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();
        }        
    }    
}