I upgraded two production applications to Spring Boot 4.0 the week it went GA. I read the migration guide, skimmed the release notes, and thought the hardest part would be the Jackson 3 change everyone was talking about.
The Jackson 3 change was not the hardest part.
Two applications, a few days of debugging, and one very confusing test suite later, I have a clear picture of what actually breaks — and what the official guide glosses over.
This is not a walkthrough of the migration guide. This is the stuff that costs you real time.
This is a preview. Read the full article on Medium: Spring Boot 4.0 Migration: What Nobody Tells You About the Breaking Changes
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