code: public static Element createRootElement(String tagName) throws AlipayApiException {
          Document doc = newDocument();
          Element root = doc.createElement(tagName);
          doc.appendChild(root);
          return root;
      }
docs: Creates a root element as well as a new document with specific tag name.
  
  @param tagName the name of the root element
  @return a new element instance
  @throws ApiException problem generating a new document