- 深入解析Android 虚拟机
- 钟世礼
- 1749字
- 2020-06-28 05:36:03
4.2 分析Java层
在MediaScanner系统中，JNI的调用关系为：
MediaScanner -------------libmedia_jni.so -------------libmedia.so
在Android系统中，MediaScanner的功能是扫描媒体文件，得到诸如歌曲时长、歌曲作者等信息，并将这些信息存放到媒体数据库中，以供其他应用程序使用。
在JNI应用中，Java层MediaScanner的实现文件为：
/frameworks/base/media/java/android/media/MediaScanner.java
在本节的内容中，将详细讲解MediaScanner系统中Java层的具体实现过程。
4.2.1 加载JNl库
在文件MediaScanner.java中，首先定义类MediaScanner并加载JNI库，然后定义JNI的Native（本地）函数。主要代码如下所示：
public class MediaScanner
{
static {
System.loadLibrary("media_jni");
native_init();
}
private final static String TAG = "MediaScanner";
private static final String[] FILES_PRESCAN_PROJECTION = new String[] {
Files.FileColumns._ID,//0
Files.FileColumns.DATA,//1
Files.FileColumns.FORMAT,//2
Files.FileColumns.DATE_MODIFIED,//3
};
private static final String[] ID_PROJECTION = new String[] {
Files.FileColumns._ID,
};
private static final int FILES_PRESCAN_ID_COLUMN_INDEX = 0;
private static final int FILES_PRESCAN_PATH_COLUMN_INDEX = 1;
private static final int FILES_PRESCAN_FORMAT_COLUMN_INDEX = 2;
private static final int FILES_PRESCAN_DATE_MODIFIED_COLUMN_INDEX = 3;
private static final String[] PLAYLIST_MEMBERS_PROJECTION = new String[] {
Audio.Playlists.Members.PLAYLIST_ID,//0
};
private static final int ID_PLAYLISTS_COLUMN_INDEX = 0;
private static final int PATH_PLAYLISTS_COLUMN_INDEX = 1;
private static final int DATE_MODIFIED_PLAYLISTS_COLUMN_INDEX = 2;
private static final String RINGTONES_DIR = "/ringtones/";
private static final String NOTIFICATIONS_DIR = "/notifications/";
private static final String ALARMS_DIR = "/alarms/";
private static final String MUSIC_DIR = "/music/";
private static final String PODCAST_DIR = "/podcasts/";
……
private static native final void native_init(); //声明一个native函数，native为关键字
private native final void native_setup();
……
}
函数native_init位于包android.media中，其完整路径名为：
android.media.MediaScanner.nantive_init
根据规则其对应的JNI层函数名称为：
android_media_MediaScanner_native_init
在调用函数native之前需要先加载JNI库，一般在类的static中加载调用函数System.loadLibrary()。在加载了相应的JNI库之后，如果要使用相应的native函数，只需使用native声明需要被调用的函数即可：
private native void processDirectory(String path, String extensions, MediaScannerClient client);
private native void processFile(String path, String mimeType, MediaScannerClient client);
public native void setLocale(String locale);
4.2.2 实现扫描工作
在文件MediaScanner.java中，通过函数scanDirectories实现扫描工作，具体实现代码如下所示：
public void scanDirectories(String[] directories, String volumeName) {
try {
long start = System.currentTimeMillis();
initialize(volumeName); //初始化
prescan(null, true); //扫描前的预处理
long prescan = System.currentTimeMillis();
if (ENABLE_BULK_INSERTS) {
//create MediaInserter for bulk inserts
mMediaInserter = new MediaInserter(mMediaProvider, 500);
}
//函数processDirectory是一个Native函数，功能是对目标文件夹进行扫描
for (int i = 0; i < directories.length; i++) {
processDirectory(directories[i], mClient);
}
if (ENABLE_BULK_INSERTS) {
//flush remaining inserts
mMediaInserter.flushAll();
mMediaInserter = null;
}
long scan = System.currentTimeMillis();
postscan(directories); //扫描后的处理
long end = System.currentTimeMillis();
if (false) {
Log.d(TAG, " prescan time: " + (prescan - start) + "ms\n");
Log.d(TAG, " scan time: " + (scan - prescan) + "ms\n");
Log.d(TAG, "postscan time: " + (end - scan) + "ms\n");
Log.d(TAG, " total time: " + (end - start) + "ms\n");
}
} 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);
}
}
在上述代码中用到了函数initialize，此函数的功能是实现初始化操作，具体实现代码如下所示：
private void initialize(String volumeName) {
//打开MediaProvider，获得它的一个实例
mMediaProvider = mContext.getContentResolver().acquireProvider("media");
//得到一些uri
mAudioUri = Audio.Media.getContentUri(volumeName);
mVideoUri = Video.Media.getContentUri(volumeName);
mImagesUri = Images.Media.getContentUri(volumeName);
mThumbsUri = Images.Thumbnails.getContentUri(volumeName);
mFilesUri = Files.getContentUri(volumeName);
//如果需要外部存储的话，则可以支持播放列表，用缓存池实现，例如mGenreCache等
if (! volumeName.equals("internal")) {
//we only support playlists on external media
mProcessPlaylists = true;
mProcessGenres = true;
mPlaylistsUri = Playlists.getContentUri(volumeName);
mCaseInsensitivePaths = true;
}
}
4.2.3 读取并保存信息
在文件MediaScanner.java中，函数prescan的功能是读取之前扫描的数据库中和文件相关的信息并保存起来。此函数创建了一个FileCache，用来缓存扫描文件的一些信息，例如last_modified等。这个FileCache是从MediaProvider中已有信息构建出来的，也就是历史信息。后面根据扫描得到的新信息来对应更新历史信息。函数prescan的具体实现代码如下所示：
private void prescan(String filePath, boolean prescanFiles) throws RemoteException {
Cursor c = null;
String where = null;
String[] selectionArgs = null;
//mPlayLists保存从数据库中获取的信息
if (mPlayLists == null) {
mPlayLists = new ArrayList<FileEntry>();
} else {
mPlayLists.clear();
}
if (filePath ! = null) {
//只有一个文件查询
where = MediaStore.Files.FileColumns._ID + ">? " +
" AND " + Files.FileColumns.DATA + "=? ";
selectionArgs = new String[] { "", filePath };
} else {
where = MediaStore.Files.FileColumns._ID + ">? ";
selectionArgs = new String[] { "" };
}
//告诉提供者不删除文件．
//如果不需要删除文件，则需要避免意外删除这个文件的机制
//这可能在系统未被安装和未安装在扫描仪之前发生
Uri.Builder builder = mFilesUri.buildUpon();
builder.appendQueryParameter(MediaStore.PARAM_DELETE_DATA, "false");
MediaBulkDeleter deleter = new MediaBulkDeleter(mMediaProvider, builder.build());
//根据内容提供者建立文件列表
try {
if (prescanFiles) {
//首先从文件表读到现有文件
//因为可能存在删除不存在文件的情况，所以要小批量的实现数据库查询以避免这个问题
long lastId = Long.MIN_VALUE;
Uri limitUri = mFilesUri.buildUpon().appendQueryParameter("limit", "1000").build();
mWasEmptyPriorToScan = true;
while (true) {
selectionArgs[0] = "" + lastId;
if (c ! = null) {
c.close();
c = null;
}
c = mMediaProvider.query(limitUri, FILES_PRESCAN_PROJECTION,
where, selectionArgs, MediaStore.Files.FileColumns._ID, null);
if (c == null) {
break;
}
int num = c.getCount();
if (num == 0) {
break;
}
mWasEmptyPriorToScan = false;
while (c.moveToNext()) {
long rowId = c.getLong(FILES_PRESCAN_ID_COLUMN_INDEX);
String path = c.getString(FILES_PRESCAN_PATH_COLUMN_INDEX);
int format = c.getInt(FILES_PRESCAN_FORMAT_COLUMN_INDEX);
long lastModified = c.getLong(FILES_PRESCAN_DATE_MODIFIED_COLUMN_INDEX);
lastId = rowId;
//Only consider entries with absolute path names.
//This allows storing URIs in the database without the
//media scanner removing them.
if (path ! = null && path.startsWith("/")) {
boolean exists = false;
try {
exists = Libcore.os.access(path, libcore.io.OsConstants.F_OK);
} catch (ErrnoException e1) {
}
if (! exists && ! MtpConstants.isAbstractObject(format)) {
//do not delete missing playlists, since they may have been
//modified by the user.
//The user can delete them in the media player instead.
//instead, clear the path and lastModified fields in the row
MediaFile.MediaFileType mediaFileType = MediaFile.getFileType(path);
int fileType = (mediaFileType == null ? 0 : mediaFileType.fileType);
if (! MediaFile.isPlayListFileType(fileType)) {
deleter.delete(rowId);
if (path.toLowerCase(Locale.US).endsWith("/.nomedia")) {
deleter.flush();
String parent = new File(path).getParent();
mMediaProvider.call(MediaStore.UNHIDE_CALL, parent, null);
}
}
}
}
}
}
}
}
finally {
if (c ! = null) {
c.close();
}
deleter.flush();
}
//计算图像的原始尺寸
mOriginalCount = 0;
c = mMediaProvider.query(mImagesUri, ID_PROJECTION, null, null, null, null);
if (c ! = null) {
mOriginalCount = c.getCount();
c.close();
}
}
4.2.4 删除SD卡外的信息
在文件MediaScanner.java中，函数postscan的功能是删除不存在于SD卡中的文件信息。函数postscan的具体实现代码如下所示：
private void postscan(String[] directories) throws RemoteException {
//触发播放列表后能够知道对应存储的媒体文件
if (mProcessPlaylists) {
processPlayLists();
}
if (mOriginalCount == 0 && mImagesUri.equals(Images.Media.getContentUri("external")))
pruneDeadThumbnailFiles();
//允许GC清理
mPlayLists = null;
mMediaProvider = null;
}
4.2.5 直接转向JNl
在文件MediaScanner.java中，processDirectory是一个本地方法，能够直接转向JNI。具体实现代码如下所示：
static void android_media_MediaScanner_processDirectory(JNIEnv ＊env, jobject thiz, jstring path,
jstring extensions, jobject client)
{ //获取MediaScanner
MediaScanner ＊mp = (MediaScanner ＊)env->GetIntField(thiz, fields.context);
//参数判断，并抛出异常
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;
}
//初始化client实例
MyMediaScannerClient myClient(env, client);
//mp调用processDirectory
mp->processDirectory(pathStr, extensionsStr, myClient, ExceptionCheck, env);
//gc
env->ReleaseStringUTFChars(path, pathStr);
env->ReleaseStringUTFChars(extensions, extensionsStr);
}
4.2.6 扫描函数scanFile
在此讲解Java层的函数scanFile，功能是调用函数doScanFile对指定的文件进行扫描，具体实现代码如下所示：
public void scanFile(String path, long lastModified, long fileSize,
boolean isDirectory, boolean noMedia) {
//这是来自本地代码的回调函数
//Log.v(TAG, "scanFile: "+path);
doScanFile(path, null, lastModified, fileSize, isDirectory, false, noMedia);
}
4.2.7 JNl中的异常处理
为了处理Java实现的方法中或者C/C++实现的方法中抛出的Java异常，JNI也提供了一套异常处理机制函数集，专门用于检查、分析和处理异常情况。例如在文件jni.h中，定义了主要的异常函数，具体代码如下所示：
//抛出异常
jint (＊Throw)(JNIEnv＊, jthrowable);
//抛出新的异常
jint (＊ThrowNew)(JNIEnv ＊, jclass, const char ＊);
//异常产生
jthrowable (＊ExceptionOccurred)(JNIEnv＊);
void (＊ExceptionDescribe)(JNIEnv＊);
//清除异常
void (＊ExceptionClear)(JNIEnv＊);
void (＊FatalError)(JNIEnv＊, const char＊);
例如在Camera模块中也用到了异常处理，在文件android_hardware_Camera.cpp中，也涉及到了异常操作，具体代码例如下所示：
void JNICameraContext::copyAndPost(JNIEnv＊ env, const sp<IMemory>& dataPtr, int
msgType)
{
……
if (obj == NULL) {
LOGE("Couldn't allocate byte array for JPEG data");
env->ExceptionClear();
} else {
env->SetByteArrayRegion(obj, 0, size, data);
}
} else {
LOGE("image heap is NULL");
}
}
……
}
在文件android_hardware_Camera.cpp中，函数android_hardware_Camera_startPreview()也同样用到了异常处理机制，具体代码如下所示：
static void android_hardware_Camera_startPreview(JNIEnv ＊env, jobject thiz)
{
LOGV("startPreview");
sp<Camera> camera = get_native_camera(env, thiz, NULL);
if (camera == 0) return;
if (camera->startPreview() ! = NO_ERROR) {
jniThrowRuntimeException(env, "startPreview failed");
return;
}
}
在上述代码中，android_hardware_Camera_startPreview()如果发现startPreview()函数返回错误，则会抛出异常并返回。这里的异常与Java中的异常机制很相似，读者可以对比分析它们的原理。