Spring Boot 4 Migration

Migrate from the latest Boot 3.5 release and keep business changes outside the upgrade diff.

Required baseline

• Use Java 17 or newer; prefer Java 21 for application builds.
• Use Spring Framework 7, Jakarta EE 11, and a Servlet 6.1-compatible container.
• Upgrade Kotlin to the supported baseline when applicable.
• Use a Boot 4-compatible Spring Cloud release train.
• Use GraalVM 25 or newer for native images.

Migrate modules and starters

Boot 4 splits infrastructure into focused modules and starters.

• Replace spring-boot-starter-web with spring-boot-starter-webmvc.
• Use dedicated technology starters such as spring-boot-starter-flyway.
• Use technology-specific test starters; they bring the core test starter transitively.
• Use spring-boot-starter-security-test for Spring Security test support.
• Use spring-boot-starter-batch-jdbc when jobs require persistent metadata and restartability.
• Use classic starters only as a temporary migration bridge, then remove them.

Migrate code

• Replace Jackson 2 com.fasterxml.jackson customization with Jackson 3 tools.jackson APIs.
• Replace removed @MockBean and @SpyBean with framework Mockito bean overrides.
• Add @AutoConfigureMockMvc when a @SpringBootTest requires MockMvc.
• Replace removed or moved Boot package imports and deleted Boot 3 deprecations.
• Remove Undertow configuration and verify the selected server supports Servlet 6.1.

Configuration workflow

1. Add spring-boot-properties-migrator temporarily.
2. Start every supported profile and fix each reported property.
3. Remove the migrator before release.
4. Compare /actuator/configprops, logging, JSON, and endpoint behavior against the baseline.

Verification

• Run unit, slice, integration, native, startup, and database migration tests as applicable.
• Verify JSON contracts, security failure responses, actuator exposure, and test slice composition.
• Inspect the dependency tree for old starters, duplicate Jackson generations, and unmanaged versions.

Examples

• See examples/good-migration-plan.md and examples/bad-migration-plan.md.

Gotchas

• Agent keeps old starter names because compilation succeeds transitively - use Boot 4 dedicated starters.
• Agent adds both a technology test starter and the classic test starter - avoid duplicate test graphs.
• Agent leaves Jackson 2 and Jackson 3 customizations together - migrate the complete JSON boundary.
• Agent expects plain Batch starter metadata to persist - use the JDBC starter when restartability matters.
• Agent leaves the properties migrator or classic starters installed - remove temporary migration aids.