HTTP文件断点续传原理解析(源码)

生活中，有许多事物，在没有被揭开面纱之前，我们往往会觉得很神秘很高深，认为它一定很难，进而望而却步，失去了解它的机会。然而，很多事，只要我们自己能沉下心来，细细研究，那些神秘高深的，也会变得简单明了。"HTTP文件断点续传"就是这样一个好例子，深入了解背后之理，“HTTP文件断点续传原理”其实很简单。

一、什么是断点续传

1.定义：

可以从下载或上传断开点继续开始传输，就叫断点续传。

2.核心实现原理：

i.RandomAccessFile(文件任意位置保存)
方法seek():可以移动到保存文件任意位置，在该位置发生下一个读取或写入操作

ii.HttpURLConnection.setRequestProperty()(任意位置请求返回剩余文件)

HttpURLConnection.setRequestProperty(“Range”, “bytes=” + start + “-” + end)

二、实例分析

流程图

实现步骤

1.建立数据库：保存文件下载信息
2.下载服务类(DownloadService)
3.两个线程：文件信息线程(FileInfoThread)和文件下载线程(DownloadThread)
4.广播(BroadcastReceiver)：UI进度更新

1.建立数据库
按常规数据库建立方法，具体(略)。数据保存信息为：

/*** 下载信息类*/
public class DownloadInfo {private int id;private String url;//下载链接private long start;//开始大小private long end;//最终大小private long progress;//下载进度
}

2.下载服务类
利用service多次启动只调用onStartCommand()方法，处理开始或暂停下载逻辑。

    public int onStartCommand(Intent intent, int flags, int startId) {if (intent.getAction().equals(ACTION_START)) {FileInfo fileInfo = (FileInfo) intent.getSerializableExtra(TAG_FILEINFO);mFileInfoThread = new FileInfoThread(fileInfo,mHandler);mFileInfoThread.start();} else if (intent.getAction().equals(ACTION_PAUSE)) {if (mDownloadThread != null) {mDownloadThread.setPause(true);}}return super.onStartCommand(intent, flags, startId);}

3.两个线程
i.文件信息线程(FileInfoThread)

通过网络获取下载文件大小，并建立对应大小的保存文件路径。

 HttpURLConnection conn = null;RandomAccessFile raf = null;try {URL url = new URL(mFileInfo.getUrl());conn = (HttpURLConnection) url.openConnection();//连接网络文件conn.setConnectTimeout(3000);conn.setRequestMethod("GET");int length = -1;Log.e(TAG,"HttpResponseCode=="+ conn.getResponseCode() + "");if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {//获取文件长度length = conn.getContentLength();}if (length < 0) {return;}File dir = new File(DownloadService.DOWNLOAD_PATH);if (!dir.exists()) {dir.mkdir();}//在本地创建文件File file = new File(dir, mFileInfo.getFileName());raf = new RandomAccessFile(file, "rwd");//设置本地文件长度raf.setLength(length);mFileInfo.setLength(length);Log.e(TAG,"下载文件大小(size)"+ mFileInfo.getLength() + "");mHandler.obtainMessage(DownloadService.MSG_FILEINFO, mFileInfo).sendToTarget();} catch (Exception e) {e.printStackTrace();} finally {try {if (conn != null && raf != null) {raf.close();conn.disconnect();}} catch (IOException e) {e.printStackTrace();}}

ii.文件下载线程(DownloadThread)
断点续传原理核心类。
1.判断下载进度是否有保存，若无，数据插入一条数据。

        if (!mDatabaseOperation.isExists(downloadInfo.getUrl(), downloadInfo.getId())) {mDatabaseOperation.insert(downloadInfo);}

2.设置网络请求Range参数，从请求位置返回数据

   //设置下载位置long start = downloadInfo.getStart() + downloadInfo.getProgress();connection.setRequestProperty("Range", "bytes=" + start + "-" + downloadInfo.getEnd());

3.通过RandomAccessFile从进度保存位置保存文件

  RandomAccessFile raf;File file = new File(DownloadService.DOWNLOAD_PATH, mFileInfo.getFileName());raf = new RandomAccessFile(file, "rwd");raf.seek(start);...//写入文件raf.write(buffer, 0, len);

4.用户暂停时，保存下载进度

 //下载暂停时，保存进度if (isPause) {Log.e(TAG,"保存进度文件(size):"+progress + "");mDatabaseOperation.update(mFileInfo.getUrl(), mFileInfo.getId(), progress);return;}

4.广播(BroadcastReceiver)：
每秒广播一次，刷新UI

 long time = System.currentTimeMillis();......if (System.currentTimeMillis() - time > 1000) {//超过一秒，就刷新UItime = System.currentTimeMillis();sendBroadcast(intent,(int)(progress * 100 / mFileInfo.getLength()));Log.e(TAG,"进度：" + progress * 100 / mFileInfo.getLength() + "%");}

DownloadThread类源码：

/*** 文件下载线程* Created by AwenZeng on 2017/9/6.*/public class DownloadThread extends Thread {private DownloadInfo downloadInfo;private FileInfo mFileInfo;private long progress = 0;private boolean isPause;private DatabaseOperation mDatabaseOperation;private Context mContext;private static final String TAG = "DownloadThread";public DownloadThread(Context context, DatabaseOperation databaseOperation, DownloadInfo threadInfo, FileInfo fileInfo) {this.downloadInfo = threadInfo;mContext = context;mDatabaseOperation = databaseOperation;mFileInfo = fileInfo;}public void setPause(boolean pause) {isPause = pause;}@Overridepublic void run() {if (!mDatabaseOperation.isExists(downloadInfo.getUrl(), downloadInfo.getId())) {mDatabaseOperation.insert(downloadInfo);}HttpURLConnection connection;RandomAccessFile raf;InputStream is;try {URL url = new URL(downloadInfo.getUrl());connection = (HttpURLConnection) url.openConnection();connection.setConnectTimeout(3000);connection.setRequestMethod("GET");//设置下载位置long start = downloadInfo.getStart() + downloadInfo.getProgress();connection.setRequestProperty("Range", "bytes=" + start + "-" + downloadInfo.getEnd());//设置文件写入位置File file = new File(DownloadService.DOWNLOAD_PATH, mFileInfo.getFileName());raf = new RandomAccessFile(file, "rwd");raf.seek(start);progress += downloadInfo.getProgress();Log.e(TAG,"下载文件进度(size)："+ downloadInfo.getProgress() + "");Log.e(TAG,"HttpResponseCode ==="+connection.getResponseCode() + "");//开始下载if (connection.getResponseCode() == HttpURLConnection.HTTP_PARTIAL) {Log.e(TAG,"剩余文件(size):"+connection.getContentLength() + "");Intent intent = new Intent(DownloadService.ACTION_UPDATE);//广播intentis = connection.getInputStream();byte[] buffer = new byte[1024 * 4];int len = -1;long time = System.currentTimeMillis();while ((len = is.read(buffer)) != -1) {//下载暂停时，保存进度if (isPause) {Log.e(TAG,"保存进度文件(size):"+progress + "");mDatabaseOperation.update(mFileInfo.getUrl(), mFileInfo.getId(), progress);return;}//写入文件raf.write(buffer, 0, len);//把下载进度发送广播给Activityprogress += len;if (System.currentTimeMillis() - time > 1000) {//超过一秒，就刷新UItime = System.currentTimeMillis();sendBroadcast(intent,(int)(progress * 100 / mFileInfo.getLength()));Log.e(TAG,"进度：" + progress * 100 / mFileInfo.getLength() + "%");}}sendBroadcast(intent,100);/***  删除下载信息（重新下载）*/mDatabaseOperation.delete(mFileInfo.getUrl(), mFileInfo.getId());is.close();}raf.close();connection.disconnect();} catch (Exception e) {e.printStackTrace();}}private void sendBroadcast(Intent intent,int progress){intent.putExtra("progress",progress);mContext.sendBroadcast(intent);}
}