# Spring Boot Version

**Status:**
- proposed: 2026-06-09
- accepted: -
- rejected: -
- superseded: -

**Decision [proposed]:** Spring Boot 4.0 — only actively-supported version; springmockk workaround via @TestConfiguration until a compatible release is published.

## Context and Problem Statement

GitTally is a greenfield Kotlin/Spring Boot project.
A Spring Boot version must be chosen for the initial setup.

The choice is constrained by the support lifecycle: as of June 2026,
almost the entire Spring Boot 3.x release train is already end-of-life.

### Technical Background

[Spring Boot support lifecycle](https://endoflife.date/spring-boot)
([HeroDevs April 2026 overview](https://www.herodevs.com/blog-posts/spring-boot-versions-eol-dates-and-latest-releases-april-2026))
as of June 2026:

| Version | EOL date       | Status                          |
|---------|----------------|---------------------------------|
| 3.5.x   | June 30, 2026  | Expires this month              |
| 4.0.x   | December 2026  | **Active** (current: 4.0.6)    |

Spring Boot 3.5.x is the final 3.x release; there will be no 3.6.
The Spring project ended the 3.x train at 3.5 and continues in the 4.x line
([Spring Boot milestones](https://github.com/spring-projects/spring-boot/milestones)).
Spring Boot 4.0 moves to Spring Framework 7 and Jakarta EE 11.

The key ecosystem dependencies and their Spring Boot 4.0 compatibility status:

| Dependency | Role | Spring Boot 4.0 status |
|---|---|---|
| [picocli-spring-boot-starter](https://picocli.info/) | CLI wiring | Untested; last documented target is Spring Boot 3.1. The integration is minimal (one `IFactory` bean), so it likely works in practice. |
| [springmockk](https://github.com/Ninja-Squad/springmockk) | `@MockkBean` in tests | **Incompatible** — confirmed Spring Boot 3.x only. |
| [kotest-extensions-spring](https://github.com/kotest/kotest-extensions-spring) | Kotest + `@SpringBootTest` | Compatibility unconfirmed. |
| [wiremock-spring-boot](https://wiremock.org/docs/spring-boot/) | HTTP stubbing in tests | **Compatible** — Spring Boot 4.0 explicitly supported since January 2026. |

## Considered Options

* Spring Boot 3.5.x (latest 3.x)
* Spring Boot 4.0.x

### Spring Boot 3.5.x

The last actively-supported 3.x release.

#### Advantages

- Full compatibility with all current dependencies
  (picocli-spring-boot-starter, springmockk, kotest-extensions-spring).
- Well-known ecosystem; documentation and community resources are mature.

#### Disadvantages

- EOL June 30, 2026 — expires this month.
  Starting a project on a version ending within days is indefensible.
- Upgrading to Spring Boot 4.0 immediately after project start incurs migration cost
  at the worst possible time (early, before the codebase is stable).

### [Spring Boot 4.0](https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-4.0-Release-Notes)

The current and only actively-supported major release.

#### Advantages

- The only version receiving security patches and updates beyond June 2026.
- Spring Framework 7 and Jakarta EE 11: active, long-term foundation.
- Aligns with Gradle 9 when that upgrade is eventually made (ADR 0002).
- Wiremock Spring Boot integration fully supports it.

#### Disadvantages

- **springmockk is incompatible.** The `@MockkBean` / `@SpykBean` annotations are not available.
  This matters in particular for `@WebMvcTest` (controller error-behaviour) and
  `@DataJpaTest`-adjacent tests where collaborators need to be mocked in the Spring context.
  The workaround is to register MockK mocks as Spring beans via `@TestConfiguration`,
  which preserves full MockK syntax (`every { }`, `verify { }`) at the cost of one
  inner configuration class per test file and an explicit `clearMocks()` call in `beforeEach`:
  ```kotlin
  @WebMvcTest(SomeController::class)
  @Import(SomeControllerTest.Mocks::class)
  class SomeControllerTest : FunSpec() {
      @TestConfiguration
      class Mocks {
          @Bean fun someService(): SomeService = mockk()
      }
      @Autowired lateinit var someService: SomeService
      init {
          beforeEach { clearMocks(someService) }
          // full MockK syntax available
      }
  }
  ```
  This is more verbose than `@MockkBean` but avoids mixing Mockito and MockK.
  Revisit once springmockk publishes a Spring Boot 4.0-compatible release.
- **picocli-spring-boot-starter is untested against Spring Boot 4.0.**
  The risk is low (the auto-configuration is a single `IFactory` bean with no framework-version-specific API),
  but it cannot be assumed safe until verified by a running context test.
- **kotest-extensions-spring** compatibility needs verification before the first
  `@SpringBootTest` test is written.
- Relatively new GA; fewer community examples for the 4.0 generation.

## Decision Outcome

**Spring Boot 4.0** is chosen.

The main reasons:

- Spring Boot 3.5.x expires this month; starting on it is not a viable option.
- Spring Boot 4.0 is the only release with active support through the foreseeable project lifetime.
- The ecosystem blockers are manageable:
  - picocli-spring-boot-starter risk is low and will be confirmed by the existing
    context-wiring smoke test (`ApplicationContextTest`).
  - springmockk is a test-only dependency; the short-term workaround
    (`@TestConfiguration` with `mockk()` beans, `clearMocks()` in `beforeEach`)
    preserves full MockK syntax in `@WebMvcTest` and `@DataJpaTest` tests
    without falling back to Mockito.
  - kotest-extensions-spring compatibility must be verified before the
    first `@SpringBootTest` test is written.

Open items before the code is upgraded:
1. Bump Spring Boot to 4.0.6 and verify `ApplicationContextTest` still passes.
2. Confirm picocli-spring-boot-starter works under the new context.
3. Confirm kotest-extensions-spring works with Spring Boot 4.0.
4. Replace `@MockkBean` usages with the `@TestConfiguration` + `mockk()` pattern
   until springmockk publishes a compatible release.