List query parameters are exploded on the Feign client even when explode is false

[rvinzent]

Problem

I have the following OpenAPI 3.1 schema:

{
  ..., 
  "/v1/resource/": {
    "get": {
      "operationId": "getPage",
      "parameters": [
        {
          "in": "query",
          "name": "slug",
          "schema": {
            "type": "array",
            "items": {
              "type": "string"
            }
          },
       "explode": false,
       "style": "form"
      }
    ]
  }
}

According to the OpenAPI docs, "style": "form" with "explode": false should produce a comma separated list of values, e.g. ?slug=one,two,three, but the generated Feign client ends up creating a request with exploded parameters.

Generated client method looks something like this:

@RequestLine("GET /v1/reagent/?slug={slug}")
@Headers("Accept: application/json")
public fun getResourcePage(
  @Param("slug") slug: List<String>? = null,
  @HeaderMap additionalHeaders: Map<String, String> = emptyMap(),
  @QueryMap additionalQueryParameters: Map<String, String> = emptyMap(),
): PaginatedResource

The suggested fix for Spring OpenFeign clients is to use the @CollectionFormat(feign.CollectionFormat.CSV) annotation to the @FeignClient class (docs), however, this seems to only work for methods/classes annotated with @RequestMapping (source).

@RequestLine instead accepts a collectionFormat parameter on the annotation to provide this configuration (source), however, each parameter in an OpenAPI spec can define explode independently, and feign doesn't seem to have the same collectionFormat configuration available on the @Param annotation. Instead, to configure the behavior on a per parameter basis, we'd probably have to utilize the expand parameter on the @Param annotation.

[rvinzent]

@cjbooms I can try to address this if we can confirm that it's indeed a bug and should be fixed as proposed

[cjbooms]

Certainly open to any fix you can make. I personally have never used the feign client so I'd defer to your or others expertise on how this should be handled.

[rvinzent]

Hey @cjbooms I finally got around to this (or at least let Claude take a stab). Looks like it has landed on a decent solution. I'd appreciate your review, thanks!