public function __construct($access_id = NULL, $access_key = NULL, $endpoint = NULL)
 {
     parent::__construct();
     $this->accessKeyId = $access_id ? $access_id : OSS_ACCESS_ID;
     $this->accesKeySecret = $access_key ? $access_key : OSS_ACCESS_KEY;
     $this->endpoint = $endpoint ? $endpoint : OSS_ENDPOINT;
 }