private Header spnegoAuthenticate( boolean stripPort, URI uri ) throws Exception {
  SPNegoSchemeFactory spNegoSchemeFactory = new SPNegoSchemeFactory( stripPort );
  // using newInstance method instead of create method to be compatible httpclient library from 4.2 to 4.5
  // the create method was introduced at version 4.3
  SPNegoScheme spNegoScheme = (SPNegoScheme) spNegoSchemeFactory.newInstance( null );
  spNegoScheme.processChallenge( AUTHENTICATE_HEADER );
  return spNegoScheme.authenticate( credentials, new HttpGet( "" ), getContext( uri ) );
}