Java实现多线程下载

 package cn.test.DownLoad;

 import java.io.File;
 import java.io.InputStream;
 import java.io.RandomAccessFile;
 import java.net.HttpURLConnection;
 import java.net.URL;

 public class MultiPart {
     public void down() throws Exception
     {
         //1、声明URL
         String fileName="a.rar";
         String path="http://localhost:8080/day23_MultiThreadDownLoad/file/"+fileName;
         URL url=new URL(path);
         //2、返回连接对象
         HttpURLConnection conn=(HttpURLConnection) url.openConnection();
         //3、设置请求类型
         conn.setRequestMethod("GET");
         //4、设置允许接收消息
         conn.setDoInput(true);
         //5、连接
         conn.connect();
         //6、状态码
         int code=conn.getResponseCode();
         if(code==200)
         {
             int sum=conn.getContentLength();//总长度
             String downFile="d:\\"+fileName;
             //7、创建一个相同大小的空文件
             RandomAccessFile file=new RandomAccessFile(new File(downFile), "rw");
             file.setLength(sum);
             file.close();
             //8、声明线程数量
             int threadCount=3;
             //9、声明每个线程的下载量
             int threadSize=sum/threadCount+((sum%threadCount==0)?0:1);
             for(int i=0;i<threadCount;i++)
             {
                 int start=i*threadSize;
                 int end=start+threadSize-1;
                 System.out.println("线程： "+i+" : "+start+" : "+end);
                 //10、启动线程
                 new myThread(start,end,downFile,url).start();
             }
         }
         //11、关闭连接
         conn.disconnect();
     }

     public static void main(String[] args) {
         try {
             new MultiPart().down();
         } catch (Exception e) {
             e.printStackTrace();
         }
         System.out.println("OK");
     }
 }

 class myThread extends Thread
 {
     private int start;
     private int end;
     private String downFile;
     private URL url;
     public myThread(int start, int end, String downFile, URL url) {
         this.start = start;
         this.end = end;
         this.downFile = downFile;
         this.url = url;
     }

     public void run() {
         try {
             HttpURLConnection conn=(HttpURLConnection) url.openConnection();
             conn.setRequestMethod("GET");
             conn.setDoInput(true);
             //设置从哪里下载。断点
             conn.setRequestProperty("range", "bytes="+start+"-"+end);
             conn.connect();
             int code=conn.getResponseCode();
             if(code==206)
             {
                 int size=conn.getContentLength();
                 InputStream in=conn.getInputStream();
                 //写同一文件
                 RandomAccessFile file=new RandomAccessFile(new File(downFile), "rw");
                 //设置从文件的哪里开始写
                 file.seek(start);
                 byte[] b=new byte[1024];
                 int len=-1;
                 while((len=in.read(b))!=-1)
                 {
                     file.write(b, 0, len);
                 }
                 file.close();
             }
             conn.disconnect();            

         } catch (Exception e) {
             e.printStackTrace();
         }
     }
 }