Migrate Kotlin extension to Axon Framework 5

[vcanuel]

Closes AxonFramework/extension-kotlin#393. Takes over #3878 by @jangalinski.

Context

@jangalinski opened #3878 to import the extension-kotlin repository into
extensions/kotlin/ (with full commit history) and wire it into the monorepo build.
That PR intentionally stopped there: incompatible AF4 files were moved to _todo/
rather than rewritten, leaving the actual AF5 migration as follow-up work.

This PR completes that migration. The reference for every change is
extension-kotlin#394,
which performed the same migration against the standalone extension-kotlin
repository (targeting AF 5.1.0).

---

Differences from extension-kotlin#394

The monorepo integration required a few adjustments beyond a straight copy of the patch.

extensions/kotlin/pom.xml — parent and coordinates

PR#394 kept the standalone axon-kotlin-parent / 4.12.x-SNAPSHOT parent and
simply updated axon.version to 5.0.3.

Our branch replaces the entire parent with the monorepo coordinates:

• parent → axon-extensions / 5.1.0-SNAPSHOT
• version → 5.1.0-SNAPSHOT (inherited)
• All standalone release/deploy/SCM infrastructure removed (sonatype, gpg, dokka,
  release-plugin, pluginRepositories, scm, developers, .github/, .editorconfig,
  mvnw, coverage-report/, etc.)
• docs/ removed — AF4 Antora reference content imported via history, no AF5
  equivalent yet; reference docs update is a separate effort

Additionally the <log4j.version>2.13.3</log4j.version> property present in the
standalone pom was removed. In the monorepo it overrides the managed 2.25.3 version
and causes a Maven resolution failure (log4j-slf4j2-impl:2.13.3 does not exist in
Maven Central — only introduced in log4j 2.20).

CommandGatewayExtensions.kt — sendAndWait null safety

CommandGateway.sendAndWait(Object, Class<R>) is @Nullable. PR#394 silently trusts
the platform type. Our branch adds ?: error(...) to fail fast with a clear message
pointing callers to sendForResult as the nullable alternative.

QueryUpdateEmitterExtensions.kt — type bound and SAM lambda

Two changes vs PR#394: Q : Any bound added (required to resolve the correct Java
overload), and filter wrapped in an explicit lambda to satisfy Predicate<? super Q>
unambiguously. @Suppress("UNCHECKED_CAST") covers the remaining compiler warning.

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What was done (full summary)

Build (extensions/kotlin/pom.xml)

• Parent changed to monorepo axon-extensions / 5.1.0-SNAPSHOT
• Removed <log4j.version>2.13.3</log4j.version> (would break monorepo resolution)
• Removed AF4 compile dependencies: jackson-module-kotlin,
  kotlinx-serialization-core, javax.xml.bind:jaxb-api
• Removed all-open:annotation=...AggregateRoot compiler plugin option
• axon-configuration → axon-messaging in dependency management

Deleted — AF4-only, no AF5 equivalent (yet)

File	Reason
AggregateLifecycleExtensions.kt	AggregateLifecycle / @AggregateRoot removed in AF5
BuilderExtensions.kt	EventSourcingRepository / GenericJpaRepository builders not present in AF5
ResultDiscriminatorCommandCallback.kt	CommandCallback / CommandResultMessage replaced by CommandResult
EventUpcaster.kt	SingleEventUpcaster not available in AF5.0/5.1; tracked for 5.2
messaging/responsetypes/ArrayResponseType.kt	AbstractResponseType removed; ResponseType hierarchy reworked
serialization/ (entire module)	Serializer replaced by Converter; full rewrite out of scope — tracked as follow-up
META-INF/spring-devtools.properties	Spring Boot DevTools dependency removed
All corresponding tests	Deleted with their production classes

Rewritten for AF5

CommandGatewayExtensions.kt — send, sendForResult, sendAndWait over
CommandResult<R> (replaces AF4 CommandCallback / CommandResultMessage pattern).

QueryGatewayExtensions.kt — query, queryMany, subscriptionQuery retained;
scatterGather* replaced by streamingQuery; type bound R : Any on all functions
where R is used as Class<R>.

QueryUpdateEmitterExtensions.kt — package import updated to AF5;
Q : Any bound added; explicit SAM lambda for Predicate.

kotlin-test/FixtureExtensions.kt — AggregateTestFixture / SagaTestFixture /
TestExecutor / ResultValidator replaced by the AF5 phase-based
AxonTestFixture / AxonTestPhase API; whenever(), whenever(command),
exception(KClass), exception(KClass, message) rewritten accordingly.

Tests

18 tests pass (12 in kotlin, 6 in kotlin-test). Test files are identical to
the ones in PR#394.

Documentation

README.md: versions 4.6.0 → 5.1.0-SNAPSHOT, CI/SonarCloud/CodeTriage badges
removed (standalone-repo artefacts), issue tracker URL updated to monorepo,
docs profile name corrected (javadoc-and-sources → docs-and-sources).

---

Out of scope / follow-up

Item	Notes
KotlinSerializer / KotlinConverter	Requires full rewrite against AF5 Converter API
EventUpcaster DSL	SingleEventUpcaster absent from AF5.0/5.1; scheduled for 5.2

[jangalinski]

Thanks for picking this up @vcanuel , I am on vacation this week but will review on Monday.

[jangalinski]

Running the archtest in axon-common fails with this state of the file?

Reverting the file to the contents in "main" solved this.

[jangalinski]

Naming convention: Axon Extension - Kotlin - Parent

[jangalinski]

Thanks @vcanuel . Looking great. I will take over and fix some minor things as it is probably faster than round-tripping.

Short update: We had a team meeting this morning and decided to not include this in the upcoming 5.1.0-RC1, as it would really benefit from a kotlin example application showing how to get the most out of this extension and we do not have the time to do so..

So might I ask you to change ths PR to go against my original feature branch feature/introduce-kotlin-extension? We then have it it the repo already and it is much easier to merge it after our internal changes, that would be great.

We could also need your help finalizing this extension if you are interested. I will copy the https://github.com/AxonIQ/university-demo-kotlin to the examples section of this repository on the feature/introduce-kotlin-extension and it would be great to adjust that to the current state of the framework.

[smcvb]

Setting this PR to milestone 5.2.0, as we do not have sufficient time to round of this contribution before our foreseen release date of 5.1.0.

[jangalinski]

I manually merged this PR into #3878 as we won't necessarily want to merge this to main soon.

I also restructured the packages a bit and imported the university-demo-kotlin into examples for reference.

@vcanuel - use this Draft PR as reference for further improvements if you like, and thanks again for your contributions.

[jangalinski]

Manually merged this PR into #3878 to continue the work.