code: public String asJson(T instance) throws JAXBException {
      Marshaller marshaller = getMarshaller(instance);
      marshaller.setProperty(MarshallerProperties.MEDIA_TYPE, "application/json");
      marshaller.setProperty(MarshallerProperties.JSON_INCLUDE_ROOT, true);
      String result = getString(marshaller, instance);
      return result;
    }
docs: create a Json representation for the given <T> instance
  
  @param instance
  - the instance to convert to json
  @return a Json representation of the given <T>
  @throws JAXBException