code: public static synchronized NameResolverRegistry getDefaultRegistry() {
      if (instance == null) {
        List<NameResolverProvider> providerList = ServiceProviders.loadAll(
            NameResolverProvider.class,
            getHardCodedClasses(),
            NameResolverProvider.class.getClassLoader(),
            new NameResolverPriorityAccessor());
        if (providerList.isEmpty()) {
          logger.warning("No NameResolverProviders found via ServiceLoader, including for DNS. This "
              + "is probably due to a broken build. If using ProGuard, check your configuration");
        }
        instance = new NameResolverRegistry();
        for (NameResolverProvider provider : providerList) {
          logger.fine("Service loader found " + provider);
          if (provider.isAvailable()) {
            instance.addProvider(provider);
          }
        }
        instance.refreshProviders();
      }
      return instance;
    }
docs: Returns the default registry that loads providers via the Java service loader mechanism.