code: private static void notifyUserDataHandlers(
              short operation, Node source, NodeImpl destination) {
          if (!(source instanceof NodeImpl)) {
              return;
          }
  
          NodeImpl srcImpl = (NodeImpl) source;
          if (srcImpl.document == null) {
              return;
          }
  
          for (Map.Entry<String, UserData> entry
                  : srcImpl.document.getUserDataMapForRead(srcImpl).entrySet()) {
              UserData userData = entry.getValue();
              if (userData.handler != null) {
                  userData.handler.handle(
                          operation, entry.getKey(), userData.value, source, destination);
              }
          }
      }
docs: Calls {@link UserDataHandler#handle} on each of the source node's
  value/handler pairs.
  
  <p>If the source node comes from another DOM implementation, user data
  handlers will <strong>not</strong> be notified. The DOM API provides no
  mechanism to inspect a foreign node's user data.