code: void setRuleMessage(XmlModelWriter xw, ValidatorRule rule, Element element)
      {
          String messageKey = rule.getMessageKey();
          String message = rule.getMessage();
  
          if ( messageKey != null || message != null )
          {
              Element msgElementToUse = findChildElement(xw, element, "msg", "name", rule.getRuleName(), true, null);
              setElementAttribute(msgElementToUse, "resource", true);
  
              if ( messageKey != null )
              {
                  setElementAttribute(msgElementToUse, "key", messageKey);
                  if (_isValidatorOneOne) {
                      setElementAttribute(msgElementToUse, "bundle", rule.getBundle());
                  }
              }
              else // message != null (it's a hardcoded message)
              {
                  // Add our special constant as the message key, append the hardcoded message to it.
                  setElementAttribute(msgElementToUse, "key", ValidatorConstants.EXPRESSION_KEY_PREFIX + message);
              }
          }
      }
docs: Set up the desired &lt;msg&gt; element and attributes for the given rule.
  
  @param rule the rule with the message to use