A practical Spring Boot 3 to 4 migration checklist: start with 3.5.x, review modular starters, adopt Jackson 3, update tests and configuration, and validate production behavior.
Upgrade path
Move through the latest 3.5.x release before upgrading to a 4.0.x maintenance release.
Dependencies
Review the Java baseline, modular starters, moved imports, and third-party compatibility.
Jackson 3
Update JSON integration and verify payloads and custom serializers.
Tests
Migrate mock annotations, review test dependencies, and verify application startup.
Configuration
Use the properties migrator temporarily, fix settings, and remove it after cleanup.
Release gates
Check contracts, health, latency, and recovery before rolling out.