Spring Boot 4 Migration Skill

Migrate Spring Boot 2.7.x applications to 3.5.x, then 3.5.x to 4.x,
and stay current across 4.x minor versions, anchored to the official
Spring Boot migration guides and release notes for each leg.

Scope: 2.7.x → 3.5.x → 4.x and 4.x Minor Versions

This skill covers three scenarios:

1. Legacy migration (2.7.x → 3.5.x) — the prelude. Covered in
   references/spring-boot-2-to-3-migration.md: Java 8/11 → 17 baseline,
   Jakarta EE namespace migration (javax.* → jakarta.*), Spring
   Security 5 → 6, Hibernate 5 → 6, observability migration to Micrometer
   Tracing/Observation, OpenRewrite recipe automation for every step.
   Reach Boot 3.5 latest patch here, then chain into the next scenario.
2. Major migration (3.5.x → 4.0) — the bulk of this skill. All 9 phases,
   the gradual upgrade strategy, and the bridge system below.
3. Minor version upgrades (4.0 → 4.1, 4.1 → 4.2, etc.) — tracked in
   references/minor-version-changes.md. Minor versions may deprecate
   APIs, remove compatibility bridges, change defaults, and introduce new
   features. Check that file before bumping to any new 4.x minor version.

If your project is on Boot 2.7.x or earlier, start with §
"Coming from Spring Boot 2.7?" below, complete the 2 → 3 leg, then
return here for the 3 → 4 phases.

Verify APIs Against Current Documentation

Many APIs change across these migrations. When unsure whether an API,
property, or pattern is still valid for the target version, look it up
rather than relying on model knowledge:

• Use Context7 (mcp__claude_ai_Context7__resolve-library-id then
  mcp__claude_ai_Context7__query-docs) for library docs.
• Use WebSearch / WebFetch for breaking-change announcements, CVEs,
  and community migration reports.
• Use gh for release notes (e.g.,
  gh api repos/spring-projects/spring-boot/releases/latest).

Toolchain Version Check (Do This First)

Before starting any migration, detect the project's Java, Kotlin, Maven,
and Gradle versions. If any are below the minimums for the target Boot
version, upgrade them BEFORE bumping Boot.

Read references/toolchain-versions.md for the minimum/recommended
versions table and per-tool upgrade commands.

Coming from Spring Boot 2.7? Start here.

If your project is on Spring Boot 2.7.x (or earlier), do the 2.7 → 3.5
leg FIRST. The two scenarios chain:

Boot 2.7.x  →  Boot 3.5.x latest  →  Boot 4.x
              (this section's prelude)   (the 9 phases below)

Read references/spring-boot-2-to-3-migration.md for the complete
2.7 → 3.5 walkthrough. It covers:

• Toolchain bump: Java 8/11 → 17 (re-run the 2.7 build on Java 17
  BEFORE bumping Boot, so JDK incompats surface separately from Spring
  upgrade churn)
• Pre-flight on 2.7.18 latest patch: clear all deprecation warnings
  and add spring-boot-properties-migrator for runtime diagnostics
• OpenRewrite automation: one-shot via
  org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_5 (composes
  every 3.0 → 3.5 step, including Jakarta migration), or step-by-step
  per minor for large codebases
• Jakarta EE namespace migration (javax.* → jakarta.*)
• Spring Security 5 → 6 (WebSecurityConfigurerAdapter removed,
  lambda DSL required, requestMatchers over antMatchers)
• Hibernate 5 → 6 (org.hibernate.orm group ID, ID generator
  changes, naming strategy, query handling)
• Per-minor highlights: 3.0, 3.1, 3.2, 3.3, 3.4, 3.5
• Property migration via Properties Migrator + OpenRewrite
• Observability migration to Micrometer Tracing / Observation API

When the 2 → 3 leg is verified (build passes on Boot 3.5 latest patch
and Properties Migrator prints no warnings), return to the
Prerequisites section below and proceed with Phases 1–9 for 3.5 → 4.0.

Prerequisites

For 3.x → 4.0 migration:

• Toolchain versions meet minimums (see Toolchain Version Check above)
• Source project compiles and tests pass on Spring Boot 3.5.x (latest patch)
• Java 17+ is available (Java 21+ recommended, Java 25 supported)
• All deprecated API calls from Boot 3.x are resolved where possible
• If on Boot 3.4 or earlier, first upgrade to 3.5.x before proceeding

For 4.x → 4.y minor version upgrade:

• Project is on the latest patch of the current minor version (e.g., 4.0.x latest)
• Review references/minor-version-changes.md for the target version
• Check the official release notes for the target version
• Resolve any deprecation warnings from the current version

Choose Your Migration Strategy

Strategy 1 — Gradual Upgrade (Recommended for enterprise/large codebases)
Read references/gradual-upgrade-strategy.md FIRST. This models migration
as a dependency graph: a Day-1 baseline using compatibility bridges, then
6 independent tracks (Starters, Jackson 3, Properties, Security, Testing,
Framework 7) completed at your own pace. Key bridges:

• spring-boot-starter-classic — restores 3.x monolithic auto-configuration
• spring-boot-jackson2 — keeps Jackson 2 code working alongside Boot 4
• spring-security-access — bridges legacy AccessDecisionManager/Voter
  Use this when: multiple teams, many services, phased rollouts, or when
  complete Jackson 3 / Security 7 migration will take more than one sprint.

Strategy 2 — All-at-Once (below)
Execute all 9 phases sequentially in one effort. Best for greenfield
projects, small codebases, or single-team ownership.

Automated Migration with OpenRewrite

Before doing manual migration, consider using OpenRewrite recipes to
automate the mechanical changes. Run OpenRewrite FIRST to handle bulk
find-replace operations, then use this skill's phases to address the
remaining manual changes (Security DSL rewrites, behavioral differences,
property semantics, etc.).

For the 2.7 → 3.5 leg (run before any 3 → 4 work):

Recipe ID	Coverage
org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_5	One-shot 2.x → 3.5: composes 3.0 → 3.1 → 3.2 → 3.3 → 3.4 → 3.5, including Jakarta migration, Spring Security 6, Hibernate 6 coordinates, property renames
org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_0	Step-by-step: 2.7 → 3.0 only (Jakarta + Boot 3.0 baseline)
org.openrewrite.java.spring.boot3.UpgradeSpringBoot_3_1 … _3_5	Per-minor incremental upgrades
org.openrewrite.java.spring.boot3.SpringBootProperties_3_X	Property key renames per minor version
org.openrewrite.java.migrate.jakarta.JakartaEE10	Standalone Jakarta EE namespace migration (javax.* → jakarta.*) when applied outside the Boot upgrade

Catalog: docs.openrewrite.org/recipes/java/spring/boot3. End-to-end 2 → 3 walkthrough: docs.openrewrite.org/running-recipes/popular-recipe-guides/migrate-to-spring-3.

For the 3.5 → 4.0 leg:

Recipe ID	Coverage
org.openrewrite.java.spring.boot4.UpgradeSpringBoot_4_0 (Community Edition variant available)	Boot 3.5 → 4.0
org.openrewrite.java.jackson.UpgradeJackson_2_3	Jackson 2 → 3 package/import migration
org.openrewrite.java.spring.boot3.MigrateMockBeanToMockitoBean (or the equivalent in rewrite-spring)	@MockBean/@SpyBean → @MockitoBean/@MockitoSpyBean

See: moderne.ai/blog/spring-boot-4x-migration-guide. For the 4.0 recipe specifically: docs.openrewrite.org/recipes/java/spring/boot4/upgradespringboot_4_0-community-edition.

Invocation:

# Maven
./mvnw org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-spring:RELEASE \
  -Drewrite.activeRecipes=<recipe-id>

# Gradle
./gradlew rewriteRun -Drewrite.activeRecipes=<recipe-id>

Migration Workflow (All-at-Once)

Execute these phases IN ORDER. Each phase must compile and pass tests
before proceeding to the next.

Phase 1: Build File Migration

Update Boot/Framework versions, build plugins, and replace deprecated
starters with modular equivalents. Add modular test starters for each
technology used in tests. Use classic starters as a stopgap if needed.

Read references/build-and-dependencies.md for complete starter mappings,
build plugin changes, and step-by-step instructions.

Compile check: Run mvn compile or gradle compileJava — fix any
dependency resolution errors before continuing.

Phase 2: Property Migration

Scan all application.properties, application.yml, profile-specific
variants, and @SpringBootTest(properties = ...) annotations. Rename
changed property keys (Jackson, MongoDB, session, actuator, Hibernate).

Read references/property-changes.md for the complete property mapping.

Phase 3: Jackson 3 Migration

Jackson 3 is the default in Boot 4. Migrate group IDs, packages, and
renamed Boot classes (@JsonComponent → @JacksonComponent, etc.).
Use spring-boot-jackson2 bridge as a temporary stopgap if needed.

Read references/jackson3-migration.md for complete details.

Phase 4: Package and API Relocations

Fix relocated imports (@EntityScan, BootstrapRegistry, etc.), removed
APIs (PropertyMapper.alwaysApplyingNotNull, path matching options), and
deprecated converters. Also migrate HTTP client code if applicable.

Read references/api-changes.md for the full list of relocated packages
and removed APIs.
Also read references/http-clients.md if your project uses RestClient,
WebClient, @HttpExchange, or Feign.

Phase 5: Observability Migration

Replace individual Micrometer/OTel dependencies with the consolidated
spring-boot-starter-opentelemetry starter, update OTLP properties, and
rename observability modules. Actuator is now optional for OTel export.

Read references/observability-migration.md for complete details.

Phase 6: Spring Security 7 Migration

Migrate to Security 7 DSL (lambda-only, no and()), replace removed
AuthorizationManager#check, switch to PathPatternRequestMatcher, and
update Jackson/SAML integrations. Use spring-security-access bridge if
legacy AccessDecisionManager/Voter code cannot migrate immediately.

Read references/spring-security7.md for complete details.

Phase 7: Testing Infrastructure Migration

Replace @MockBean/@SpyBean with @MockitoBean/@MockitoSpyBean
(removed, not just deprecated). Add explicit auto-configure annotations
for test HTTP clients. Migrate Testcontainers 2 module names/packages
and adopt JUnit 6. Add modular test starters for each technology.

Read references/testing-migration.md for complete details.

Phase 8: Spring Framework 7 Specific Changes

Address JSpecify nullability (Kotlin impact), deprecated AntPathMatcher,
MVC XML config removal, SpringExtension scope changes, Hibernate 7.1
entity mapping changes, and Spring Retry → Framework core retry migration.

Read references/spring-framework7.md for complete details.
Also read references/resilience-migration.md if your project uses
Spring Retry, @Retryable, @ConcurrencyLimit, or Resilience4j.
Optionally read references/api-versioning.md for new API versioning
capabilities introduced in Framework 7.

Phase 9: Final Verification

Run the verification script if available, otherwise manually check:

1. mvn clean verify or gradle clean build — full compile + tests
2. Verify application starts: mvn spring-boot:run or gradle bootRun
3. Check actuator health: curl localhost:8080/actuator/health
4. Verify liveness/readiness probes (now enabled by default)
5. Check structured logging output format
6. Run integration tests against each active Spring profile
7. Verify Docker image builds if using buildpacks or Jib

Optionally read references/aot-native.md if you plan to adopt AOT
processing, GraalVM native images, or the new AOT cache feature.

Minor Version Upgrades (4.0 → 4.1, 4.1 → 4.2, etc.)

When upgrading between Spring Boot 4.x minor versions, follow this process:

1. Check What Changed

Read references/minor-version-changes.md for the target version. Also
consult the official release notes:

• https://github.com/spring-projects/spring-boot/wiki (Release Notes per version)
• https://docs.spring.io/spring-boot/upgrading.html

2. Bridge Removal Awareness

Minor versions are where compatibility bridges get removed. Before
upgrading, check whether any bridges you depend on are being dropped:

Bridge	Introduced	Expected Removal
spring-boot-jackson2	4.0	4.1 or 4.2
spring-boot-starter-classic	4.0	5.0
spring-boot-starter-test-classic	4.0	5.0
Deprecated starter names	4.0	5.0

If you are still using a bridge that is being removed in the target
version, complete the corresponding migration track BEFORE upgrading.

3. Upgrade Process

1. Update the Spring Boot version in your build file to the target
   minor version's latest patch release.
2. Run mvn compile / gradle compileJava — fix any new compilation errors.
3. Run the full test suite — fix any test failures.
4. Review deprecation warnings in both build output and application logs.
   These signal what will break in the NEXT minor version.
5. Run verify_migration.sh to confirm migration state.

4. New Features

Each minor version introduces new features and auto-configurations.
These are opt-in and don't require action, but you may want to adopt
them. Check the "New and Noteworthy" section of each release's notes.

Troubleshooting

Common Compilation Errors

Error	Cause	Fix
ClassNotFoundException: ...autoconfigure...	Modular starters needed	Add specific spring-boot-starter-X
NoSuchMethodError: PropertyMapper.alwaysApplyingNotNull	API removed	Use always() instead
Cannot resolve symbol JsonComponent	Renamed	Use @JacksonComponent
Package com.fasterxml.jackson does not exist	Jackson 3 packages	Change to tools.jackson
Cannot resolve symbol MockBean	Deprecated/removed	Use @MockitoBean
ClassNotFoundException: RestClientBuilderCustomizer	Elasticsearch change	Use Rest5ClientBuilderCustomizer

Quick Fixes

• If build won't compile at all after version bump, use spring-boot-starter-classic and spring-boot-starter-test-classic to get running, then incrementally migrate to modular starters. See references/gradual-upgrade-strategy.md for the full Day-1 baseline.
• If Jackson 3 migration is blocking, add spring-boot-jackson2 temporarily. This is a first-class bridge — see Track B in the gradual strategy.
• If Spring Security changes are extensive, add spring-security-access bridge and upgrade to Security 6.5 preparation steps first (they provide opt-out flags for 7.0 breaking changes). See Track D in the gradual strategy.
• For enterprise rollouts across many services, use the Wave 1-4 approach in references/gradual-upgrade-strategy.md to minimize blast radius.

Reference File Index

File	When to read
references/toolchain-versions.md	Toolchain check — Java/Kotlin/Maven/Gradle minimums and per-tool upgrade commands
references/spring-boot-2-to-3-migration.md	Boot 2.7.x → 3.5.x prelude — Java baseline, Jakarta migration, Hibernate 5 → 6, Security 5 → 6, OpenRewrite recipes, per-minor highlights
references/gradual-upgrade-strategy.md	FIRST (for 3 → 4) — migration dependency graph, bridges, independent tracks, enterprise rollout
references/build-and-dependencies.md	Phase 1 / Track A — full starter mapping tables, build plugin changes
references/property-changes.md	Phase 2 / Track C — all property key renames and value changes
references/jackson3-migration.md	Phase 3 / Track B — Jackson 3 packages, APIs, compatibility mode
references/api-changes.md	Phase 4 — package relocations, removed APIs, renamed classes
references/observability-migration.md	Phase 5 — OpenTelemetry starter, OTLP properties, module renames, Actuator decoupling
references/spring-security7.md	Phase 6 / Track D — Security 7 breaking changes and DSL migration
references/testing-migration.md	Phase 7 / Track E — MockBean, Testcontainers 2, JUnit 6, RestTestClient
references/spring-framework7.md	Phase 8 / Track F — Framework 7 changes, JSpecify, path matching
references/http-clients.md	HTTP clients — RestClient, WebClient, @HttpExchange, Feign migration, RestTestClient
references/api-versioning.md	API versioning — strategies, semantic ranges, client-side, deprecation, testing
references/resilience-migration.md	Resilience — Spring Retry → Framework 7, @Retryable, @ConcurrencyLimit, Resilience4j
references/aot-native.md	AOT/Native — BeanRegistrar, RuntimeHints, Spring Data AOT, GraalVM 25, AOT Cache
references/minor-version-changes.md	4.x minor upgrades — changes per minor version, bridge removals, new features
scripts/verify_migration.sh	Phase 9 — bridge-aware verification with PASS/FAIL/WARN/BRIDGE

Authoritative external sources for each leg are listed at the bottom of
the corresponding reference file.