code: @Override
      @Nullable
      public Object convertRequest(ServiceRequestContext ctx, AggregatedHttpMessage request,
                                   Class<?> expectedResultType) throws Exception {
  
          final MediaType contentType = request.contentType();
          if (contentType != null && (contentType.is(MediaType.JSON) ||
                                      contentType.subtype().endsWith("+json"))) {
              final ObjectReader reader = readers.computeIfAbsent(expectedResultType, mapper::readerFor);
              if (reader != null) {
                  final String content = request.content(contentType.charset().orElse(StandardCharsets.UTF_8));
                  try {
                      return reader.readValue(content);
                  } catch (JsonProcessingException e) {
                      throw new IllegalArgumentException("failed to parse a JSON document: " + e, e);
                  }
              }
          }
          return RequestConverterFunction.fallthrough();
      }
docs: Converts the specified {@link AggregatedHttpMessage} to an object of {@code expectedResultType}.