JRuby is a 100% Java implementation of the Ruby programming language. It is Ruby
for the JVM.

JRuby provides a complete set of core "builtin" classes and syntax for the Ruby
language, as well as most of the Ruby Standard Libraries. The standard libraries
are mostly Ruby's own complement of .rb files, but a few that depend on C
language-based extensions have been reimplemented. Some are still missing, but
we hope to implement as many as is feasible.