Java SE 多线程下载网络上的文件代码+注释

import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;import org.junit.Test;public class DownLoader {    @Test    public void download() throws Throwable{        String filename = "baiduTool.exe";        String path ="http://dlsw.baidu.com/sw-search-sp/soft/e2/28619/BaiduExpert_Setup_2.0.201.1910.1422515416.exe";        URL url = new URL(path);        HttpURLConnection conn =(HttpURLConnection)url.openConnection();        conn.setConnectTimeout(5*1000);        conn.setRequestMethod("GET");        int filelength = conn.getContentLength();//文件的长度        System.out.println(filelength);        RandomAccessFile file = new RandomAccessFile(filename, "rw");//得到随机文件类，第一个参数为文件名称，第二个参数是对文件的操作，rw是读写操作        file.setLength(filelength);//设置本地文件的长度，等于本地文件的长度        file.close();//关闭这个文件        conn.disconnect();//先断掉这个链接，这个方法不调用也可以的        int threadsize = 3;//设置要下载的线程数        int threadlength = filelength %3 == 0?filelength/3:filelength/3+1;//每条线程下载的长度        for(int i= 0;i<threadsize;i++){            int startposition = i* threadlength;//计算每条线程应该从文件的什么位置开始下载            RandomAccessFile threadfile = new RandomAccessFile(filename, "rw");            threadfile.seek(startposition);//从文件的什么位置开始写入数据            new DownLoadThread(i,path,startposition,threadfile,threadlength).start();        }        int quit =System.in.read();        while('q' != quit){            Thread.sleep(2*1000);        }    }    private class DownLoadThread extends Thread{        private int threadid;        private int startpositon;        private RandomAccessFile threadFile;        private int threadlength;        private String path;        public DownLoadThread(int threadid,String path,int startposition,RandomAccessFile threadFile,int threadlength){            this.threadid = threadid;            this.path= path;            this.startpositon= startposition;            this.threadFile = threadFile;            this.threadlength =threadlength;        }        @Override        public void run(){            try {                URL url = new URL(path);                HttpURLConnection conn = (HttpURLConnection) url.openConnection();                conn.setConnectTimeout(5*1000);                conn.setRequestMethod("GET");                conn.setRequestProperty("Range","bytes="+startpositon+"-");//从指定文件从什么位置开始下载                InputStream inStream = conn.getInputStream();                byte[] buffer = new byte[1024];                int len =-1;                int length =0;//指的是当前线程下载的值，之后将下载的长度和每个线程下载的长度进行比较，如果小于就继续下载，如果大于就停止下循环                while(length<threadlength &&(len = inStream.read(buffer))!=-1){                    threadFile.write(buffer,0,len);                    length +=len;//累计下载长度                    //当复制到第二条线程的时候，已经制定了开始写位置，写到自己的长度就应该断掉                }                threadFile.close();                inStream.close();                System.out.println("线程"+(threadid+1)+"已经下载完成");            } catch (Exception e) {                System.out.println("线程"+(threadid+1)+"下载出错了"+e);            }        }    }}