JRuby Workshop at RubyConf Austria 2026

Slides for the JRuby Workshop presented at RubyConf Austria 2026

Transcript

1. Do More with JRuby! Workshop and Of fi ce Hours QR CODE HERE
2. @headius headius.com Hello! • Charles Oliver Nutter ("headius") • Full-time JRuby dev since 2006 • Co-founder of Headius Enterprises • "The JRuby Company" • My fi rst time in Vienna! • Second time running a workshop!
3. HEADIUS.COM
4. Agenda • Short introductory talk about JRuby • Workshop topics and experimentation • https://github.com/headius/tutorial.git • Of fi ce hours
5. Workshop Topics • Getting started with JRuby • Simple Java/JVM integration • Packaging apps for redistribution • Glimmer for JRuby • JRuby on Rails • Building and contributing to JRuby
7. @headius headius.com What is JRuby? • Ruby on the Java Virtual Machine • Ruby implementation fi rst, JVM language second • Many bene fi ts from JVM ecosystem • Ruby code should "just work" • Try your code, your library • If something doesn't work, let us know!
8. @headius headius.com Ruby Compatibility • JRuby 10 (LTS) supports Ruby 3.4 • Language and core specs: 98% passing • Pure-Ruby standard library shared with CRuby • JRuby support for most native stdlib • JRuby 10.1 supports Ruby 4.0 • Leading edge, more experimental changes
9. @headius headius.com JVM Language • JRuby is a fi rst-class JVM language • Extensions are written in Java, not insecure C code • JVM can optimize Ruby code just like Java • JVM garbage collector is concurrent and ef fi cient • We write Java so you don't have to! • But you can if you want to! (or Scala, Clojure, Kotlin...)
10. @headius headius.com JRuby Architecture Ruby (.rb) JIT Java Instructions (java bytecode) Ruby Instructions (IR) parse interpret interpreter interpret C1 compile native code better native code java bytecode interpreter execute C2 compile Java Virtual Machine JRuby Internals
11. @headius headius.com A World of Libraries • JVM ecosystem has hundreds of thousands of libraries • Graphics, GUIs, servers, document formats, ML/AI/LLM • One of the largest collections in the dev world • All available to JRuby users! • Easy integration into Ruby apps and code • "Magic" to make Java APIs look like Ruby
12. @headius headius.com Example: 3D Bar Chart with JFreeChart • Chart-generating library with multiple output formats • Easily used from JRuby • https://blog.headius.com/ 2025/05/3d-charts-and-more-with- jruby-and-jfreechart.html
13. @headius headius.com
14. @headius headius.com Visual VM
15. @headius headius.com Threads vs Ractors 0 1.5 3 4.5 6 6x 3.7x 1.6x CRuby threads CRuby ractors JRuby threads
16. @headius headius.com "JRuby is excellent. I don't even code for MRI anymore and don't miss it." - "G. Gibson" on JRuby's Matrix chat
17. HEADIUS.COM
18. @headius headius.com Headius Enterprises • Expert support for your JRuby apps • New dev, migrations, upgrades • Bug prioritization, security SLAs • Help with your code under NDA • Optimizing Ruby libraries • Using JVM tools to improve perf • headius.com/welcome
19. @headius headius.com Thank You! • Please get in touch! • @headius on socials • headius@headius.com • JRuby Matrix chat • headius.com/welcome • USERS.md
20. Getting Started
21. Getting Started with JRuby • Install a Java Development Kit (JDK) • Install JRuby • Run script or IRB to verify
22. Install a JDK
23. Install a JDK • getting_started/install_jdk.md • Multiple choice! • JDK vendor? • JDK version? • What is a "JRE"? • Any JDK for Java 21+ will do
24. Getting the JDK • Option 1: OS-speci fi c package installer • apt, yum, pkg, brew • Option 2: download package or archive • Recommendation: Eclipse Temurin (by IBM) • Least encumbered, most "vanilla" build of OpenJDK
25. https://adoptium.net/temurin
26. Verify JDK Installation • java -version • jshell is IRB for Java • java in PATH is fi ne • Recommend setting JAVA_HOME $ java -version openjdk version "25" 2025-09-16 LTS OpenJDK Runtime Environment Zulu25.28+85-CA (build 25+36-LTS) OpenJDK 64-Bit Server VM Zulu25.28+85-CA (build 25+36-LTS, mixed mode, sharing) $
27. Install JRuby
28. Install JRuby • getting_started/install_jruby.md • Option 1: Installer/switcher tools • ruby-install, ruby-build, rvm, mise, asdf, ... • Option 2: Download from jruby.org • tarballs, zips, Windows installer • Unpack and set PATH
29. ruby-install $ ruby-install jruby >>> Updating jruby versions ... >>> Installing jruby 10.1.0.0 into /Users/headius/.rubies/jruby-10.1.0.0 ... >>> Downloading https://repo1.maven.org/maven2/org/jruby/jruby-dist/10.1.0.0/jruby- dist-10.1.0.0-bin.tar.gz into /Users/headius/src ... >>> Verifying jruby-dist-10.1.0.0-bin.tar.gz ... *** No md5 checksum for /Users/headius/src/jruby-dist-10.1.0.0-bin.tar.gz >>> Extracting jruby-dist-10.1.0.0-bin.tar.gz to /Users/headius/src/jruby-10.1.0.0 ... >>> Installing jruby 10.1.0.0 ... >>> Symlinking bin/ruby to bin/jruby ... >>> Successfully installed jruby 10.1.0.0 into /Users/headius/.rubies/jruby-10.1.0.0
30. Verify JRuby
31. Verify JRuby Install • getting_started/verify_jruby.md • ruby -v • irb • Run a simple script • Try to bundle install your app or library
32. Verify with IRB $ ruby -v jruby 10.1.0.0 (4.0.0) 2026-04-20 32f988b78c OpenJDK 64-Bit Server VM 25+36-LTS on 25+36-LTS +indy +jit [arm64-darwin] $ irb irb(main):001> RUBY_ENGINE => "jruby" irb(main):002> RUBY_VERSION => "4.0.0" irb(main):003> RUBY_ENGINE_VERSION => "10.1.0.0" irb(main):004> java.lang.Runtime.runtime.available_processors => 10
33. Java and JVM Integration
34. Simple Swing GUI • java_integration/swing.md • Try adding a second button • Try adding a JLabel with a simple .new("text")
36. JConsole • java_integration/jconsole.md • JConsole provides a monitoring/management tool for JVM apps • Monitor memory/heap, CPU usage, threads, and other details • The "MBeans" tab provides JVM and app-speci fi c tools • Run JRuby with --manage for JRuby tools • Try starting jconsole and connecting to your JRuby app
38. JDK Mission Control • "Enterprise-grade" pro fi ling for JVM-based applications • Method calls • Memory allocations • Thread contention • Based on "JDK Flight Recorder" output
39. JDK Flight Recorder • Built-in low-overhead monitoring and pro fi ling for JVM • Less than 1% overhead for monitoring • Less than 2% overhead for basic pro fi ling • "Almost unnoticeable" • Enable at command line or let JDK Mission Control enable it
47. jruby -Xreify.classes
49. Packaging JRuby Apps
50. Packaging and Deploying • JRuby applications can be deployed like Ruby • Whatever you use today probably works fi ne • The JVM world provides other ways to deploy apps • "Executable" Java Archive (JAR) fi les that need only a JDK • Web Application Archive (WAR) fi les to deploy on an app server
51. Warbler • packaging_apps/warbler • JRuby's tool for packaging apps • Combine your app plus JRuby plus dependencies in a single fi le • Executable JAR or deployable WAR • Generate con fi g, tweak settings, and go
52. Using Warbler • gem install warbler • mkdir config • warble config and tweak settings • warble jar
54. [] warbler $ java -jar hello.jar Hello, world! [] warbler $
55. [] warbler $ java -jar hello.jar WARNING: A restricted method in java.lang.System has been called WARNING: java.lang.System::load has been called by com.kenai.jffi.internal.StubLoader in an unnamed module (file:/var/folders/ 5n/y5_kd6x94rndnyjh4519bp2w0000gn/T/jruby15925531817024339324extract/jruby- core-10.0.5.0-complete.jar) WARNING: Use --enable-native-access=ALL-UNNAMED to avoid a warning for callers in this module WARNING: Restricted methods will be blocked in a future release unless native access is enabled Hello, world! [] warbler $
56. Embedding JRuby • JRuby also includes Java APIs for embedding into apps • Load JRuby, request a new instance, execute code • Used by large Java apps and plugins to leverage Ruby features
57. JRuby on Rails
58. JRuby on Rails • JRuby has supported Rails since 2006 (Rails 1.x) • ActiveRecord is the only part that needs JRuby-speci fi c logic • Native adapters replaced with Java's JDBC API • The rest of Rails generally just works • Rails on JRuby can scale much better due to threads
59. Bootstrapping JRoR • Rails 7.0-7.1: Just "rails new" • Rails 7.2: Use "rails new --skip-bundle" and tweak Gem fi le • Rails 8.0-8.1 • Use template from workshop repo or https://jruby.org/rails/8.0.rb • Pass `--skip-kamal` due to an unsupported dependency
60. Workshop Ideas • Generate app • Scaffold something basic and migrate DB • Run server... it works • Run console... it works too • What else?
61. Building and Contributing
62. Building JRuby • Easiest Ruby to build by far • Install JDK • Clone https://github.com/jruby/jruby.git • Run ./mvnw • The cloned repository is now a fully-functional JRuby install!
63. Codebase Structure • bin - startup scripts and installed Ruby binstubs • core/src/main/java - Java sources of JRuby core • core/src/main/ruby - Ruby sources of JRuby core • lib/jruby.jar - the built JRuby core jar • lib/ruby/stdlib - Ruby core stdlib • lib/ruby/gems/shared/gems - Ruby gem home • test and spec - many, many test suites
64. Glimmer for JRuby