p RUBY_ENGINE
# => "jruby"

JRuby is an implementation of Ruby that runs inside the runs on vm::Java Virtual Machine and can call Java libraries from Ruby code. JRuby 1.6.x implements both Ruby 1.8.7 and Ruby 1.9.2 in one install.

JRuby has preemptive concurrency like Java; each Ruby Thread is a Java Thread. JRuby can run multiple threads in parallel on multiple CPUs, if the JVM can do so. MRI has a Global VM Lock and can run these threads on only one CPU. In contrast, JRuby can never fork processes; MRI can fork with some platforms.

JRuby implements the interpreter and most of the core library in implemented in language::Java. The standard library is a mix of Java and Ruby.

• http://jruby.org