public static XPathFactory instance() {
    final XPathFactory ret = defaultreference.get();
    if (ret != null) {
      return ret;
    }
    XPathFactory fac = new JaxenXPathFactory();
    if (defaultreference.compareAndSet(null, fac)) {
      return fac;
    }
    // someone else installed a different instance before we added ours.
    // return that other instance.
    return defaultreference.get();
  }
public static XPathFactory instance()