HTTP文件断点续传原理解析

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：

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/**
 * 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。

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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。

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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。

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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

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//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

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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

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//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

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

DownloadThread类源码：

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/**
 * 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;
    }
    @Override
    public 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intent
                is = 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Activity
                    progress += len;
                    if (System.currentTimeMillis() - time > 1000) {//超过一秒，就刷新UI
                        time = 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);
    }
}

三、源码地址

如果你觉得还不错，欢迎star或fork。
https://github.com/awenzeng/BreakPointDemo

四、参考文献

RandomAccessFiley详解

http断点续传原理：http头 Range、Content-Range

InputStream中read()与read(byte[] b)

赏

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