@Test
  void responseMapperIsAppliedBeforeDelegate() throws IOException {
    ResponseMappingDecoder decoder =
        new ResponseMappingDecoder(upperCaseResponseMapper(), new StringDecoder());
    String output = (String) decoder.decode(responseWithText("response"), String.class);

    assertThat(output).isEqualTo("RESPONSE");
  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/FeignTest.java