code: public static byte[] sha256Bytes(byte[] bs) {
          if (bs == null) return null;
          MessageDigest digester;
          try {
              digester = MessageDigest.getInstance("SHA-256");
          } catch (NoSuchAlgorithmException ex) {
              throw new RuntimeException(ex);
          }
          return digester.digest(bs);
      }
docs: SHA-256
  
  @param bs 待hash数据
  
  @return hash值