// null = online, Throwable = offline
  public static Throwable checkIfHttpServerIsOnline(String serverName) {
    try {
      String url = serverName.startsWith("http") ? serverName : "http://" + serverName + "/";
      //HttpURLConnection.setFollowRedirects(false);
      // note : you may also need
      //        HttpURLConnection.setInstanceFollowRedirects(false)
      HttpURLConnection con =
         (HttpURLConnection) new URL(url).openConnection();
      con.setRequestMethod("HEAD");
      int responseCode = con.getResponseCode();
      //print("Response code for " + url + ": " + responseCode);
      // If we came this far, assume server is online.
      return null;
      //return (con.getResponseCode() == HttpURLConnection.HTTP_OK);
    }
    catch (Exception e) {
       return e;
    }
  }