code: protected GsonJsonEngine createGsonJsonEngine(JsonEngineResource resource) {
          final boolean serializeNulls = isGsonSerializeNulls();
          final boolean prettyPrinting = isGsonPrettyPrinting();
          final Consumer<GsonBuilder> builderSetupper = prepareGsonBuilderSetupper(serializeNulls, prettyPrinting);
          final Consumer<JsonMappingOption> optionSetupper = prepareMappingOptionSetupper(resource.getMappingOption());
          return resource.getYourEngineCreator().map(creator -> { // rare option
              return createYourEngine(creator, builderSetupper, optionSetupper);
          }).orElseGet(() -> { // mainly here
              return newGsonJsonEngine(builderSetupper, optionSetupper);
          });
      }
docs: mappingOption is specified for another engine