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{{implementation|Ruby}} {{infobox begin}}
p RUBY_ENGINE # => "jruby"
{{infobox end}}
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