[REQ] [kotlin-spring] Add clientRegistrationId for spring-declarative-http-interface (parity with Java spring)

[rkudryashov]

Is your feature request related to a problem? Please describe.

When generating a Kotlin HTTP Interface client with kotlin-spring + library=spring-declarative-http-interface (Spring Boot 4 / Security 7), the generated API interfaces do not include @ClientRegistrationId. Without that annotation, Spring Security’s OAuth2 HTTP Service Client integration does not attach tokens for client_credentials (or other registrations).

The Java spring generator already supports this via clientRegistrationId (library=spring-http-interface, useSpringBoot4=true) — see PR #22726 and the spring generator docs. kotlin-spring has no equivalent option, so Kotlin users must post-process generated *Api.kt files.

Describe the solution you'd like

Add a clientRegistrationId config option to kotlin-spring with the same semantics as the Java spring generator:

• When set, and library=spring-declarative-http-interface (with Boot 4 / Security 7 as required), emit @ClientRegistrationId("<id>") on the generated interface.
• Document the option in kotlin-spring docs.

Example:

generatorName: kotlin-spring
library: spring-declarative-http-interface
additionalProperties:
  useSpringBoot4: "true"
  interfaceOnly: "true"
  clientRegistrationId: assistant

Expected:

@ClientRegistrationId("assistant")
interface DocumentsApi {
    // ...
}

Describe alternatives you've considered

1. Gradle doLast workaround: inject @ClientRegistrationId into generated *Api.kt after generation (what we do today).
2. Switch the client generator to Java spring + spring-http-interface (works, but mixes Java clients into a Kotlin service and splits server/client generators).
3. Manual/non-generated HTTP interfaces.

Additional context

• Spring Security docs: HTTP Service Clients Integration (@ClientRegistrationId on the interface or methods).
• Java support: spring generator clientRegistrationId, PR [Spring] Add clientRegistrationId option for OAuth2 HTTP Interface #22726.
• Kotlin declarative HTTP interfaces: PR [kotlin-spring][server] Feat: Add Spring Declarative HTTP Interface support for easy client instantiation #22302 / spring-declarative-http-interface; no clientRegistrationId in kotlin-spring docs.
• openapi-generator version: 7.25.0.

[Picazsoo]

Just to note: putting @ClientRegistrationId on the generated interface was never actually a blocker.

The same binding can be configured centrally when the client bean is built, without touching generated code, by attaching an OAuth2ClientHttpRequestInterceptor with a fixed ClientRegistrationIdResolver:

@Bean
fun documentsApi(manager: OAuth2AuthorizedClientManager): DocumentsApi {
    val interceptor = OAuth2ClientHttpRequestInterceptor(manager) { _ -> "assistant" }
    val restClient = RestClient.builder().requestInterceptor(interceptor).build()
    val factory = HttpServiceProxyFactory
        .builderFor(RestClientAdapter.create(restClient))
        .build()
    return factory.createClient(DocumentsApi::class.java)
}

This keeps the generated *Api.kt free of security concerns and lets the id change without regenerating. The annotation mainly helps when the id must differ per endpoint, which OpenAPI doesn't express anyway, so one registration per client is the norm.

None of this is a reason not to add the option, though. Supporting clientRegistrationId as an opt-in (interface-level, for parity with the Java spring generator) is still a nice convenience. I just wanted to make clear it's a convenience, not a missing capability.

See HTTP Service Clients Integration for details.