Ruby implementation on the Java Virtual Machine. Enables Ruby code to call Java and Kotlin libraries as if they were Ruby classes (Java case auto-converted to Ruby snake_case). Charles Nutter has worked on it full-time since 2006, originally hired by Sun Microsystems at the start of the Rails explosion. Focused first on being a faithful Ruby implementation (same tools, same command lines, same workflow) and second on JVM benefits. Currently supports Ruby 3.4 (98–99% of Ruby core specs pass; missing pieces are things that can't be done on the JVM like forking); JRuby 10.1 (due ~April 2026) will support Ruby 4.0. Ships pre-compiled Java-bytecode extensions so no C build tools are needed on target systems. Packages as executable JAR files (run via `java -jar`) or WAR files deployable on any Java application server. JRuby Rack bridges the Java Servlet API to any Rack application (Rails, Sinatra, Hanami, etc.). The JIT compiles Ruby → JRuby IR bytecode → JVM bytecode → optimized native code (first native JIT for Ruby, ~2008). Benefits from world-class JVM garbage collectors (including pauseless configurations), monitoring via VisualVM, and trivially parallel threads with shared mutable state — no GIL. Used to drive a Kotlin roguelike framework from Ruby (Game Hunter lightning talk). JRuby 10.1 also integrates new JVM features (Leyden-style pre-optimization, Lilliput object shrinking — JRuby objects now ~half the size of C Ruby objects in memory).