private ResponseMapper upperCaseResponseMapper() {
    return (response, type) -> {
      try {
        return response.toBuilder()
            .body(Util.toString(response.body().asReader()).toUpperCase().getBytes())
            .build();
      } catch (IOException e) {
        throw new RuntimeException(e);
      }
    };
  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/AsyncFeignTest.java