在Java项目中如何利用多线程实现文件下载功能

import java.io.File; import java.io.InputStream; import java.io.RandomAccessFile; import java.net.HttpURLConnection; import java.net.URL;  public class MulThreadDownload {   public static void main(String[] args) throws Exception {     String path = "http://192.168.1.100:8080/Hello/Big.exe";     new MulThreadDownload().download(path, 3);   }       private void download(String path, int threadSize) throws Exception {     URL url = new URL(path);     HttpURLConnection connection = (HttpURLConnection) url.openConnection();     connection.setRequestMethod("GET");     connection.setConnectTimeout(5000);     if (connection.getResponseCode() == 200) {       int length = connection.getContentLength();// 获取网络文件长度       File file = new File(getFileName(path));       // 在本地生成一个长度与网络文件相同的文件       RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");       accessFile.setLength(length);       accessFile.close();        // 计算每条线程负责下载的数据量       int block = length % threadSize == 0 &#63; length / threadSize : length           / threadSize + 1;       for (int threadId = 0; threadId < threadSize; threadId++) {         new DownloadThread(threadId, block, url, file).start();       }     } else {       System.out.println("download fail");     }   }    private class DownloadThread extends Thread {      private int threadId;     private int block;     private URL url;     private File file;      public DownloadThread(int threadId, int block, URL url, File file) {       this.threadId = threadId;       this.block = block;       this.url = url;       this.file = file;     }      @Override     public void run() {       int start = threadId * block; // 计算该线程从网络文件什么位置开始下载       int end = (threadId + 1) * block - 1; // 计算下载到网络文件什么位置结束       try {         RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");         accessFile.seek(start); //从start开始          HttpURLConnection connection = (HttpURLConnection) url             .openConnection();         connection.setRequestMethod("GET");         connection.setConnectTimeout(5000);         //设置获取资源数据的范围，从start到end         connection.setRequestProperty("Range", "bytes=" + start + "-"             + end);         //注意多线程下载状态码是 206 不是200         if (connection.getResponseCode() == 206) {           InputStream inputStream = connection.getInputStream();           byte[] buffer = new byte[1024];           int len = 0;           while ((len = inputStream.read(buffer)) != -1) {             accessFile.write(buffer, 0, len);           }           accessFile.close();           inputStream.close();         }         System.out.println("第" + (threadId + 1) + "条线程已经下载完成");       } catch (Exception e) {         e.printStackTrace();       }     }   }       private String getFileName(String path) {     return path.substring(path.lastIndexOf("/") + 1);   } }