最近在安卓4.4上遇到一个断开wifi后重新连接wifi， downloadProvider继续下载文件失败的问题。于是开始了解下载管理模块的断点续载功能：
1、首先，分析android log, 当将网络断开之后，下载会中止，出现如下信息：
W/DownloadManager(29473): Aborting request for download 5: Failed reading response: java.net.SocketException: recvfrom failed: ETIMEDOUT (Connection timed out)
I/DownloadManager(29473): Download 5 finished with status WAITING_FOR_NETWORK
在代码中搜索Failed reading response， 发现是在下载数据中不断读取网络数据流时抛出的异常：
/**
* Transfer as much data as possible from the HTTP response to the
* destination file.
*/
private void transferData(State state, InputStream in, OutputStream out)
throws StopRequestException {
final byte data[] = new byte[Constants.BUFFER_SIZE];
for (;;) {
int bytesRead = readFromResponse(state, data, in);
if (bytesRead == -1) { // success, end of stream already reached
handleEndOfStream(state);
return;
}
state.mGotData = true;
writeDataToDestination(state, data, bytesRead, out);
state.mCurrentBytes += bytesRead;
reportProgress(state);
checkPausedOrCanceled(state);
}
在循环中不停读取网络那边的响应，当网络断开后，InputStream的读接口应该就会抛出异常，代码中进行捕捉，并且判断之后是否能够断点续载，然后抛出相应信息：
/**
* Read some data from the HTTP response stream, handling I/O errors.
* @param data buffer to use to read data
* @param entityStream stream for reading the HTTP response entity
* @return the number of bytes actually read or -1 if the end of the stream has been reached
*/
private int readFromResponse(State state, byte[] data, InputStream entityStream)
throws StopRequestException {
try {
return entityStream.read(data);
} catch (IOException ex) {
ContentValues values = new ContentValues();
values.put(Downloads.Impl.COLUMN_CURRENT_BYTES, state.mCurrentBytes);
mContext.getContentResolver().update(mInfo.getAllDownloadsUri(), values, null, null);
if (cannotResume(state)) {
throw new StopRequestException(STATUS_CANNOT_RESUME,
"Failed reading response: " + ex + "; unable to resume", ex);
} else {
throw new StopRequestException(STATUS_HTTP_DATA_ERROR,
"Failed reading response: " + ex, ex);
}
}
}
这里的判断是否能够续载，有很多条件， 主要是两个方面，下载字节数是否大于0 或者 是否DRM 下载需要转换：
D/DownloadManager( 9658): state.mCurrentBytes=5257536 state.mHeaderETag=69b8155f8ae29636cec71afb21637c92 mInfo.mNoIntegrity=false state.mMimeType=application/vnd.android.package-archive
导出数据库，查看此时下载管理该文件状态：
这个状态 status = 195 是怎么来的呢？
我们可以继续跟踪代码，前面说了，当网络断开后，代码开始抛出异常StopRequestException， 并且带有错误码，仔细阅读代码，这个异常是各个方法，
一层一层网上抛出，最后达到下载管理线程 DownloadThread 类中的 run中， 它在catch这个异常后，也会打印出log信息，并且增加了处理：
catch (StopRequestException error) {
// remove the cause before printing, in case it contains PII
errorMsg = error.getMessage();
String msg = "Aborting request for download " + mInfo.mId + ": " + errorMsg;
Log.w(Constants.TAG, msg);
if (Constants.LOGV) {
Log.w(Constants.TAG, msg, error);
}
finalStatus = error.getFinalStatus();
从代码中可以看出其增加了下载文件在数据库中存放的Id信息，然后在加上出错新消息，也就我们最终看到的log：
W/DownloadManager(29473): Aborting request for download 5: Failed reading response: java.net.SocketException: recvfrom failed: ETIMEDOUT (Connection timed out)
在输出完信息之后，其会对错误码判断进行处理，想断网这种问题，会有个继续尝试，然后确定最终的错误码。最初抛出异常的错误码是STATUS_HTTP_DATA_ERROR ， 即495.
W/DownloadManager(11584): Aborting request for download 5: Failed reading response: java.net.SocketException: recvfrom failed: ETIMEDOUT (Connection timed out)
D/DownloadManager(11584): -----finalStatus=495
最后经过代码转换：
// Some errors should be retryable, unless we fail too many times.
if (isStatusRetryable(finalStatus)) {
if (state.mGotData) {
numFailed = 1;
} else {
numFailed += 1;
}
if (numFailed it = mActiveNotifs.keySet().iterator();
while (it.hasNext()) {
final String tag = it.next();
if (!clustered.containsKey(tag)) {//没有包含在tag列表中的，需要清除
mNotifManager.cancel(tag, 0);
it.remove();
}
}
log信息， 构建好的tag形式就是type: id, 当然这是已经下载完成的：
D/DownloadManager(32155): =====tag=3:15
D/DownloadManager(32155): =====tag=3:14
D/DownloadManager(32155): =====tag=3:13
D/DownloadManager(32155): =====tag=3:12
D/DownloadManager(32155): =====tag=3:6
D/DownloadManager(32155): =====tag=3:19
D/DownloadManager(32155): =====tag=3:18
D/DownloadManager(32155): =====tag=3:17
D/DownloadManager(32155): =====tag=3:16
D/DownloadManager(32155): =====tag=3:20
D/DownloadManager(32155): =====tag=3:11
D/DownloadManager(32155): =====tag=3:10
D/DownloadManager(32155): =====tag=3:21
D/DownloadManager(32155): =====tag=1:com.android.browser
D/DownloadManager(32155): =====remove tag=1:com.android.browser
还有就是那种执行过一键清理后，那种更新信息也不会再显示在通知栏中了，因为其tag为null, 也已经不包含在tag列表中了。