http多线程下载

转载自：传智播客教程

package cn.itcast.net.download;

import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class MulThreadDownload {

    /**
     * @param args
     */
    public static void main(String[] args) {
        String path = "http://down10.zol.com.cn/zoldownload/QQ8.2@81_428274.exe";
        try {
            new MulThreadDownload().download(path, 5);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    /**
     * 从路径中获取文件名称
     * @param path 下载路径
     * @return
     */
    public static String getFilename(String path){
        return path.substring(path.lastIndexOf('/')+1);
    }
    /**
     * 下载文件
     * @param path 下载路径
     * @param threadsize 线程数
     */
    public void download(String path, int threadsize) throws Exception{
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection)url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5 * 1000);
        int filelength = conn.getContentLength();//获取要下载的文件的长度
        String filename = getFilename(path);//从路径中获取文件名称
        File saveFile = new File(filename);
        RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");//d：立刻写到硬盘中
        accessFile.setLength(filelength);//设置本地文件的长度和下载文件相同
        accessFile.close();
        //计算每条线程下载的数据长度
        int block = filelength%threadsize==0? filelength/threadsize : filelength/threadsize+1;
        for(int threadid=0 ; threadid < threadsize ; threadid++){
            new DownloadThread(url, saveFile, block, threadid).start();
        }
    }
    
    private final class DownloadThread extends Thread{
        private URL url;
        private File saveFile;
        private int block;//每条线程下载的数据长度
        private int threadid;//线程id

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

        @Override
        public void run() {
            //计算开始位置公式：线程id*每条线程下载的数据长度= ？
            //计算结束位置公式：（线程id +1）*每条线程下载的数据长度-1 =?
            int startposition = threadid * block;
            int endposition = (threadid + 1 ) * block - 1;
            try {
                RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");
                accessFile.seek(startposition);//设置从什么位置开始写入数据
                HttpURLConnection conn = (HttpURLConnection)url.openConnection();
                conn.setRequestMethod("GET");
                conn.setConnectTimeout(5 * 1000);
                conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);//请求下载文件的范围
                InputStream inStream = conn.getInputStream();
                byte[] buffer = new byte[1024];
                int len = 0;
                while( (len=inStream.read(buffer)) != -1 ){
                    accessFile.write(buffer, 0, len);
                }
                inStream.close();
                accessFile.close();
                System.out.println("线程id:"+ threadid+ "下载完成");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }        
    }

}