JRuby: Professional-Grade Ruby

Opening talk at wroclove.rb 2026 by Charles Nutter. Explains what JRuby is (Ruby on the JVM), why it opens Ruby up for enterprise adoption, and demonstrates: installing JRuby alongside any JDK via ruby-install; calling Java classes (java.lang.Runtime#availableProcessors, #freeMemory) like Ruby classes with case conversion; Ruby 3.4 compatibility today and Ruby 4.0 arriving in JRuby 10.1 'next week'; deploying via Puma (workers→threads), executable JARs and WAR files on Java application servers via JRuby Rack; the JRuby architecture (Ruby → IR/bytecode → interpreter → JVM bytecode → native via HotSpot, native JIT shipped in 2008); benchmarks (Object.new ~10.3M/s on C Ruby 3.4 + YJIT, ~30% faster with Ruby 4.0's opt_new, orders-of-magnitude faster on JRuby, Mandelbrot >2× faster than C Ruby with YJIT and another 2× on GraalVM); pulling JVM libraries (JFreeChart for 3D charts via jar-file + lock_jars); Glimmer + SWT desktop GUIs; Ruboto for Android; pauseless JVM garbage collectors, VisualVM monitoring; trivial true parallelism with threads vs. C Ruby Ractors (JSON-parse demo: C Ruby threads ~1×, Ractors ~3.7×, JRuby threads ~6× on a 10-core machine); JRuby 10.1 futures (Project Leyden-style pre-optimization, Lilliput object shrinking). Production user spotlights: Quantum Inventions (ships JAR/WAR tools to customers, AOT obfuscation), Poshmark (high-throughput e-commerce flash sales), Kami (Rails front-end with JRuby backend processing millions of PDFs/day), Logstash (massively parallel log aggregation), Substitute Alert (fan-out notifications to substitute teachers in the US). Charles's company Headius Enterprises now funds JRuby development after 18 years of big-company sponsorship ended in 2024. Q&A: what's bad about JRuby (startup time — now ~400ms baseline / 1–1.5s for big rake tasks — and missing C extensions, big ones like Nokogiri/JSON/Psych shipped as JRuby versions), difference vs TruffleRuby (TruffleRuby needs GraalVM, focuses on C extension compat and micro-benchmarks, uses more memory, poor startup; JRuby targets whole ecosystem and has way more deployments), Rails compatibility (yes, adapters for Rails 8.0/8.1 almost ready), deployment on Heroku/Railway (Heroku build pack released each version, works like regular Ruby).