Feign client returns null for non-nullable return value

[Lunkers]

Hello,
I'm using a feign client that looks something like this:

@FeignClient("api", url = "\${api-url}", decode404 = true)
interface Client {

    @GetMapping("/api/article/{id}")
    fun getLatestRevision(
        @PathVariable("id") id: String,
    ): Optional<ArticleRevision>


    @GetMapping("/api/video/{id}/revision/{revision}")
    fun getRevision(
        @PathVariable("id") id: String,
        @PathVariable("revision") revision: Int
    ): ArticleRevision
}

The assumption that I'm making is that when the first method gets a 404 response, it should return Optional.empty, which it does. But since the return type of the second method is not nullable, I would assume that an exception would be thrown if the response is 404. However, this does not happen and the function just returns null. Is this expected behavior? Can I not use decode404 and expect a non-nullable return value to not be null?

[vitalijr2]

The documentation says:

This flag indicates that the decoder should process responses with 404 status, specifically returning null or empty instead of throwing....
All first-party (ex gson) decoders return well-known empty values defined by Util.emptyValueOf()...

I think than the term empty is regarding to collections, maps, strings and instances of Optional, see Util class

feign/core/src/main/java/feign/Util.java

Line 263 in c4686e0

empties.put(Optional.class, Optional::empty);

On the other hand, only three decoders (Decoder, JAXBDecoder, SOAPDecoder) uses this method.

Could you say what decoder you use, please?

[vitalijr2]

And I have found the commit that changes behavior: #1012

Also here are another issue and PR about decode404 flag: #1528 and #1529