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UI
time = 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;
}
@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);
}
}
参考文章：http://blog.ncmem.com/wordpress/2023/10/18/http%e6%96%87%e4%bb%b6%e6%96%ad%e7%82%b9%e7%bb%ad%e4%bc%a0%e5%8e%9f%e7%90%86%e8%a7%a3%e6%9e%90%e6%ba%90%e7%a0%81/
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