public class jions {
    public static int runnling=3;
     public static int threadcount=3;
 public static  void  main(String[]args) throws Exception{
    String path="http://down9.869v.com/setup_445.exe";
     URL url=new URL(path);
             HttpURLConnection con= (HttpURLConnection) url.openConnection();
              con.setReadTimeout(6000);
              con.setRequestMethod("GET");
           int code= con.getResponseCode();
           if(code==200){
              int length= con.getContentLength();//已经设置了请求的位置//返回的是当前位置对应文件的输入流
              RandomAccessFile raf=new RandomAccessFile("421.exe", "rwd");//记录当前线程下载的数据长度
              raf.close();
              
              //System.out.print("文件长度:"+length);
               int bolak =length/threadcount;
               for(int threadID=1;threadID<=threadcount;threadID++){
                   int starIndex=(threadID-1)*bolak;
                   int endIndex=threadID*bolak-1;
                   if(threadID==threadcount){
                       endIndex=length;
                    
                       
                   }
                   System.out.println("线程"+threadID+"下载---->"+starIndex+"----->"+endIndex);
                   new MyThread(path, threadID, starIndex, endIndex).start();
               }
           }else{
               System.out.print("读取长度失败");
               
               
           }
    
 }
 public static class MyThread extends Thread{
       public MyThread(String path, int threadID, int starIndex, int endIndex) {
        super();
        this.path = path;
        this.threadID = threadID;
        this.starIndex = starIndex;
        this.endIndex = endIndex;
    }
    private String path;  
       private int threadID;
       private int starIndex;
       private int endIndex;
    
    
    
 
  public void run(){
       
    try {
        //检查是否在记录下载长度的文件 如果存在读取这个文件的数据
       File tempFile =new File(threadID+".txt");
        if(tempFile.exists()&&tempFile.length()>0){
        FileInputStream fis=new FileInputStream(tempFile);
        byte[] tpem=new byte[1024];
        int leng =fis.read(tpem);
        String dowmlod =new String(tpem,0,leng);
        int downlodalenInt= Integer.parseInt(dowmlod);
        starIndex=downlodalenInt;
        fis.close();
        }    
        
        URL url=new URL(path);
       HttpURLConnection con =(HttpURLConnection) url.openConnection();
       con.setRequestMethod("GET");
       con.setRequestProperty("Range", "byte="+starIndex+"-"+endIndex);
       System.out.println("线程真实下载位置"+threadID+"下载----"+starIndex);
       con.setReadTimeout(8000);
       
       int code=con.getResponseCode();
       System.out.println("code"+code);
       InputStream is= con.getInputStream();
       RandomAccessFile raf=new RandomAccessFile("421.exe", "rwd");
       raf.seek(starIndex);
       int len=0;
       byte[]buffer=new byte[1024];
       int total=0;
       while((len=is.read(buffer))!=-1){
         RandomAccessFile flie =new RandomAccessFile(threadID+".txt ","rwd");
         raf.write(buffer,0,len);
           total+=len;
           System.out.println("线程"+threadID+"total:"+total);
          flie.write((total+starIndex+"").getBytes());//记录下载的位置
          flie.close();
       }
       is.close();
       raf.close();
       System.out.println("线程ID"+threadID+"下载完毕......");
    } catch (Exception e) {
        
        e.printStackTrace();
    }finally{
        runnling--;
        if(runnling==0){
            for(int i=1 ;i<=3;i++){
                File file=new File(i+".txt");
                file.delete();
                
            }
            System.out.println("文件下载半壁删除所有下载记录");
        }
        
    }
      
  }
  }
 
}

JAVA实现多线程断点下载