java多线程下载网络资源

在java中，我们可以通过URL和HttpURLConnection实现对资源的多线程下载，以此来减少资源的下载时间，下面是一个实现多线程下载的例子，不足的是没实现断点下载的功能

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


public class DownLoadTool{

	public void resourceDownLoad(String path) throws Exception {
		//网络资源路径
		//如path = "http://www.hackvip.com/desk/d/0/3/2006110911245141091.jpg";
		URL url = new URL(path);
		HttpURLConnection conn = (HttpURLConnection)url.openConnection();
		conn.setConnectTimeout(6*1000);
		conn.setRequestMethod("GET");
		//获得网络上文件的大小
		int fileSize = conn.getContentLength();
		//线程的数量
		int threadNum = 4;
		//我用三个线程对文件进行下载，每个线程下载量为
		int block = fileSize/threadNum + 1;
		conn.disconnect();
		//保存在项目的根目录下,如我下载的是一张图片
		File file = new File("a.jpg");
		RandomAccessFile rfile = new RandomAccessFile(file,"rw");
		rfile.setLength(fileSize);
		rfile.close();
		for (int i = 0; i < threadNum; i++) {
			int startPosition = i*block;
			RandomAccessFile randomFile = new RandomAccessFile(file,"rw");
			//从文件的什么位置开始写入
			randomFile.seek(startPosition);
			new DownLoadThread(i,url,randomFile,startPosition,block).start();
		}
		byte[] b = new byte[1];
		System.in.read(b);
		while(!(b[0]=='q')){
			Thread.sleep(5*1000);
		}

	}

	private class DownLoadThread extends Thread{
		private URL url = null;
		private RandomAccessFile rfile = null;
		private int startPosition = 0;
		private int block = 0;
		private int i = 0;
		public DownLoadThread(int i,URL url, RandomAccessFile rfile,
				int startPosition, int block) {
			this.i = i;
			this.url = url;
			this.rfile = rfile;
			this.startPosition = startPosition;
			this.block = block;
		}

		@Override
		public void run() {
			try {
				HttpURLConnection conn = (HttpURLConnection)url.openConnection();
				conn.setRequestMethod("GET");
				conn.setRequestProperty("Range", "bytes" + startPosition + "-");
				conn.setConnectTimeout(6*1000);
				InputStream is = conn.getInputStream();
				byte[] buffer = new byte[1024];
				int len = -1;
				int readSize = 0;
				while(readSize<block && ((len=is.read(buffer))!=-1)){
					rfile.write(buffer, 0, len);
					readSize += len;
				}
				rfile.close();
				is.close();
				conn.disconnect();
				System.out.println("第" + (++i) + "个线程下载完成");
			} catch (IOException e) {
				e.printStackTrace();
			}

		}
	}

}

编写一个测试类，传入一个资源路径，当然前提是该资源网站没有设置防盗链

import org.junit.Test;

public class DownLoadTest {

	@Test
	public void test() {
		String path = "http://www.hackvip.com/desk/d/0/3/2006110911245141091.jpg";
		try {
			new DownLoadTool().resourceDownLoad(path);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}