Android 多线程下载网络文件

public class MulThreadDownLoad {
    public static void main(String[] args) {
        String path = "http://localhost:4484/WebSite1/21.bmp";// 一个本地web服务，指向一张图片
        try {
            new MulThreadDownLoad().download(path, 3);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void download(String path, int threadsize) throws Exception {
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setConnectTimeout(5000);
        conn.setRequestMethod("GET");
        if (conn.getResponseCode() == 200) {
            int length = conn.getContentLength();
            File file = new File(getFilename(path));
            RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");
            // 在本地生成一个长度为length的文件
            accessFile.setLength(length);
            accessFile.close();
            // 每条线程负责下载的数据量
            int block = length / threadsize == 0 ? length / threadsize : length / threadsize + 1;
            for (int threadid = 0; threadid < threadsize; threadid++) {
                new Thread(new Runa(threadid, block, url, file)).start();
            }
        } else
            System.out.println(conn.getResponseCode() + "");
    }

    private class Runa implements Runnable {
        private int threadid;
        private int block;
        private URL url;
        private File file;

        public Runa(int threadid, int block, URL url, File file) {
            this.threadid = threadid;
            this.block = block;
            this.url = url;
            this.file = file;
        }

        @Override
        public void run() {
            int start = threadid * block;// 计算该线程从网络文件的什么位置开始下载
            int end = (threadid + 1) * block - 1;// 计算该线程下载网络文件的结束位置
            try {
                RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");// 创建从中读取和向其中写入（可选）的随机访问文件流
                accessFile.seek(start);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setConnectTimeout(5000);
                conn.setRequestMethod("GET");
                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);// 设置从start开始下载，到end结束
                // if (conn.getResponseCode() == 200) {
    // 如果是分段下载，请求状态码不是200而是206！
                InputStream ins = conn.getInputStream();
                byte[] buffer = new byte[1024];
                int length = 0;
                while ((length = ins.read(buffer)) != -1) {
                    accessFile.write(buffer, 0, length);
                }
                accessFile.close();
                ins.close();
                // }

            } catch (Exception e) {

                e.printStackTrace();
            }
        }
    }
    private String getFilename(String path) {
        return path.substring(path.lastIndexOf("/") + 1);
    }
}