try (FileOutputStream fos = new FileOutputStream("C://file/2.txt");)
    {
        BufferedInputStream bis = new BufferedInputStream(sin);
        BufferedOutputStream bos = new BufferedOutputStream(fos);
        int n;
        int length = 16;//известна, задается извне
        byte[] buffer = new byte[8192];
        while (((n = bis.read(buffer)) > -1) && (length > 0)) {
            if (n > length) n = length;
            length -= n;
            bos.write(buffer, 0, n);
            bos.flush();
        //    out.flush();
        }

    } catch (IOException ex) {
    }