🧵 From JRuby + WebLogic Panic to Peace: A Debugging Story

How a tiny git: in Gemfile broke our WAR deployment — and cost me a sleepless night. 😰 The Situation We have multiple JRuby applications deployed on Oracle WebLogic. One of them suddenly stopped working. It had been: ✅ Working in production ✅ Working earlier on this same server ❌ Suddenly failing during startup The error: org.jruby.exceptions.StandardError: (GitError) https://github.com/jruby/warbler (at master@973d14c) is not yet checked out. Run `bundle install` first. And not just one error, And not just one error. Multiple layers of errors. That’s when the tension started. 🥊 Phase 1: Class Conflict Error The first error looked like a classic classloader issue: undefined method `bytesize' for Rack::Utils:Module That’s usually a symptom of: Different Rack versions loaded Library collision Parent-first classloading override We had: Multiple JRuby apps in the same WebLogic domain Different JRuby versions (The other app is using 9.3.5.0 and this one 9.4.5.0) Different Rack versions WebL