/**
  * 获取fileName的地址
  *
  * @param fileName
  * @return
  */
 @Override
 public String getUrl(String fileName) {
   if (BAIDU_BCS == null) {
     BAIDU_BCS = new BaiduBCS(new BCSCredentials(AK, SK), HOST);
   }
   GenerateUrlRequest generateUrlRequest =
       new GenerateUrlRequest(HttpMethodName.GET, BUCKET, fileName);
   return BAIDU_BCS.generateUrl(generateUrlRequest);
 }