Spring Boot Migration

Purpose

Use this skill for phased upgrade work on existing Spring Boot applications. This skill adds value through the migration scanner, the Boot 4 / Modulith 2 / Testcontainers 2 reference guides, and a strict migration order that avoids mixing too many changes at once.

Critical rules

• Never migrate blindly. Scan the codebase first.
• Never apply every migration at once. Follow phased upgrades.
• Treat Spring Boot 4, Spring Modulith 2, and Testcontainers 2 as the intended target stack for this skill unless the user asks for a narrower target. Boot 4's Java baseline is 17; recommend Java 25 for new compile targets but do not force an upgrade past what the migration actually requires. For Java 26+ targets, read ../creating-springboot-projects/references/java-version-policy.md.
• Verify after each phase and stop when failures appear.

Workflow

Step 1: Scan the project

Use the migration scanner before planning or editing. It reads pom.xml, build.gradle, or build.gradle.kts (whichever the project has) plus Java sources, properties files, and Flyway migrations:

python3 "${CLAUDE_SKILL_DIR}/scripts/scan_migration_issues.py" /path/to/project

${CLAUDE_SKILL_DIR} is this skill's folder. Codex doesn't set it, so there use the folder this SKILL.md is in.

Use the scan output to identify:

• current Spring Boot version
• starter rename work
• annotation and import migrations
• configuration changes
• Spring Modulith compatibility
• Testcontainers compatibility
• Java preview APIs (StableValue, StructuredTaskScope) that break on a newer JDK

Step 2: Identify which migrations apply

Load only the references that match the codebase:

Step 3: Plan the migration in phases

Use the reference guides to plan and execute in this order.

#### Phase 1: Dependencies

• update pom.xml or build.gradle
• rename starters where required
• add or remove dependencies needed by the target stack
• align version properties

#### Phase 2: Source-code changes

• update imports and package names
• migrate test annotations
• fix Jackson 3 issues
• fix Testcontainers API changes where relevant

#### Phase 3: Configuration

• update application.properties or application.yml
• apply Boot 4 defaults intentionally
• update Spring Modulith event-store configuration if relevant

#### Phase 4: Verification

• run the scanner again with --check; it exits with 1 while any CRITICAL issue remains (a missing modular starter, for example, compiles but fails at startup):

  python3 "${CLAUDE_SKILL_DIR}/scripts/scan_migration_issues.py" /path/to/project --check

• run unit tests
• run integration tests
• run container-based tests where present
• check for deprecations and startup failures

Step 4: Use the right migration order for mixed upgrades

When multiple ecosystems are involved, use this order:

1. Spring Boot 4
2. Spring Modulith 2
3. Testcontainers 2

Read references/migration-overview.md before deviating from this sequence.

Step 5: Report progress after each phase

After each phase, report:

• what changed
• what remains
• what failed, if anything
• whether it is safe to continue

Reference loading guide

• Spring Boot 4 migration details: references/spring-boot-4-migration.md
• Spring Modulith 2 migration details: references/spring-modulith-2-migration.md
• Testcontainers 2 migration details: references/testcontainers-2-migration.md
• Mixed scenarios and common issues: references/migration-overview.md

Available script

• scripts/scan_migration_issues.py: --check exits with 1 while CRITICAL issues remain. It skips build output and hidden directories, including project-level skill installs.

Output format

When planning or reporting the migration, return:

## Migration scope
- Current versions:
- Target versions:

## Planned phases
1. ...
2. ...
3. ...

## Files expected to change
- `path/to/file`

## Verification
- Tests or checks to run

When not to use this skill

• Creating a new Spring Boot project from scratch
• Broad architecture advice without an actual migration task