JRuby is a JavaTM implementation of the Ruby interpreter. While retaining many of the popular characteristics of Ruby, such as dynamic-typing, JRuby is integrated with the Java platform. With JRuby on Rails, you get the simplicity and productivity offered by Ruby and Rails and the power of the Java platform offered by JRuby, thereby giving you many benefits as a Rails developer, including these:
You can access the rich set of Java libraries from your Rails application.
You can use the powerful and secure support of Java Unicode strings with your Rails application.
Your JRuby on Rails application can spin off multiple threads because JRuby uses Java threads, which map to native Ruby threads. Furthermore, you can pool these threads.
To learn more about JRuby, see JRuby.