Spring Boot 4.0 Migration Guide

Spring Boot 4.0 isn't a routine upgrade. The Spring team made deliberate, sweeping changes, clearing out years of deprecated APIs, reorganizing package namespaces, and tightening the component model.

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The result: 83 documented breaking changes with an estimated 200–500 hours of migration effort, depending on your codebase.

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The catch? Most teams don't know what they don't know.

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At Spring I/O 2026, roughly 100 developers took a 15-question migration quiz. The mean score was 66%, meaning the average Spring developer is walking into a migration with about 50 hours of unplanned work already built in.

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This guide, authored by Java Champion and HeroDevs Developer Advocate Steve Poole, makes the hidden work visible before it becomes a problem.

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Inside you will find:

1. Tier 1 – Won't Build: 39 changes that stop the compiler cold
2. Tier 2 – Won't Run: 27 changes that pass the build but crash at startup or runtime
3. Tier 3 – Wrong Results: 17 silent behavioral changes that can sit undetected in production for months — the most dangerous category
4. A Migration Playbook for sequencing the work
5. A Spring Security 7 Annex covering the security-layer changes specifically
6. "Harder Than It Looks" callouts for changes whose per-card effort estimate understates the real risk

Whether you're planning to migrate now, migrate incrementally, or need time before your team can take it on, this guide gives you an honest picture of what's actually involved.

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Download the full guide to get your team ahead of it.