Java多线程下载文件

package com.test.download;

 

import java.io.File;

import java.io.InputStream;

import java.io.RandomAccessFile;

import java.net.HttpURLConnection;

import java.net.URL;

 

/*

 * 多线程下载

 */

public class MulThreadDownload {

 

    public static void main(String[] args) throws Exception {

        String path = "http://pic.4j4j.cn/upload/pic/20130909/681ebf9d64.jpg";

        new MulThreadDownload().download(path,3);

    }

     

    public 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){

            //获取网络文件长度

            int length = conn.getContentLength();  

            //新建本地文件保存下载数据

            File file = new File(getFilename(path));

            //计算每条线程负责下载的数据量

            int block = length%threadsize==0 ? length/threadsize : length/threadsize+1;

            //开启指定数目的线程同时下载

            for(int threadid = 0; threadid < threadsize; threadid++){

                new DownloadThread(threadid,block,url,file).start();

            }

        }else{

            System.out.println("下载失败！");

        }

    }

     

    private class DownloadThread extends Thread{

        private int threadid;   //线程编号

        private int block;      //下载块大小

        private URL url;        //下载链接

        private File file;      //下载数据保存文件

        public DownloadThread(int threadid, int block, URL url, File file) {

            this.threadid = threadid;

            this.block = block;

            this.url = url;

            this.file = file;

        }

 

        public void run() {

            int start = threadid * block;       //本线程下载数据写入文件开始位置

            int end = (threadid+1) * block - 1; //本线程下载数据写入文件结束位置

            try {

                //创建一个随机访问文件流对象

                RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");

                //文件指针偏移至正确写入位置

                accessFile.seek(start);

                HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                conn.setConnectTimeout(5000);

                conn.setRequestMethod("GET");

                //设置请求数据的范围

                conn.setRequestProperty("Range", "bytes="+start+"-"+end);

                if(conn.getResponseCode() == 206){//状态码206:（部分内容） 服务器成功处理了部分 GET 请求

                    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)+"部分下载完成");

            } catch (Exception e) {

                e.printStackTrace();

            }

        }

    }

 

    private String getFilename(String path) {

        return path.substring(path.lastIndexOf("/")+1);

    }

}