多线程断点下载文件

所谓多线程断点下载 ：就是当某个文件下了一部分后突然断电了，或者下载程序暂停了，再次运行下载 可以继续下载，而不是重新下载 的一个执行过程。

基本模拟了迅雷下载的原理

本次功能用java实现 ：

利用txt文件记录下载的状态  如果在安卓端的话 可以改成用数据库记录下载状态

本程序可以扩展为具有良好用户体验的 下载程序

package mutiDownload;

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

/**
 * 多线程断点下载文件
 * 
 * @author Administrator
 * 
 */
public class BreakpointDoloadDemo {

	public  static int threadcount = 3;
	public  static int threadRunning = 3;
	
	
	
	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub

		// 获取服务器上的文件长度，在本地创建一个和服务器上的文件一样大的临时文件
		String pathString = "http://192.168.1.84:8091/androidServer/test.mkv";
		URL url = new URL(pathString);
		HttpURLConnection connection = (HttpURLConnection) url.openConnection();
		connection.setConnectTimeout(5000);
		connection.setRequestMethod("GET");
		int code = connection.getResponseCode();
		if (code == 200) { // 请求成功
			int length = connection.getContentLength();
			System.out.println("文件长度:" + length);
			RandomAccessFile ref = new RandomAccessFile("test.mkv", "rwd");
			// 指定创建的临时文件的长度
			ref.setLength(length);
			ref.close();

			int blocksize = length / threadcount;
			for (int threadId = 1; threadId <= threadcount; threadId++) {
				int startIndex = (threadId - 1) * blocksize; // 线程开始位置
				int endIndex = threadId * blocksize - 1;
				if (threadId == threadcount) {
					endIndex = length;
				}
				System.out.println("线程" + threadId + "下载---" + startIndex
						+ "-->" + endIndex);
				new Downhread(threadId, startIndex, endIndex, pathString)
						.start();
				;

			}
		} else {
			System.out.println("服务器错误");
		}

	}

	/**
	 * 下载文件子线程
	 * 
	 * @author Administrator
	 * 
	 */
	public static class Downhread extends Thread {
		private int threadId;
		private int startIndex;
		private int endIndex;
		private String path;

		/**
		 * 
		 * @param threadId
		 *            线程id
		 * @param startIndex
		 *            线程下载的开始位置
		 * @param endIndex
		 * @param path
		 *            下载文件在服务器上的位置
		 */
		public Downhread(int threadId, int startIndex, int endIndex, String path) {
			super();
			this.threadId = threadId;
			this.startIndex = startIndex;
			this.endIndex = endIndex;
			this.path = path;
		}

		@Override
		public void run() {
			// TODO Auto-generated method stub
			// super.run();
			try {
				
				//检查是否存在记录下载长度的文件,如果存在读取文件的数据
				File tempFile=new File(threadId+".txt");
				if(tempFile.exists()&&tempFile.length()>0){
					FileInputStream fis=new FileInputStream(tempFile);
					byte [] temp=new byte[1024];
					int leng=fis.read(temp);
					String downloadLen=new String(temp,0,leng);
					int downloadInt=Integer.parseInt(downloadLen);
					startIndex=downloadInt;	//修改下载的真实开始位置
				}
				
				
				
				
				URL url = new URL(path);
				HttpURLConnection connection = (HttpURLConnection) url
						.openConnection();
				connection.setConnectTimeout(5000);
				connection.setRequestMethod("GET");
				connection.setRequestProperty("Range", "bytes=" + startIndex
						+ "-" + endIndex);
				
				System.out.println("线程真实下载位置:" + threadId + "startIndex:" + startIndex+"-->"+endIndex);
				int code = connection.getResponseCode();
				System.out.println("code:" + code);
				// if(code==200){ //请求成功
				//
				// }
				InputStream is = connection.getInputStream();
				RandomAccessFile raf = new RandomAccessFile("test.mkv", "rwd");
				// 写文件的时候指定从哪个位置开始写
				raf.seek(startIndex);

				int len = 0;
				byte[] buffer = new byte[1024];
				int total = 0; // 已经下载的数据长度
				// File file=new File(threadId+".txt"); //作用：记录当前线程下载的数据长度

				while ((len = is.read(buffer)) != -1) {
					RandomAccessFile file = new RandomAccessFile(threadId
							+ ".txt", "rwd");
					raf.write(buffer, 0, len);
					total += len;
					//System.out.println("线程:" + threadId + "total:" + total);
					file.write(String.valueOf(total+startIndex).getBytes());	//记录下载的位置
					file.close();
				}
				is.close();
				raf.close();
				System.out.println("线程:" + threadId + "下载完毕了...");
				
//			    File delfile =new File(threadId+ ".txt");
//			    delfile.delete();	//当线程下载完毕后清除记录文件
			    
			} catch (MalformedURLException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}finally{
				threadRunning--;
				
				if(threadRunning==0){	//所有线程执行完毕
					for(int i=0;i<=3;i++){
						 File file =new File(i+ ".txt");
						 file.delete();
					}
					System.out.println("文件下载完毕，删除所有记录文件");
				}
			}
		}
	}

}