Kaushik Vejju

How Uber Migrated 75K JUnit4 Test Classes to JUnit5 Using OpenRewrite

At Uber, we migrated our entire Java test codebase from JUnit4 to JUnit5 using OpenRewrite, fully automating one of the largest test migrations in the industry. The migration covered over 75,000 test classes, 400,000 tests, and 15M+ lines of code, with 1.5 million lines modified completed in just a few months with minimal developer disruption.

This session covers the engineering approach, tooling design, and validation strategies that made this possible. Topics include customizing OpenRewrite for Ubers monorepo, ensuring migration correctness at scale, managing rollout safety, and integrating automated refactoring into CI pipelines. Attendees will learn practical strategies for applying large-scale code transformations safely and efficiently in complex production environments.