code: public static String nodeToString(Node node) throws AlipayApiException {
          String payload = null;
  
          try {
              Transformer tf = TransformerFactory.newInstance().newTransformer();
  
              Properties props = tf.getOutputProperties();
              props.setProperty(OutputKeys.INDENT, LOGIC_YES);
              props.setProperty(OutputKeys.ENCODING, DEFAULT_ENCODE);
              tf.setOutputProperties(props);
  
              StringWriter writer = new StringWriter();
              tf.transform(new DOMSource(node), new StreamResult(writer));
              payload = writer.toString();
              payload = payload.replaceAll(REG_INVALID_CHARS, " ");
          } catch (TransformerException e) {
              throw new AlipayApiException("XML_TRANSFORM_ERROR", e);
          }
  
          return payload;
      }
docs: Converts the Node/Document/Element instance to XML payload.
  
  @param node the node/document/element instance to convert
  @return the XML payload representing the node/document/element
  @throws ApiException problem converting XML to string