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 );
    }
}