各位前辈麻烦你们帮小弟我解决上这个有关问题，是关于HttpURLConnection类的有关问题

package cn.zcp.downlod;import java.io.File;import java.io.FileNotFoundException;import java.io.IOException;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;import java.util.Scanner;public class DownlodMessage{    public static void main(String[] args) throws Exception    {        Scanner can = new Scanner(System.in);        String path = can.next();        new DownlodMessage().dowload(path,4);    }    private void dowload(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)        {            int len = conn.getContentLength();            File file = new File(this.getFilename(path));            RandomAccessFile ran = new RandomAccessFile(file, "rwd");            ran.setLength(len);            ran.close();                    int block = len%threadsize ==0 ? len/threadsize:len/threadsize+1;             for(int i = 0 ; i< threadsize; i++)            {                new MyThread(threadsize,file,url,block).start();            }        }        else        {            System.out.println("下载失败");        }    }        private class MyThread extends Thread    {        private int threadid;        private File file;        private URL url;        private int block;        public MyThread(int threadsize, File file, URL url, int block)        {            this.block = block;            this.file = file;            this.threadid = threadsize;            this.url = url;        }        @Override        public void run()        {            int start = threadid*block;            int end = (threadid+1)*block - 1;                        try            {                RandomAccessFile accessFile = new RandomAccessFile(file,"rwd");                accessFile.seek(start);                HttpURLConnection http = (HttpURLConnection)url.openConnection();                http.setConnectTimeout(5000);                http.setRequestMethod("GET");                http.setRequestProperty("Range", "bytes="+start+"-"+end);                if(http.getResponseCode() == 206)                {                System.out.println(http.getResponseCode());                    InputStream iStream =  http.getInputStream();                    int lenght = 0;                    byte[] buffer = new byte[2048];                    while((lenght = iStream.read(buffer , 0, buffer.length))!=-1)                    {                        accessFile.write(buffer, 0, lenght);                    }                    accessFile.close();                    iStream.close();                    System.out.println("第"+(threadid+1)+"个线程下载完成");                }                else                {                    System.out.println("下载失败");                }            }            catch (FileNotFoundException e)            {                // TODO Auto-generated catch block                e.printStackTrace();            }            catch (IOException e)            {                // TODO Auto-generated catch block                e.printStackTrace();            }                    }                    }    private String getFilename(String path)    {        String name = path.substring(path.lastIndexOf("/")+1);                return name;    }        }