多线程分段下载文件

http://aokunsang.iteye.com/blog/1069794

断点续传的原理：

把一个文件分为n个小文件，程序分n个线程去下载每一个小文件。阅读有关资料，IE浏览器传递给服务端的时候可以传递一个请求信息"Range"：如：RANGE: bytes=2000070-  一看就知道告诉服务器下载的时候从2000070字节开始下载。值得注意的是response.getResponseCode()为206,不再是200了。

 

上代码：

/**
 * 下载工具类
 * 
 * @author aokunsang
 * 
 */
public class FileDownloader {

    private HttpURLConnection conn;

    public static void main(String[] args) throws Exception {
        FileDownloader load = new FileDownloader();
        load.download();
    }
    
    /**
     * 开始下载
     */
    public void download() throws Exception {
        HttpConnect connect = new HttpConnect(1);
        int fileSize = 0, threadSize = 3; // 文件长度和线程数
        conn = connect.initConnction();
        conn.connect();
        if (conn.getResponseCode() == 200) {
            fileSize = conn.getContentLength();
            conn.disconnect();
        }
        int perthThreadSize = fileSize / 3 + 1;   //每个线程需要下载的文件大小
        RandomAccessFile raf = new RandomAccessFile(new File("D://ca.mp3"), "rw");
        raf.setLength(fileSize);
        raf.close();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < threadSize; i++) {
            int startpos = i * perthThreadSize;  //线程开始下载的位置
            RandomAccessFile raff = new RandomAccessFile(new File("D://ca.mp3"), "rw");
            raff.seek(startpos);
            new myThread(startpos, perthThreadSize, i, raff).start();
        }
        long endTime = System.currentTimeMillis();
        
        while(Thread.activeCount()>1){}  //防止main线程结束而导致下载线程被销毁
        System.out.println("--------------下载完成，共耗时："+(endTime-startTime)/(60*1000)+"分钟--------");
    }

    class myThread extends Thread {
        private int startpos;
        private int perThreadSize;
        private int threadId;
        private RandomAccessFile raf;

        public myThread(int startpos, int perThreadSize, int threadId,
                RandomAccessFile raf) {
            this.startpos = startpos;
            this.perThreadSize = perThreadSize;
            this.threadId = threadId;
            this.raf = raf;
        }

        @Override
        public void run() {
            HttpConnect connect = new HttpConnect(0);
            HttpURLConnection con = connect.initConnction();
            con.setRequestProperty("Range", "bytes=" + this.startpos + "-");
            try {
                InputStream is = con.getInputStream();
                byte[] bt = new byte[1024];
                int length = 0;
                int len = 0;
                while (length < this.perThreadSize && (len = is.read(bt)) != -1) {
                    raf.write(bt, 0, len);
                    length += len;
                }
                raf.close();
                is.close();
                System.out.println("线程"+threadId+"完成！");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    class HttpConnect {
        private int flag;

        public HttpConnect(int flag) {
            this.flag = flag;
        }

        public HttpURLConnection initConnction() {
            HttpURLConnection connection = null;
            try {
                URL url = new URL(HttpConnectParams.URLSTRING.content);
                connection = (HttpURLConnection) url.openConnection();
                connection.setConnectTimeout(Integer
                        .parseInt(HttpConnectParams.CONNECTTIEMEDOUT.content));
                connection.setRequestMethod(HttpConnectParams.POST.content);
                connection.setRequestProperty(HttpConnectParams.ACCECT.header,
                        HttpConnectParams.ACCECT.content);
                connection.setRequestProperty(
                        HttpConnectParams.ACCECT_LANGAGE.header,
                        HttpConnectParams.ACCECT_LANGAGE.content);
                connection.setRequestProperty(HttpConnectParams.CHARSET.header,
                        HttpConnectParams.CHARSET.content);
                if (flag == 1) {
                    connection.setRequestProperty(
                            HttpConnectParams.KEEPCONNECT.header,
                            HttpConnectParams.KEEPCONNECT.content);
                }
            } catch (Exception e) {
                connection = null;
            }
            return connection;
        }
    }
}

---/---

 * HTTP请求的一些常量信息
 * @author aokunsang
 *
 */
public enum HttpConnectParams {
    /**
     * ＵＲＬ地址(mp3格式的音乐文件)
     */
    URLSTRING("http://www.61mp3.cn/music/%B6%F9%CD%AF%B8%E8%C7%FA/%CA%AE%B6%FE%C9%FA%D0%A4%B8%E8.mp3"),
    /**
     * 请求的方式
     */
    POST("GET"),
    /**
     *  请求的格式
     */
    ACCECT("Accept","image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*"),
    /**
     * 请求的语言
     */
    ACCECT_LANGAGE("Accept-Language","zh-CN"),
    /**
     * 请求的字符集编码
     */
    CHARSET("Charset","UTF-8"),
    /**
     * 链接的超时数
     */
    CONNECTTIEMEDOUT("5000"),
    /**
     * 保持链接
     */
    KEEPCONNECT("Connection","Keep-Alive");
    
    public String header; //标题
    public String content; //内容
    
    private HttpConnectParams(String header,String content) {
        this.header = header;
        this.content = content;
    }
    private HttpConnectParams(String content){
        this.content = content;
    }
}