code: String readBodyAndValidateForNumber() {
          String messageContent = readBodyAsString();
          if (messageContent == null || messageContent.isEmpty()) {
              throw new RuntimeException("Message is empty");
          }
          // Check if content is a number
          for (int i = 0; i < messageContent.length(); i++) {
              if (! Character.isDigit(messageContent.charAt(i))) {
                  throw new RuntimeException("Message is not a number");
              }
          }
          return messageContent;
      }
docs: Extracts the charset of the message content. If
  no charset is provided in the message properties
  then the default charset (UTF-8) is used.
  
  @return The message content charset