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Add boot4-migration module: one Boot 3.5.16 service migrated to 4.1.2, with the real compile, start-up, silent and test failures captured at each step

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Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b4VQVEJWetTT1dYzyJ
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asmhatre
2026-10-29 05:57:26 +00:00
co-authored by Claude Sonnet 5.5
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boot4-migration/README.md
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# boot4-migration
Companion module for the ankurm.com post
[Spring Boot 3 to 4 Migration Guide (Updated for 4.1 and 4.2)](https://ankurm.com/spring-boot-3-to-4-migration-guide/).
One small service (Web MVC, Security, Actuator, JDBC + Flyway, AOP aspect, `RestClient.Builder`,
a Jackson serializer, three tests) in two forms:
| Directory | What it is | Built by |
|---|---|---|
| `before/` | the service on Spring Boot 3.5.16 | never by a normal build; `scripts/run-all.sh` copies it to a scratch dir |
| `after/` | the same service migrated to Spring Boot 4.1.2 | `cd after && mvn verify` |
`run-all.sh` builds `before/` as written, then bumps only the parent version and records what
breaks at each step: the build error, the javac errors, the start-up failure report, the
things that start fine but behave differently, and the test failures. Every file in `output/` is
a real transcript from that script (JDK 25.0.5, Maven 3.9).
```bash
export JAVA_HOME=/path/to/jdk-25
./scripts/run-all.sh # 15-25 minutes on a shared 2-vCPU box; needs Maven Central
```
| Output | Shows |
|---|---|
| `01-before-boot35.txt` | `before/` builds and runs on 3.5.16 (the baseline every other file is compared with) |
| `02-bump-only-compile-errors.txt` | only the parent version changed: Maven POM error, then 12 javac errors |
| `03-compiles-but-will-not-start.txt` | imports ported, starters not: compiles, fails at start-up (no `RestClient.Builder`) |
| `04-starts-but-silently-wrong.txt` | starts, answers 200, but Flyway never ran, logs never rotate |
| `05-properties-migrator.txt` | what `spring-boot-properties-migrator` does and does not report |
| `06-after-boot41.txt` | `after/`: 8 tests pass, same endpoints, new health groups |
| `07-jackson-property-order.txt` | bean property order on Jackson 2, Jackson 3, and `use-jackson2-defaults` |
| `08-test-compile-errors.txt` | `@MockBean` and the old `@AutoConfigureMockMvc` package |
| `09-test-runtime-failure.txt` | a plain `@Mock` field is `null`: compiles, fails at run time |
| `10-security-dsl-compile-errors.txt` | the real javac message for the removed `authorizeHttpRequests()` chain |
Not covered: Gradle, Kotlin, native image, and any library that has not shipped a Boot 4 release.