Android中 多线程下载原理

计算每个线程的下载起始终止位置公式如下

文件读写方式4中类型

工程源码目录

package cn.itcast.download;import java.io.File;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;public class MulThreadDownloader {    public static void main(String[] args) throws Exception {        String path = "http://219.245.72.241:8080/web3/gaosu.jsp";//服务器文件的地址        int threadsize = 3;//开启三个线程，android应用中开启的线程数不能太多        new MulThreadDownloader().download(path, threadsize);    }    private void download(String path, int threadsize) throws Exception {        URL downpath = new URL(path);        HttpURLConnection conn = (HttpURLConnection) downpath.openConnection();        conn.setConnectTimeout(5000);        conn.setRequestMethod("GET");        if(conn.getResponseCode() == 200){            int length = conn.getContentLength();//获取网络文件的长度            File file = new File(getFileName(path));            RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");//生成本地文件            accessFile.setLength(length);            accessFile.close();            //计算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 DownloadThread(threadid, downpath, block, file).start();            }        }    }    //负责下载操作    private final class DownloadThread extends Thread{        private int threadid;        private URL downpath;        private int block;        private File file;        public DownloadThread(int threadid, URL downpath, int block, File file) {            this.threadid = threadid;            this.downpath = downpath;            this.block = block;            this.file = file;        }        public void run() {            int startposition = threadid * block;//从网络文件的什么位置开始下载数据            int endposition = (threadid+1) * block - 1;//下载到网络文件的什么位置结束            //指示该线程要从网络文件的startposition位置开始下载，下载到endposition位置结束            //Range:bytes=startposition-endposition            try{                RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");                accessFile.seek(startposition);//移动指针到文件的某个位置                HttpURLConnection conn = (HttpURLConnection) downpath.openConnection();                conn.setConnectTimeout(5000);                conn.setRequestMethod("GET");                conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);//区域下载                //分段下载请求码不是200，而是206                //if(conn.getResponseCode() == 206){                InputStream inStream = conn.getInputStream();                byte[] buffer = new byte[1024];                int len = 0;                while( (len = inStream.read(buffer)) != -1 ){                    accessFile.write(buffer, 0, len);                }                accessFile.close();                inStream.close();                System.out.println("第"+ (threadid+1)+ "线程下载完成");            }catch (Exception e) {                e.printStackTrace();            }        }    }    /**     * 获取文件名称     * @param path 下载路径     * @return     */    private static String getFileName(String path) {        return path.substring(path.lastIndexOf("/")+ 1);    }}

客户端要下载服务器web3目录里的gaosu.jsp文件

启动tomcat，挂起服务器web3

运行上面的java程序，结果展示

此时会发现工程源码目录下生成了下载完成的gaosu.jsp文件