code: public void setMessageFormat(String messageFormat) {
          if (!"string".equals(messageFormat)
                  && !"message".equals(messageFormat)) {
              throw new IllegalArgumentException("Unsupported message.format=" + messageFormat + ", only supported \"string\" or \"message\" format.");
          }
          this.messageFormat = messageFormat;
      }
docs: httl.properties: message.format=string