fix(kotlin-spring): fix Flow<String> and Flux<T> array return types for reactive generators

[Picazsoo]

Just for documentation purposes as the change has been already merged:

Why string arrays return suspend List<String> even when useFlowForArrayReturnType = true

When targeting a reactive Kotlin Spring server, useFlowForArrayReturnType = true normally
makes array responses return Flow<T> (a streamed publisher). For string arrays
(T = kotlin.String) we deliberately override this and emit suspend fun ...: List<String>
(or Set<String> for uniqueItems) instead.

The reason

Spring WebFlux's StringDecoder intercepts a reactive stream of String before Jackson
runs. Instead of decoding each JSON array element, it hands back the entire JSON array as a
single raw string blob. As a result, Flow<String> array endpoints silently produce
corrupted / undecoded data.

This is a limitation specific to T = String. Flow<Pet>, Flow<Long>, etc. decode
correctly and are left unchanged.

Why not just honor the flag?

For T = String, useFlowForArrayReturnType = true is an unsatisfiable request: there is
no Flow<String> shape that decodes correctly. So the generator's only options are:

1. Emit Flow<String> anyway → compiles, silently corrupts data. ❌
2. Emit suspend List<String> → correct, but changes the method signature. ✅ (chosen)

Correct, non-corrupting option was chosen. A signature that silently corrupts data is worse than a compile-time
signature change the user can adapt to.

Why there is no opt-out flag to force Flow<String> back on

Such a flag would be a supported, documented switch whose only effect is to re-enable known-broken decoding - a
footgun, not a legitimate preference. A generator option should let users choose between two working shapes, not between "correct" and "corrupt."

Summary

Config	Element type	Output	Correct?
useFlowForArrayReturnType = true	non-String	Flow<T>	✅
useFlowForArrayReturnType = false	String	suspend List<String>	✅
useFlowForArrayReturnType = true	String	suspend List<String> (overridden)	✅

Note: this is a breaking signature change for users who previously generated
Flow<String> array endpoints. It is intentional — those endpoints did not decode
correctly to begin with.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Fixes reactive array return types in Kotlin Spring to avoid broken decoding and invalid signatures. Arrays of strings now return List<String>/Set<String> with suspend (not Flow<String>), and declarative Reactor interfaces return Mono<List<T>>/Mono<Set<T>> (not Flux<T>).

• Bug Fixes

  • Coroutines (kotlin-spring): for array-of-string responses, generate suspend + List<String>/Set<String> to avoid Spring WebFlux StringDecoder swallowing JSON arrays (issue 22662).
  • Declarative Reactor (spring-declarative-http-interface): arrays now use Mono<{{returnContainer}}<T>> (e.g., Mono<List<T>>, Mono<Set<T>>), including Mono<ResponseEntity<...>> when enabled; removed reactor.core.publisher.Flux import.
  • Kotlin type handling: unwrap Set/MutableSet like List so returnType is the inner type and returnContainer is Set, fixing cases like Mono<set<kotlin.collections.Set<...>>>.
  • Added x-reactive-array-string-return vendor extension and updated templates to conditionally emit the new signatures.
  • Updated samples to use Mono<List<...>>/Mono<Set<...>> and drop Flux imports.

• Migration

  • Coroutines server: replace Flow<String> with List<String> (or Set<String> for uniqueItems) and mark methods suspend.
  • Declarative Reactor: replace Flux<T> with Mono<List<T>>/Mono<Set<T>> (and Mono<ResponseEntity<...>> when used); update imports.

^{Written for commit a10b418. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 14 files

_{Re-trigger cubic}