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;}@Overridepublic 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 blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.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");}}}}}