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JUnit Migration at Scale

1 post · Apr 2026

What this trail covers

Uber has successfully migrated a significant portion of its Java monorepo from JUnit 4 to JUnit 5, enabling adoption of modern testing capabilities and ensuring future framework support. This involved developing a custom Bazel integration for the JUnit Platform and leveraging OpenRewrite for automated code transformations. The migration processed over 75,000 test classes, modifying 1.25 million lines of code within four months, demonstrating a scalable approach to large-scale framework transitions.

All Uber trails
  1. How Uber Executed A JUnit Migration at Massive ScaleThis post details the engineering effort behind migrating Uber's Java monorepo from JUnit 4 to JUnit 5. Key contributions include: 1. Developing a custom Bazel test runner to integrate the JUnit Platform (supporting both Vintage and Jupiter engines) with Bazel. 2. Building a custom Bazel aspect to integrate OpenRewrite for automated code refactoring, enabling it to collect source files and compilation classpaths for type resolution. 3. Authoring custom OpenRewrite recipes to handle Uber-specific JUnit 4 test conventions (rules, runners, base classes) and contributing upstream improvements. 4. Implementing precondition recipes within OpenRewrite to filter test files, ensuring only fully transformable targets were processed, thereby maximizing migration scale and minimizing manual intervention. 5. Describing the end-to-end workflow for migrating test targets, including the generation of patch files and diagnostic datatables.