code: public static boolean validateIsBase64String(final String data) {
  
          if (data == null || data.length() % 4 != 0) {
              return false;
          }
  
          for (int m = 0; m < data.length(); m++) {
              final byte charByte = (byte) data.charAt(m);
  
              // pad char detected
              if (DECODE_64[charByte] == -2) {
                  if (m < data.length() - 2) {
                      return false;
                  }
                  else if (m == data.length() - 2 && DECODE_64[(byte) data.charAt(m + 1)] != -2) {
                      return false;
                  }
              }
  
              if (charByte < 0 || DECODE_64[charByte] == -1) {
                  return false;
              }
          }
  
          return true;
      }
docs: Determines whether the given string contains only Base64 characters.
  
  @param data
  the string, as a <code>String</code> object, to validate
  @return <code>true</code> if <code>data</code> is a valid Base64 string, otherwise <code>false</code>