android Media Playerback（2）

As you may know, when the user changes the screen orientation (or changes the device configuration in another way), the system handles that by restarting the activity (by default), so you might quickly consume all of the system resources as the user rotates the device back and forth between portrait and landscape, because at each orientation change, you create a newMediaPlayer that you never release. (For more information about runtime restarts, see Handling Runtime Changes.) audio focus 

Notice: You should always use wake locks sparingly and hold them only for as long as truly necessary, because they significantly reduce the battery life of the device.

持有Wifi锁：

WifiLock wifiLock = ((WifiManager) getSystemService(Context.WIFI_SERVICE))
    .createWifiLock(WifiManager.WIFI_MODE_FULL, "mylock");

wifiLock.acquire();

wifiLock.release();

In order to turn your service into a foreground service, you must create a Notification for the status bar and call startForeground() from the Service. For example:

String songName;
// assign the song name to songName
PendingIntent pi = PendingIntent.getActivity(getApplicationContext(), 0,
                new Intent(getApplicationContext(), MainActivity.class),
                PendingIntent.FLAG_UPDATE_CURRENT);
Notification notification = new Notification();
notification.tickerText = text;
notification.icon = R.drawable.play0;
notification.flags |= Notification.FLAG_ONGOING_EVENT;
notification.setLatestEventInfo(getApplicationContext(), "MusicPlayerSample",
                "Playing: " + songName, pi);
startForeground(NOTIFICATION_ID, notification);

Even though only one activity can run at any given time, Android is a multi-tasking environment. 

 You can do that by querying the ContentResolver for external media:

ContentResolver contentResolver = getContentResolver();
Uri uri = android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
Cursor cursor = contentResolver.query(uri, null, null, null, null);
if (cursor == null) {
    // query failed, handle error.
} else if (!cursor.moveToFirst()) {
    // no media on the device
} else {
    int titleColumn = cursor.getColumnIndex(android.provider.MediaStore.Audio.Media.TITLE);
    int idColumn = cursor.getColumnIndex(android.provider.MediaStore.Audio.Media._ID);
    do {
       long thisId = cursor.getLong(idColumn);
       String thisTitle = cursor.getString(titleColumn);
       // ...process entry...
    } while (cursor.moveToNext());
}