code: public T fromJson(String json) throws Exception {
      Unmarshaller u = this.getUnmarshaller();
      u.setProperty(MarshallerProperties.MEDIA_TYPE, "application/json");
      T result = this.fromString(u, json);
      return result;
    }
docs: get an instance of T for the given json string
  
  @param json
  - the json representation of the <T> instance
  @return T
  @throws Exception