怎样计算下载速度？解决方案

            URL url = new URL(urls);
            URLConnection conn = url.openConnection();
            conn.connect();
            System.out.println(urls + " 长度：" + conn.getContentLength() / 1024 + "KB");
            InputStream fis = conn.getInputStream();
            long a = System.currentTimeMillis();
            FileOutputStream out = new FileOutputStream(path);
            byte buf[] = new byte[10240];
            int n;
            while ((n = fis.read(buf)) != -1) {
                out.write(buf, 0, n);
            }
            fis.close();
            out.close();
            long b = System.currentTimeMillis();
            System.out.println("[+]: 下载费时："+ String.valueOf(b-a) + "毫秒");