code: private void ensureContentAvailable() throws IOException {
          if (nodeContentBytes.size() > 0) {
              return;
          }
          try {
              Transform serializeTransform = new Transform(rootNode);
              if (outputProperties!=null) {
                  serializeTransform.setOutputProperties(outputProperties);
              }
              StreamResult byteResult = new StreamResult(nodeContentBytes);
              serializeTransform.transformTo(byteResult);
          } catch (Exception e) {
              throw new IOException("Unable to serialize document to outputstream: "
                                    + e.toString());
          }
      }
docs: Do the actual work of serializing the node to bytes
  @throws IOException if serialization goes awry