Uber engineered an automated approach to migrate its massive Java monorepo (over 600,000 tests, 15 million lines of code) from the deprecated JUnit 4 to JUnit 5. Facing challenges like the lack of native JUnit 5 support in their Bazel build system and custom test configurations, they successfully migrated over 75,000 test classes and 1.25 million lines of code in just four months without disrupting developer workflows.

https://www.uber.com/us/en/blog/junit-migration/