code: public static byte[] computeMultipartBoundary() {
      ThreadLocalRandom random = ThreadLocalRandom.current();
      byte[] bytes = new byte[35];
      for (int i = 0; i < bytes.length; i++) {
        bytes[i] = MULTIPART_CHARS[random.nextInt(MULTIPART_CHARS.length)];
      }
      return bytes;
    }
docs: a fixed size of 35