Gem::Platform::JAVA is only provided under the bundler

Author: hsbt

lib/rubygems/basic_specification.rb

@@ -75,7 +75,7 @@ def contains_requirable_file?(file)
       if platform == Gem::Platform::RUBY || Gem::Platform.local === platform
         # RubyGems warns gem specification of C extensions. But JRuby couldn't
         # rebuild with `gem pristine` command. So, we skip the warning.
-        if RUBY_PLATFORM != Gem::Platform::JAVA
+        if RUBY_PLATFORM != "java" # JRuby
           warn "Ignoring #{full_name} because its extensions are not built. " \
               "Try: gem pristine #{name} --version #{version}"
         end