JRuby is the primary alternative implementation of Ruby available. With support for native threads, excellent garbage collection, and the introduction of Invoke Dynamic it has grown into a formidable platform. So, what can JRuby do for you? Learn how JRuby differs from MRI, how JRuby supports parallelism, how to use Java libraries from JRuby, how to use Ruby from Java and more. Java no longer means just the language anymore, the JVM is an excellent place to host Ruby applications.