POST JSON with File not working (multipart/form-data)

[will-molloy]

Using these deps: feign-core and feign-gson 12.3. feign-form 3.8.0. JDK 19.

---

I can't get JSON serialisation working when using POST multipart/form-data for POJO->JSON and File.

Example:

interface Example {
  static Example createInstance() {
    return Feign.builder()
        .encoder(new FormEncoder(new GsonEncoder()))
        .target(Example.class, "https://anything.com");
  }

  @RequestLine("POST")
  @Headers("Content-Type: multipart/form-data")
  void uploadWithDto(@Param("dto") Dto dto, @Param("file") File file);

  record Dto(String word, int number) {}

  public static void main(String[] args) throws URISyntaxException {
    Example api = Example.createInstance();
    File file = new File(Thread.currentThread().getContextClassLoader().getResource("file.txt").toURI());
    api.uploadWithDto(new Dto("hello", 123), file);
  }
}

Debugger showed the POJO is not written to the output at all:

If I serialise the POJO first then it works, (but uses text/plain):

interface Example {
  static Example createInstance() {
    return Feign.builder()
        .encoder(new FormEncoder(new GsonEncoder()))
        .target(Example.class, "https://anything.com");
  }

  @RequestLine("POST")
  @Headers("Content-Type: multipart/form-data")
  void uploadWithDto(@Param("dto") String dto, @Param("file") File file);

  record Dto(String word, int number) {}

  public static void main(String[] args) throws URISyntaxException {
    Example api = Example.createInstance();
    File file =
        new File(Thread.currentThread().getContextClassLoader().getResource("file.txt").toURI());
    api.uploadWithDto(new Gson().toJson(new Dto("hello", 123)), file);
  }
}

---

There have been previous issues (#24, #28) on this but I haven't been able to get it to work, what am I missing 😕? Also I'm not using Spring which many answers/solutions seem to use.

[will-molloy]

Btw the MultipartFormContentProcessor uses PojoWriter (which doesn't write anything) - how can I make it use Writer with GsonEncoder?

[mirzaprangon]

Facing the same issue. Basically, sending an object in the multipart/form-data, the content type becomes text/plain. Which is not desirable. Could not find a way to set the content type to application/json.