Spring Boot 3 to 4 Migration

Migrate in controlled stages. Keep behavior changes separate from the framework upgrade.

Establish the baseline

1. Upgrade to the latest Boot 3.5 maintenance release.
2. Run unit, slice, integration, startup, and migration tests before changing the major version.
3. Remove Boot 3 deprecations and record all explicitly versioned dependencies.
4. Compare the Boot 3.5 and Boot 4 dependency-management reports.
5. Verify Spring Cloud and other portfolio release-train compatibility independently.

Prepare for Boot 4

• Require Java 17 or newer; prefer Java 21 for application builds.
• Replace javax.* remnants with Jakarta APIs before the upgrade.
• Remove Undertow assumptions; Boot 4 requires a Servlet 6.1-compatible container.
• Inventory Jackson 2 custom modules, serializers, ObjectMapper beans, and package imports.
• Inventory test annotations, especially @MockBean, @SpyBean, and implicit MockMvc setup.
• Add spring-boot-properties-migrator temporarily after changing the Boot version, then remove it.

Migrate dependencies deliberately

Boot 4 is more modular. Prefer dedicated starters over relying on incidental transitive dependencies. Expect dedicated starters for web MVC, security tests, Flyway/Liquibase, and technology-specific tests. Use the classic starters only as a temporary diagnostic bridge, never as the final dependency model.

Verify the result

• Run the application with every supported profile.
• Exercise schema migration against a production-like database copy.
• Verify JSON contracts, security failures, pagination, and error responses.
• Confirm actuator exposure, logging, metrics, and tracing behavior.
• Remove the properties migrator and all classic starters before declaring the migration complete.

Examples

• See examples/good-migration-plan.md for a staged migration.
• See examples/bad-migration-plan.md for a risky one-step upgrade.

Gotchas

• Agent jumps from an old Boot 3 minor directly to Boot 4 - upgrade to current 3.5 first.
• Agent changes framework versions and business behavior together - isolate the migration diff.
• Agent assumes all Boot 3 starters keep the same names and transitive dependencies - audit each one.
• Agent leaves spring-boot-properties-migrator in production - remove it after configuration cleanup.
• Agent treats passing compilation as completion - verify runtime wiring, JSON, security, and tests.