency('nokogiri', '~> 1.4')

  s.add_development_dependency('mocha', '~> 1')
  s.add_development_dependency('pry')
  s.add_development_dependency('pry-byebug')
  s.add_development_dependency('rake')
  s.add_development_dependency('test-unit', '~> 3')
  s.add_development_dependency('thor')
end

However, I will be glad if there are resources you can point me to so I can learn more about bringing Ruby Gem to Java code as I feel what I have done is not enough to make it in a Java code.