MediaProvider使用 SQLite 数据库存储图片、视频、音频等多媒体文件的信息，供视频播放器、音乐播放器、图库使用。提供了基本的增删改查等相关方法。路径如下：
/packages/providers/MediaProvider/src/com/android/providers/media/MediaProvider.java
其中包含以下内部类：
DatabaseHelper——对于一个特殊数据库的包装类，用来管理数据的创建和版本更新，继承SQLiteOpenHelper
GetTableAndWhereOutParameter——静态类，获取相关的参数
ScannerClient——静态类，继承MediaScannerConnectionClient
ThumbData——方便对变量的操作
下面依据对数据库的操作进行分析。
1. 创建
首先在MediaProvider的onCreate方法中分别对内部存储和外部存储进行链接数据库：
//绑定内部存储数据库
attachVolume(INTERNAL_VOLUME);
...
if (Environment.MEDIA_MOUNTED.equals(state) ||
Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
//绑定外部存储数据库
attachVolume(EXTERNAL_VOLUME);
}
为内部存储和外部存储进行创建数据库，如果此存储卷已经链接上了，那么什么也不做，否则就查询存储卷的id并且建立对应的数据库。接下来分析attachVolume方法：
private Uri attachVolume(String volume) {
... ...
// Update paths to reflect currently mounted volumes
updateStoragePaths();
DatabaseHelper helper = null;
synchronized (mDatabases) {
helper = mDatabases.get(volume);
//判断是否已经attached过了
if (helper != null) {
if (EXTERNAL_VOLUME.equals(volume)) {
//确保默认的文件夹已经被创建在挂载的主要存储设备上，
//对每个存储卷只做一次这种操作，所以当用户手动删除时不会打扰
ensureDefaultFolders(helper, helper.getWritableDatabase());
}
return Uri.parse("content://media/" + volume);
}
Context context = getContext();
if (INTERNAL_VOLUME.equals(volume)) {
//如果是内部存储则直接实例化DatabaseHelper，传参，之后调用DatabaseHelper的方法
helper = new DatabaseHelper(context, INTERNAL_DATABASE_NAME, true,
false, mObjectRemovedCallback);
} else if (EXTERNAL_VOLUME.equals(volume)) {
//如果是外部存储的操作
final VolumeInfo vol = mStorageManager.getPrimaryPhysicalVolume();
if (vol != null) {
final StorageVolume actualVolume = mStorageManager.getPrimaryVolume();
final int volumeId = actualVolume.getFatVolumeId();
// Must check for failure!
// If the volume is not (yet) mounted, this will create a new
// external-ffffffff.db database instead of the one we expect. Then, if
// android.process.media is later killed and respawned, the real external
// database will be attached, containing stale records, or worse, be empty.
//数据库都是以类似 external-ffffffff.db 的形式命名的，
//后面的 8 个 16 进制字符是该 SD 卡 FAT 分区的 Volume ID。
//该 ID 是分区时决定的，只有重新分区或者手动改变才会更改，
//可以防止插入不同 SD 卡时数据库冲突。
if (volumeId == -1) {
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state) ||
Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
//已经挂载但是sd卡是只读状态
} else {
//还没有挂载
}
}
// generate database name based on volume ID
//根据volume ID设置数据库的名称
String dbName = "external-" + Integer.toHexString(volumeId) + ".db";
//通过构造方法去实现创建数据库的过程
helper = new DatabaseHelper(context, dbName, false,
false, mObjectRemovedCallback);
mVolumeId = volumeId;
} else {
//将之前的数据库名字进行转换
// external database name should be EXTERNAL_DATABASE_NAME
// however earlier releases used the external-XXXXXXXX.db naming
// for devices without removable storage, and in that case we need to convert
// to this new convention
... ...
//根据之前转换的数据库名，创建数据库
helper = new DatabaseHelper(context, dbFile.getName(), false,
false, mObjectRemovedCallback);
}
} else {
throw new IllegalArgumentException("There is no volume named " + volume);
}
//标识已经创建过了数据库
mDatabases.put(volume, helper);
if (!helper.mInternal) {
// clean up stray album art files: delete every file not in the database
File[] files = new File(mExternalStoragePaths[0],
ALBUM_THUMB_FOLDER).listFiles();
HashSet fileSet = new HashSet();
for (int i = 0; files != null && i < files.length; i++) {
fileSet.add(files[i].getPath());
}
Cursor cursor = query(MediaStore.Audio.Albums.EXTERNAL_CONTENT_URI,
new String[] { MediaStore.Audio.Albums.ALBUM_ART }, null, null, null);
try {
while (cursor != null && cursor.moveToNext()) {
fileSet.remove(cursor.getString(0));
}
} finally {
IoUtils.closeQuietly(cursor);
}
Iterator iterator = fileSet.iterator();
while (iterator.hasNext()) {
String filename = iterator.next();
if (LOCAL_LOGV) Log.v(TAG, "deleting obsolete album art " + filename);
new File(filename).delete();
}
}
}
if (EXTERNAL_VOLUME.equals(volume)) {
//给外部存储创建默认的文件夹
ensureDefaultFolders(helper, helper.getWritableDatabase());
}
return Uri.parse("content://media/" + volume);
}
下面就是分析创建数据库的源头DatabaseHelper：
@Override
public void onCreate(final SQLiteDatabase db) {
//在此方法中对63版本以下的都会新建数据库
updateDatabase(mContext, db, mInternal, 0, getDatabaseVersion(mContext));
}
@Override
public void onUpgrade(final SQLiteDatabase db, final int oldV, final int newV) {
//对数据库进行更新
mUpgradeAttempted = true;
updateDatabase(mContext, db, mInternal, oldV, newV);
}
现在已经找到创建数据库的方法updateDatabase，现在大致分析一下此方法：
private static void updateDatabase(Context context, SQLiteDatabase db, boolean internal,
int fromVersion, int toVersion) {
// sanity checks
int dbversion = getDatabaseVersion(context);
//对数据库的版本进行判断
... ...
long startTime = SystemClock.currentTimeMicro();
//对传入的数据库版本进行判断，如果小于63，或者在84到89,92到94之间的，
//都会去创建数据库
if (fromVersion < 63 || (fromVersion >= 84 && fromVersion <= 89) ||
(fromVersion >= 92 && fromVersion <= 94)) {
//下面就是执行具体的sqlite CRATE语句，创建对应的表
... ...
}
//下面也是对版本判断之后进行相应操作
... ...
//检查audio_meta的_data值是否是不同的，如果不同就删除audio_meta，
//在扫描的时候从新创建
sanityCheck(db, fromVersion);
long elapsedSeconds = (SystemClock.currentTimeMicro() - startTime) / 1000000;
}
至此，对于数据库的创建已经分析完毕。
2 更新
@Override
public int update(Uri uri, ContentValues initialValues, String userWhere,
String[] whereArgs) {
//将uri进行转换成合适的格式，去除标准化
uri = safeUncanonicalize(uri);
int count;
//对uri进行匹配
int match = URI_MATCHER.match(uri);
//返回查询的对应uri的数据库帮助类
DatabaseHelper helper = getDatabaseForUri(uri);
//记录更新的次数
helper.mNumUpdates++;
//通过可写的方式获得数据库实例
SQLiteDatabase db = helper.getWritableDatabase();
String genre = null;
if (initialValues != null) {
//获取流派的信息，然后删除掉
genre = initialValues.getAsString(Audio.AudioColumns.GENRE);
initialValues.remove(Audio.AudioColumns.GENRE);
}
// special case renaming directories via MTP.
// in this case we must update all paths in the database with
// the directory name as a prefix
... ...
//根据匹配的uri进行相应的操作
switch (match) {
case AUDIO_MEDIA_ID:
//更新音乐人和专辑字段。首先从缓存中判断是否有值，如果有直接用缓存中的
//数据，如果没有再从数据库中查询是否有对应的信息，如果有则更新，
//如果没有插入这条数据.接下来的操作是增加更新次数，并更新流派
... ...
case VIDEO_MEDIA_ID:
//更新视频，并且发出生成略缩图请求
... ...
case AUDIO_PLAYLISTS_ID_MEMBERS_ID:
//更新播放列表数据
... ...
}
... ...
}
至此，更新操作已完成。
3 插入
关于插入，有两个方法插入，一个是大量的插入bulkInsert方法传入的是ContentValues数组；一个是insert，传入的是单一个ContentValues。下面分别分析：
@Override
public int bulkInsert(Uri uri, ContentValues values[]) {
//首先对传入的Uri进行匹配
int match = URI_MATCHER.match(uri);
if (match == VOLUMES) {
//如果是匹配的是存储卷，则直接调用父类的方法，进行循环插入
return super.bulkInsert(uri, values);
}
//对DatabaseHelper和SQLiteDatabase的初始化
DatabaseHelper helper = getDatabaseForUri(uri);
if (helper == null) {
throw new UnsupportedOperationException(
"Unknown URI: " + uri);
}
SQLiteDatabase db = helper.getWritableDatabase();
if (db == null) {
throw new IllegalStateException("Couldn't open database for " + uri);
}
if (match == AUDIO_PLAYLISTS_ID || match == AUDIO_PLAYLISTS_ID_MEMBERS) {
//插入播放列表的数据，在playlistBulkInsert中是开启的事务进行插入
return playlistBulkInsert(db, uri, values);
} else if (match == MTP_OBJECT_REFERENCES) {
//将MTP对象的ID转换成音频的ID，最终也是调用到playlistBulkInsert
int handle = Integer.parseInt(uri.getPathSegments().get(2));
return setObjectReferences(helper, db, handle, values);
}
//如果不满足上述的条件，则开启事务进行插入其他的数据
db.beginTransaction();
ArrayList notifyRowIds = new ArrayList();
int numInserted = 0;
try {
int len = values.length;
for (int i = 0; i < len; i++) {
if (values[i] != null) {
//循环调用insertInternal去插入相关的数据
insertInternal(uri, match, values[i], notifyRowIds);
}
}
numInserted = len;
db.setTransactionSuccessful();
} finally {
//结束事务
db.endTransaction();
}
// Notify MTP (outside of successful transaction)
if (uri != null) {
if (uri.toString().startsWith("content://media/external/")) {
notifyMtp(notifyRowIds);
}
}
//通知更新
getContext().getContentResolver().notifyChange(uri, null);
return numInserted;
}
@Override
public Uri insert(Uri uri, ContentValues initialValues) {
int match = URI_MATCHER.match(uri);
ArrayList notifyRowIds = new ArrayList();
//只是调用insertInternal进行插入
Uri newUri = insertInternal(uri, match, initialValues, notifyRowIds);
if (uri != null) {
if (uri.toString().startsWith("content://media/external/")) {
notifyMtp(notifyRowIds);
}
}
// do not signal notification for MTP objects.
// we will signal instead after file transfer is successful.
if (newUri != null && match != MTP_OBJECTS) {
getContext().getContentResolver().notifyChange(uri, null);
}
return newUri;
}
insertInternal方法比较简单，但是类别较多，暂时不做分析。
4 删除
@Override
public int delete(Uri uri, String userWhere, String[] whereArgs) {
uri = safeUncanonicalize(uri);
int count;
int match = URI_MATCHER.match(uri);
// handle MEDIA_SCANNER before calling getDatabaseForUri()
//因为如果匹配的uri是扫描过程中的，此时uri直接通过getDatabaseForUri获取不到
//数据库，需要对uri进行重新拼装
if (match == MEDIA_SCANNER) {
if (mMediaScannerVolume == null) {
return 0;
}
DatabaseHelper database = getDatabaseForUri(
Uri.parse("content://media/" + mMediaScannerVolume + "/audio"));
if (database == null) {
Log.w(TAG, "no database for scanned volume " + mMediaScannerVolume);
} else {
database.mScanStopTime = SystemClock.currentTimeMicro();
String msg = dump(database, false);
//删除掉在数据库的log表记录
logToDb(database.getWritableDatabase(), msg);
}
mMediaScannerVolume = null;
//因为只涉及到1行，所以返回值是1
return 1;
}
if (match == VOLUMES_ID) {
//对外部存储设备进行关闭数据库的操作
detachVolume(uri);
count = 1;
} else if (match == MTP_CONNECTED) {
synchronized (mMtpServiceConnection) {
if (mMtpService != null) {
// MTP has disconnected, so release our connection to MtpService
getContext().unbindService(mMtpServiceConnection);
count = 1;
// mMtpServiceConnection.onServiceDisconnected might not get called,
// so set mMtpService = null here
mMtpService = null;
} else {
count = 0;
}
}
} else {
final String volumeName = getVolumeName(uri);
//初始化DatabaseHelper和SQLiteDatabase
... ...
synchronized (sGetTableAndWhereParam) {
//拼装字段
getTableAndWhere(uri, match, userWhere, sGetTableAndWhereParam);
if (sGetTableAndWhereParam.table.equals("files")) {
String deleteparam =
uri.getQueryParameter(MediaStore.PARAM_DELETE_DATA);
if (deleteparam == null || ! deleteparam.equals("false")) {
database.mNumQueries++;
Cursor c = db.query(sGetTableAndWhereParam.table,
sMediaTypeDataId,
sGetTableAndWhereParam.where, whereArgs, null, null, null);
String [] idvalue = new String[] { "" };
String [] playlistvalues = new String[] { "", "" };
try {
while (c.moveToNext()) {
final int mediaType = c.getInt(0);
final String data = c.getString(1);
final long id = c.getLong(2);
if (mediaType == FileColumns.MEDIA_TYPE_IMAGE) {
//判断是图片类型，直接删除源文件
deleteIfAllowed(uri, data);
MediaDocumentsProvider.onMediaStoreDelete(
getContext(),volumeName,
FileColumns.MEDIA_TYPE_IMAGE, id);
idvalue[0] = String.valueOf(id);
database.mNumQueries++;
//查询略缩图文件并删除
Cursor cc = db.query("thumbnails", sDataOnlyColumn,
"image_id=?", idvalue, null, null, null);
try {
while (cc.moveToNext()) {
deleteIfAllowed(uri, cc.getString(0));
}
database.mNumDeletes++;
//删除数据库中的信息
db.delete("thumbnails", "image_id=?", idvalue);
} finally {
IoUtils.closeQuietly(cc);
}
} else if (mediaType == FileColumns.MEDIA_TYPE_VIDEO) {
//如果是视频文件，直接删除源文件
deleteIfAllowed(uri, data);
MediaDocumentsProvider.onMediaStoreDelete(
getContext(),volumeName,
FileColumns.MEDIA_TYPE_VIDEO, id);
} else if (mediaType == FileColumns.MEDIA_TYPE_AUDIO) {
//如果是音频文件并且判断是否是外部存储
if (!database.mInternal) {
MediaDocumentsProvider.onMediaStoreDelete(
getContext(),volumeName,
FileColumns.MEDIA_TYPE_AUDIO, id);
idvalue[0] = String.valueOf(id);
database.mNumDeletes += 2; // also count the one below
//删除流派信息
db.delete("audio_genres_map","audio_id=?",idvalue);
// for each playlist that the item appears in, move
// all the items behind it forward by one
Cursor cc = db.query("audio_playlists_map",
sPlaylistIdPlayOrder,
"audio_id=?", idvalue, null, null, null);
try {
while (cc.moveToNext()) {
playlistvalues[0] = "" + cc.getLong(0);
playlistvalues[1] = "" + cc.getInt(1);
database.mNumUpdates++;
//删除对应播放列表信息
db.execSQL("UPDATE audio_playlists_map" +
" SET play_order=play_order-1" +
" WHERE playlist_id=? AND play_order>?",
playlistvalues);
}
db.delete("audio_playlists_map", "audio_id=?", idvalue);
} finally {
IoUtils.closeQuietly(cc);
}
}
} else if (mediaType == FileColumns.MEDIA_TYPE_PLAYLIST) {
// TODO, maybe: remove the audio_playlists_cleanup trigger and
// implement functionality here (clean up the playlist map)
}
}
} finally {
IoUtils.closeQuietly(c);
}
}
}
//对其他的匹配类型进行删除
switch (match) {
//删除MTP，流派信息，视频文件的略缩图
... ...
}
// Since there are multiple Uris that can refer to the same files
// and deletes can affect other objects in storage (like subdirectories
// or playlists) we will notify a change on the entire volume to make
// sure no listeners miss the notification.
Uri notifyUri = Uri.parse("content://" + MediaStore.AUTHORITY + "/" + volumeName);
getContext().getContentResolver().notifyChange(notifyUri, null);
}
}
return count;
}
5 查询
public Cursor query(Uri uri, String[] projectionIn, String selection,
String[] selectionArgs, String sort) {
uri = safeUncanonicalize(uri);
int table = URI_MATCHER.match(uri);
List prependArgs = new ArrayList();
// handle MEDIA_SCANNER before calling getDatabaseForUri()
if (table == MEDIA_SCANNER) {
if (mMediaScannerVolume == null) {
return null;
} else {
// create a cursor to return volume currently being scanned by the media scanner
MatrixCursor c = new MatrixCursor(
new String[] {MediaStore.MEDIA_SCANNER_VOLUME});
c.addRow(new String[] {mMediaScannerVolume});
//直接返回的是有关存储卷的cursor
return c;
}
}
// Used temporarily (until we have unique media IDs) to get an identifier
// for the current sd card, so that the music app doesn't have to use the
// non-public getFatVolumeId method
if (table == FS_ID) {
MatrixCursor c = new MatrixCursor(new String[] {"fsid"});
c.addRow(new Integer[] {mVolumeId});
return c;
}
if (table == VERSION) {
MatrixCursor c = new MatrixCursor(new String[] {"version"});
c.addRow(new Integer[] {getDatabaseVersion(getContext())});
return c;
}
//初始化DatabaseHelper和SQLiteDatabase
String groupBy = null;
DatabaseHelper helper = getDatabaseForUri(uri);
if (helper == null) {
return null;
}
helper.mNumQueries++;
SQLiteDatabase db = null;
try {
db = helper.getReadableDatabase();
} catch (Exception e) {
e.printStackTrace();
return null;
}
if (db == null) return null;
// SQLiteQueryBuilder类是组成查询语句的帮助类
SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
//获取uri里面的查询字符
String limit = uri.getQueryParameter("limit");
String filter = uri.getQueryParameter("filter");
String [] keywords = null;
if (filter != null) {
filter = Uri.decode(filter).trim();
if (!TextUtils.isEmpty(filter)) {
//对字符进行筛选
String [] searchWords = filter.split(" ");
keywords = new String[searchWords.length];
for (int i = 0; i < searchWords.length; i++) {
String key = MediaStore.Audio.keyFor(searchWords[i]);
key = key.replace("\\", "\\\\");
key = key.replace("%", "\\%");
key = key.replace("_", "\\_");
keywords[i] = key;
}
}
}
if (uri.getQueryParameter("distinct") != null) {
qb.setDistinct(true);
}
boolean hasThumbnailId = false;
//对匹配的其他类型进行设置查询语句的操作
switch (table) {
case IMAGES_MEDIA:
//设置查询的表是images
qb.setTables("images");
if (uri.getQueryParameter("distinct") != null)
//设置为唯一的
qb.setDistinct(true);
break;
//其他类型相类似
... ...
}
//根据拼装的搜索条件，进行查询
Cursor c = qb.query(db, projectionIn, selection,
combine(prependArgs, selectionArgs), groupBy, null, sort, limit);
if (c != null) {
String nonotify = uri.getQueryParameter("nonotify");
if (nonotify == null || !nonotify.equals("1")) {
//通知更新数据库
c.setNotificationUri(getContext().getContentResolver(), uri);
}
}
return c;
}
至此，关于MediaProvider的增删改查，创建数据库等操作的分析已经完成。