JRuby is an implementation of the Ruby programming language atop the Java Virtual Machine, written largely in Java. It is free software released under a three-way EPL/GPL/LGPL license. JRuby is tightly integrated with Java to allow the embedding of the interpreter into any Java application with full two-way access between the Java and the Ruby code (similar to Jython for the Python language). JRuby's lead developers are Charles Oliver Nutter and Thomas Enebo, with many current and past contributors including Ola Bini and Nick Sieger. In September 2006, Sun Microsystems hired Enebo and Nutter to work on JRuby full-time. In June 2007, ThoughtWorks hired Ola Bini to work on Ruby and JRuby. In July 2009, the JRuby developers left Sun to continue JRuby development at Engine Yard. In May 2012, Nutter and Enebo left Engine Yard to work on JRuby at Red Hat. History JRuby was originally created by Jan Arne Petersen, in 2001. At that time and for several years following, the code was a direct port of the Ruby 1.6 C code. With the release of Ruby 1.8.6, an effort began to update JRuby to 1.8.6 features and semantics. Since 2001, several contributors have assisted the project, leading to the current () core team of around six members. JRuby 1.1 added Just-in-time compilation and Ahead-of-time compilation modes to JRuby and was already faster in most cases than the then-current Ruby 1.8.7 reference implementation. JRuby packages are available for most platforms; Fedora 9 was among the first to include it as a standard package at JRuby 1.1.1. In July 2009, the core JRuby developers at Sun Microsystems, Charles Oliver Nutter, Thomas Enebo and Nick Sieger, joined Engine Yard to continue JRuby development. In May 2012, Nutter and Enebo left Engine Yard to work on JRuby at Red Hat. JRuby has supported compatibility with Ruby MRI versions 1.6 through 1.9.3. JRuby 1.0 supported Ruby 1.8.6, with JRuby 1.4.0 updating that compatibility to Ruby 1.8.7. JRuby 1.6.0 added simultaneous support for Ruby 1.9.2, with JRuby 1.7.0 making Ruby 1.9.3 the default execution mode (Ruby 1.8.7 compatibility is available via a command-line flag). JRuby 9.0.0.0 added support for Ruby 2.2. The current version of JRuby (9.4.1.0) targets Ruby 3.1, though some 3.1 features are still in work. Ruby on Rails JRuby has been able to run the Ruby on Rails web framework since version 0.9 (May 2006), with the ability to execute