用过Android手机的同学都知道，每次开机的时候系统会先扫描sdcard，sdcard重新插拔(挂载)也会扫描一次sdcard。
为什么要扫描sdcard，其实是为了给系统的其他应用提供便利，比如，Gallary、Music、VideoPlayer等应用，进入Gallary后会显示sdcard中的所有图片，
如果进入Gallary后再去扫描，可想而知，你会厌恶这个应用，因为我们会觉得它反应太慢了。还有Music你看到播放列表的时候实际能看到这首歌曲的时长、演唱者、专辑
等信息，这个也不是你进入应用后一下子可以读出来的。
所以Android使用sdcard挂载后扫描的机制，先将这些媒体相关的信息扫描出来保存在数据库中，当打开应用的时候直接去数据库读取(或者所通过MediaProvider去从数据库读取)并show给用户，这样用户体验会好很多，下面我们分析这种扫描机制是如何实现的。
在源码目录的packagesprovidersMediaProvider下面是MediaProvider的源码，它就是完成扫描并将数据保存于数据库中的程序。
先看下它的AndroidManifest.xml文件
application android:process="android.process.media"，也就是应用程序名称为android.process.media，我们用adb 连接到android 设备，并且进入shell后输入ps可以看到的确有应用程序app_4 2796 2075 165192 19420 ffffffff 6fd0eb58 S android.process.media 在运行。另外此程序中有三个部分，分别是provider - MediaProvider 、receiver - MediaScannerReceiver、service - MediaScannerService，它并没有activity，说明它是一直运行于后台的程序。并且从receiver中的
<intent-filter>
<action android:name="android.intent.action.BOOT_COMPLETED" />
</intent-filter>
可以看出它是开机自启动的。下面从这个广播开始看代码。
public void onReceive(Context context, Intent intent) {
String action = intent.getAction();
Uri uri = intent.getData();
String externalStoragePath = Environment.getExternalStorageDirectory().getPath();
if (action.equals(Intent.ACTION_BOOT_COMPLETED)) {
// scan internal storage
scan(context, MediaProvider.INTERNAL_VOLUME);
} else {
。 。 。
}
}
}
收到开机广播后，首先执行scan函数STEP 1，
private void scan(Context context, String volume) {
Bundle args = new Bundle();
args.putString("volume", volume);
context.startService(
new Intent(context, MediaScannerService.class).putExtras(args));
}
scan函数主要传进来一个volume卷名，MediaProvider.INTERNAL_VOLUME实际就是内置存储卡"internal"，在此我们首先理解为开机后首先扫描内置存储卡。
然后启动services MediaScannerService，这也是此服务第一次被启动。
service的启动流程就不说了，onCreate肯定是首先被调用的
public void onCreate()
{
PowerManager pm = (PowerManager)getSystemService(Context.POWER_SERVICE);
mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);
// Start up the thread running the service. Note that we create a
// separate thread because the service normally runs in the process's
// main thread, which we don't want to block.
Thread thr = new Thread(null, this, "MediaScannerService");
thr.start();
}
此处前面是申请了一把wake lock ,主要是防止CPU休眠的，然后启动了一个线程实际就是 MediaScannerService自身的线程，它继承自Runnable，下面主要看Run函数
public void run()
{
// reduce priority below other background threads to avoid interfering
// with other services at boot time.
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND +
Process.THREAD_PRIORITY_LESS_FAVORABLE);
Looper.prepare();
mServiceLooper = Looper.myLooper();
mServiceHandler = new ServiceHandler();
Looper.loop();
}
可以看出此线程的目的是为了处理hander消息ServiceHandler
执行完onCreate后就会执行onStartCommand
public int onStartCommand(Intent intent, int flags, int startId)
{
while (mServiceHandler == null) {
synchronized (this) {
try {
wait(100);
} catch (InterruptedException e) {
}
}
}
if (intent == null) {
Log.e(TAG, "Intent is null in onStartCommand: ",
new NullPointerException());
return Service.START_NOT_STICKY;
}
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent.getExtras();
mServiceHandler.sendMessage(msg);
// Try again later if we are killed before we can finish scanning.
return Service.START_REDELIVER_INTENT;
}
在这里我们通过STEP 1中传入进来的volume字符串就作为了msg.obj通过handler来处理了
private final class ServiceHandler extends Handler
{
@Override
public void handleMessage(Message msg)
{
Bundle arguments = (Bundle) msg.obj;
String filePath = arguments.getString("filepath");
String folder = arguments.getString("folder");
try {
if (filePath != null) {
IBinder binder = arguments.getIBinder("listener");
IMediaScannerListener listener =
(binder == null ? null : IMediaScannerListener.Stub.asInterface(binder));
Uri uri = scanFile(filePath, arguments.getString("mimetype"));
if (listener != null) {
listener.scanCompleted(filePath, uri);
}
} else if(folder != null) {
String volume = arguments.getString("volume");
String[] directories = null;
directories = new String[] {
new File(folder).getPath(),
};
if (directories != null) {
if (Config.LOGD) Log.d(TAG, "start scanning volume " + volume + " ; path = " + folder);
scan(directories, volume);
if (Config.LOGD) Log.d(TAG, "done scanning volume " + volume + " ; path = " + folder);
}
}else {
String volume = arguments.getString("volume");
String[] directories = null;
if (MediaProvider.INTERNAL_VOLUME.equals(volume)) {
// scan internal media storage
directories = new String[] {
Environment.getRootDirectory() + "/media",
};
}
else if (MediaProvider.EXTERNAL_VOLUME.equals(volume)) {
// scan external storage
directories = new String[] {
Environment.getExternalStorageDirectory().getPath(),
};
}
if (directories != null) {
if (Config.LOGD) Log.d(TAG, "start scanning volume " + volume);
scan(directories, volume);
if (Config.LOGD) Log.d(TAG, "done scanning volume " + volume);
}
}
} catch (Exception e) {
Log.e(TAG, "Exception in handleMessage", e);
}
stopSelf(msg.arg1);
}
};
}
很显然我们会执行到标记为红色的else中，我们是先扫描内置sdcard，很显然directories的值为/system/media ,然后调用 scan(directories, volume);函数，应该是内置sdcard中所有的媒体文件都几种存储在/system/media下面所以只需要扫描这一个路径就行了。STEP2
private void scan(String[] directories, String volumeName) {
// don't sleep while scanning
mWakeLock.acquire();
ContentValues values = new ContentValues();
values.put(MediaStore.MEDIA_SCANNER_VOLUME, volumeName);
Uri scanUri = getContentResolver().insert(MediaStore.getMediaScannerUri(), values);
Uri uri = Uri.parse("file://" + directories[0]);
sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_STARTED, uri));
try {
if (volumeName.equals(MediaProvider.EXTERNAL_VOLUME)) {
openDatabase(volumeName);
}
MediaScanner scanner = createMediaScanner();
scanner.scanDirectories(directories, volumeName);
} catch (Exception e) {
Log.e(TAG, "exception in MediaScanner.scan()", e);
}
getContentResolver().delete(scanUri, null, null);
sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_FINISHED, uri));
mWakeLock.release();
}
开始扫描和结束扫描时都会发送一个全局的广播，第三方应用程序也可以通过注册这两个广播来避开在media 扫描的时候往改扫描文件夹里面写入或删除文件，这个我在项目中就遇到过这种bug。在这一步骤中创建了MediaScanner并调用它的scanDirectories方法 STEP3
public void scanDirectories(String[] directories, String volumeName) {
try {
long start = System.currentTimeMillis();
initialize(volumeName);
prescan(null);
long prescan = System.currentTimeMillis();
for (int i = 0; i < directories.length; i++) {
processDirectory(directories[i], MediaFile.sFileExtensions, mClient);
}
long scan = System.currentTimeMillis();
postscan(directories);
long end = System.currentTimeMillis();
if (Config.LOGD) {
Log.d(TAG, " prescan time: " + (prescan - start) + "ms
");
Log.d(TAG, " scan time: " + (scan - prescan) + "ms
");
Log.d(TAG, "postscan time: " + (end - scan) + "ms
");
Log.d(TAG, " total time: " + (end - start) + "ms
");
}
} catch (SQLException e) {
// this might happen if the SD card is removed while the media scanner is running
Log.e(TAG, "SQLException in MediaScanner.scan()", e);
} catch (UnsupportedOperationException e) {
// this might happen if the SD card is removed while the media scanner is running
Log.e(TAG, "UnsupportedOperationException in MediaScanner.scan()", e);
} catch (RemoteException e) {
Log.e(TAG, "RemoteException in MediaScanner.scan()", e);
}
}
其中initialize prescan processDirectory postscan这四个函数都比较重要。STEP 4
private void initialize(String volumeName) {
mMediaProvider = mContext.getContentResolver().acquireProvider("media");
mAudioUri = Audio.Media.getContentUri(volumeName);
mVideoUri = Video.Media.getContentUri(volumeName);
mImagesUri = Images.Media.getContentUri(volumeName);
mThumbsUri = Images.Thumbnails.getContentUri(volumeName);
if (!volumeName.equals("internal")) {
// we only support playlists on external media
mProcessPlaylists = true;
mProcessGenres = true;
mGenreCache = new HashMap<String, Uri>();
mGenresUri = Genres.getContentUri(volumeName);
mPlaylistsUri = Playlists.getContentUri(volumeName);
// assuming external storage is FAT (case insensitive), except on the simulator.
if ( Process.supportsProcesses()) {
mCaseInsensitivePaths = true;
}
}
}
做一些初始化的动作，得到MediaProvider和一些URI实际也就是操作数据库的一些表名。Audio.Media.getContentUri可以在MediaStore.java中找到，此类保存了所有的媒体格式URI等信息，此处获得的mAudioUri的值为“content://media/internal//audio/media"
STEP5
private void prescan(String filePath) throws RemoteException {
。 。 。
}
此函数比较长，在此省略代码，有兴趣的可以看源码，这里所做的操作是对于之前有扫描过的，就将数据库中现有的媒体信息放到几个数据结构中临时存储起来。
然后最重要的STEP 6 processDirectory是一个native函数，先注意几个传入参数directories[i]为STEP2中传入的路径/system/media ,MediaFile.sFileExtensions 这个你可以跟到MediaFile中看看这个是如何赋值的，实际就是所有支持的媒体格式后缀以‘，’的方式串在一起的字符串”MP3，M4A，3GA，WAV。。。“最重要的mClient是MyMediaScannerClient的一个实例，此对象将是native层回调函数的接口，所有扫描完后的媒体都会通过此对象来存储到数据库中。
下面进入Native层对应文件是android_media_MediaScanner.cpp STEP 6
static void
android_media_MediaScanner_processDirectory(JNIEnv *env, jobject thiz, jstring path, jstring extensions, jobject client)
{
MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);
if (path == NULL) {
jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
return;
}
if (extensions == NULL) {
jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
return;
}
const char *pathStr = env->GetStringUTFChars(path, NULL);
if (pathStr == NULL) { // Out of memory
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}
const char *extensionsStr = env->GetStringUTFChars(extensions, NULL);
if (extensionsStr == NULL) { // Out of memory
env->ReleaseStringUTFChars(path, pathStr);
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}
MyMediaScannerClient myClient(env, client);
mp->processDirectory(pathStr, extensionsStr, myClient, ExceptionCheck, env);
env->ReleaseStringUTFChars(path, pathStr);
env->ReleaseStringUTFChars(extensions, extensionsStr);
}
这里比较重要的一个点时MyMediaScannerClient myClient(env, client);定义了一个客户端，并将java层的client传入进去，很显然，是想通过MyMediaScannerClient 再来回调client。
STEP 7
status_t MediaScanner::processDirectory(
const char *path, const char *extensions,
MediaScannerClient &client,
ExceptionCheck exceptionCheck, void *exceptionEnv) {
int pathLength = strlen(path);
if (pathLength >= PATH_MAX) {
return UNKNOWN_ERROR;
}
char* pathBuffer = (char *)malloc(PATH_MAX + 1);
if (!pathBuffer) {
return UNKNOWN_ERROR;
}
int pathRemaining = PATH_MAX - pathLength;
strcpy(pathBuffer, path);
if (pathLength > 0 && pathBuffer[pathLength - 1] != '/') {
pathBuffer[pathLength] = '/';
pathBuffer[pathLength + 1] = 0;
--pathRemaining;
}
client.setLocale(locale());
status_t result =
doProcessDirectory(
pathBuffer, pathRemaining, extensions, client,
exceptionCheck, exceptionEnv);
free(pathBuffer);
return result;
}
此函数没干什么事，具体工作是在doProcessDirectory中做的 STEP 8
status_t MediaScanner::doProcessDirectory(
char *path, int pathRemaining, const char *extensions,
MediaScannerClient &client, ExceptionCheck exceptionCheck,
void *exceptionEnv) {
. . .
}
此函数太长，在此不粘出来了，这里首先要解释下这些参数，path - 要扫描文件夹路径以'/'结尾，pathRemaining为路径长度与路径最大长度之间的差值，也就是防止扫描时路径超出范围,extensions 前面已经解释过是后缀，client是是STEP6中实例化的MyMediaScannerClient对象，后面两个参数是一些异常处理不用关心。
大家仔细看这个函数的代码就可以知道，它完成的是遍历文件夹并找到有相应extensions 里面后缀的文件fileMatchesExtension(path, extensions)，如果文件大小大于0就调用client.scanFile(path, statbuf.st_mtime, statbuf.st_size);来进行文件读取扫描 注意这里才会读文件的实际内容。
STEP 9
virtual bool scanFile(const char* path, long long lastModified, long long fileSize)
{
jstring pathStr;
if ((pathStr = mEnv->NewStringUTF(path)) == NULL) return false;
mEnv->CallVoidMethod(mClient, mScanFileMethodID, pathStr, lastModified, fileSize);
mEnv->DeleteLocalRef(pathStr);
return (!mEnv->ExceptionCheck());
}
看看，终于用到了mClient,java层传进来的client ,这就是回调到了java 类MyMediaScannerClient里面的STEP 10
public void scanFile(String path, long lastModified, long fileSize) {
// This is the callback funtion from native codes.
// Log.v(TAG, "scanFile: "+path);
doScanFile(path, null, lastModified, fileSize, false);
}
主要看doScanFile STEP 11
public Uri doScanFile(String path, String mimeType, long lastModified, long fileSize, boolean scanAlways) {
Uri result = null;
// long t1 = System.currentTimeMillis();
try {
FileCacheEntry entry = beginFile(path, mimeType, lastModified, fileSize);
// rescan for metadata if file was modified since last scan
if (entry != null && (entry.mLastModifiedChanged || scanAlways)) {
String lowpath = path.toLowerCase();
boolean ringtones = (lowpath.indexOf(RINGTONES_DIR) > 0);
boolean notifications = (lowpath.indexOf(NOTIFICATIONS_DIR) > 0);
boolean alarms = (lowpath.indexOf(ALARMS_DIR) > 0);
boolean podcasts = (lowpath.indexOf(PODCAST_DIR) > 0);
boolean music = (lowpath.indexOf(MUSIC_DIR) > 0) ||
(!ringtones && !notifications && !alarms && !podcasts);
if (!MediaFile.isImageFileType(mFileType)) {
processFile(path, mimeType, this);
}
result = endFile(entry, ringtones, notifications, alarms, music, podcasts);
}
} catch (RemoteException e) {
Log.e(TAG, "RemoteException in MediaScanner.scanFile()", e);
}
// long t2 = System.currentTimeMillis();
// Log.v(TAG, "scanFile: " + path + " took " + (t2-t1));
return result;
}
此函数里面又有三个比较重要的函数beginFile processFile endFile
先看beginFile STEP 12
public FileCacheEntry beginFile(String path, String mimeType, long lastModified, long fileSize) {
. . .
}
构建一个FileCacheEntry对象，存储文件的一些基本信息，并且放入mFileCache HashMap中。
根据此文件是否修改来觉得是否processFile ,又进入到native中
在此插入一段代码
static void
android_media_MediaScanner_native_setup(JNIEnv *env, jobject thiz)
{
MediaScanner *mp = NULL;
char value[PROPERTY_VALUE_MAX];
if (property_get("media.framework.option", value, NULL) && (!strcmp(value, "1"))){
#ifndef NO_OPENCORE
mp = new PVMediaScanner();
#else
mp = new StagefrightMediaScanner;
#endif
}else{
mp = new StagefrightMediaScanner;
}
if (mp == NULL) {
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}
env->SetIntField(thiz, fields.context, (int)mp);
}
android2.2以上mediascanner使用StagefrightMediaScanner
STEP 13
static void
android_media_MediaScanner_processFile(JNIEnv *env, jobject thiz, jstring path, jstring mimeType, jobject client)
{
MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);
if (path == NULL) {
jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
return;
}
const char *pathStr = env->GetStringUTFChars(path, NULL);
if (pathStr == NULL) { // Out of memory
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}
const char *mimeTypeStr = (mimeType ? env->GetStringUTFChars(mimeType, NULL) : NULL);
if (mimeType && mimeTypeStr == NULL) { // Out of memory
env->ReleaseStringUTFChars(path, pathStr);
jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
return;
}
MyMediaScannerClient myClient(env, client);
mp->processFile(pathStr, mimeTypeStr, myClient);
env->ReleaseStringUTFChars(path, pathStr);
if (mimeType) {
env->ReleaseStringUTFChars(mimeType, mimeTypeStr);
}
}
不再累述，直接进入 STEP 14
status_t StagefrightMediaScanner::processFile(
const char *path, const char *mimeType,
MediaScannerClient &client) {
. . .
}
由于StagefrightMediaScanner又进入到了stagefright 框架，比较复杂，鉴于篇幅限制，在下一篇blog中继续分析STEP 14