Java实现多线程下载工具类（网上下载百度图片）

DownUtil类 

package loading;

import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

import org.omg.CORBA.portable.InputStream;

public class DownUtil {	
	//定义下载资源的路径
	private String path;
	//定义所下载文件的保存位置
	private String targetFile;
	//定义需要使用多少个线程下载资源
	private int threadNum;
	//定义下载的线程对象
	private DownThreads[] threads;
	//定义下载文件的总大小
	private int fileSize;
	public DownUtil(String path,String targetFile,int threadNum)
	{
		this.path=path;
		this.targetFile=targetFile;
		this.threadNum=threadNum;
		//初始化threads数组
		threads=new DownThreads[threadNum];
	}
	//定义下载函数
	public void download() throws Exception
	{
		URL url=new URL(path);
		HttpURLConnection conn=(HttpURLConnection) url.openConnection();
		conn.setConnectTimeout(5000);
		conn.setRequestMethod("GET");
		conn.setRequestProperty("Accept", 
				"image/gif, image/jpeg, image/pjpeg, image/pjpeg, "
				+"application/x-shockwave-flash, application/xaml+xml, "
				+"application/vnd.ms-xpsdocument, application/x-ms-xbap, "
				+"appliction/x-ms-application, application/vnd.ms-excel, "
				+"application.vnds.ms-powerpoint, application/msword, */*");
				conn.setRequestProperty("Accept-Language", "zh-CN");
				conn.setRequestProperty("Charset", "UTF-8");
				conn.setRequestProperty("Connection", "Keep-Alive");
				//得到文件大小
				fileSize=conn.getContentLength();
				conn.disconnect();
				int currentPartSize=fileSize/threadNum+1;
				RandomAccessFile file=new RandomAccessFile(targetFile,"rw");
				//设置本地文件大小
				file.setLength(fileSize);
				file.close();
				for(int i=0;i<threadNum;i++)
				{
					//计算每个线程下载的开始位置
					int startPos=i*currentPartSize;
					//每个线程使用一个RandomAccessFile进行下载
					RandomAccessFile currentPart=new RandomAccessFile(targetFile, "rw");
					//定义该线程的下载位置
					currentPart.seek(startPos);
					//创建下载线程
					threads[i]=new DownThreads(startPos,currentPartSize,currentPart);
					//启动线程
					threads[i].start();
				}
		
	}
	//获取下载的完成百分比
	public double getCompleteRate()
	{
		//统计多个线程已经下载的总大小
		int sumSize=0;
		for(int i=0;i<threadNum;i++)
		{
			sumSize+=threads[i].length;
		}
		//返回已经完成的百分比
		return sumSize*1.0/fileSize;
	}
	public class DownThreads extends Thread{
		//当前线程的下载位置
		private int startPos;
		//当前线程负责下载的文件大小
		private int currentPartSize;
		//当前线程需要下载的文件块
		private RandomAccessFile currrentPart;
		//定义该线程已下载的字节数
		public int length;
		public DownThreads(int startPos, int currentPartSize, RandomAccessFile currentPart) {
			// TODO Auto-generated constructor stub
			this.startPos=startPos;
			this.currentPartSize=currentPartSize;
			this.currrentPart=currentPart;
		}
		public void run()
		{
			try
			{
				URL url=new URL(path);
				HttpURLConnection conn=(HttpURLConnection)url.openConnection();
				conn.setConnectTimeout(5*1000);
				conn.setRequestMethod("GET");
				conn.setRequestProperty("Accept",
								"image/gif, image/jpeg, image/pjpeg, image/pjpeg, "
								+"application/x-shockwave-flash, application/xaml+xml, "
								+"application/vnd.ms-xpsdocument, application/x-ms-xbap, "
								+"appliction/x-ms-application, application/vnd.ms-excel, "
								+"application.vnds.ms-powerpoint, application/msword, */*");
				conn.setRequestProperty("Accept-Language", "zh-CN");
				conn.setRequestProperty("Charset", "UTF-8");
				java.io.InputStream inStream=conn.getInputStream();
				//跳过StartPos表明该线程只下载自己负责的那一部分
				inStream.skip(this.startPos);
				byte[] buffer=new byte[1024];
				int hashRead=0;
				//读取网络数据，并写入本地文件
				while(length<currentPartSize&&(hashRead=inStream.read(buffer))!=-1)
				{
					currrentPart.write(buffer,0,hashRead);
					//累计该线程下载的总大小
					length+=hashRead;
				}
				//关闭线程
				currrentPart.close();
				inStream.close();
			}catch(Exception e){
				e.printStackTrace();
			}
			
		}
		

	}
	
}

 MultiThreadDown类

package loading;

public class MultiThreadDown {
	public static void main(String[] args) throws Exception {
		// TODO Auto-generated method stub
		//初始化DownUitl对象
		final DownUtil downUtil=new DownUtil("https://www.baidu.com/img/bd_logo1.png","D:\\bd_logo1.png",4);
		//开始下载
		downUtil.download();
		new Thread(()->{
			while(downUtil.getCompleteRate()<1)
			{
				//每隔一秒查询一次任务完成进度
				System.out.println("已完成"+downUtil.getCompleteRate());
				try{
					Thread.sleep(1000);
				}catch(Exception e)
				{
					e.printStackTrace();
				}
			}
		}
		).start();
	}

}