This example creates a MediaSession object when a Service is started. The MediaSession object is released when the Service gets destroyed:

public final class MyService extends Service {
    private static MediaSession s_mediaSession;

    @Override
    public void onCreate() {
        // Instantiate new MediaSession object.
        configureMediaSession();
    }

    @Override
    public void onDestroy() {
        if (s_mediaSession != null)
            s_mediaSession.release();
    }
}

The following method instantiates and configures the MediaSession button callbacks:

private void configureMediaSession {
    s_mediaSession = new MediaSession(this, "MyMediaSession");

    // Overridden methods in the MediaSession.Callback class.
    s_mediaSession.setCallback(new MediaSession.Callback() {
        @Override
        public boolean onMediaButtonEvent(Intent mediaButtonIntent) {
            Log.d(TAG, "onMediaButtonEvent called: " + mediaButtonIntent);
            KeyEvent ke = mediaButtonIntent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
            if (ke != null && ke.getAction() == KeyEvent.ACTION_DOWN) {
                int keyCode = ke.getKeyCode();
                Log.d(TAG, "onMediaButtonEvent Received command: " + ke);
            }
            return super.onMediaButtonEvent(mediaButtonIntent);
        }

        @Override
        public void onSkipToNext() {
            Log.d(TAG, "onSkipToNext called (media button pressed)");
            Toast.makeText(getApplicationContext(), "onSkipToNext called", Toast.LENGTH_SHORT).show();
            skipToNextPlaylistItem(); // Handle this button press.
            super.onSkipToNext();
        }

        @Override
        public void onSkipToPrevious() {
            Log.d(TAG, "onSkipToPrevious called (media button pressed)");
            Toast.makeText(getApplicationContext(), "onSkipToPrevious called", Toast.LENGTH_SHORT).show();
            skipToPreviousPlaylistItem(); // Handle this button press.
            super.onSkipToPrevious();
        }

        @Override
        public void onPause() {
            Log.d(TAG, "onPause called (media button pressed)");
            Toast.makeText(getApplicationContext(), "onPause called", Toast.LENGTH_SHORT).show();
            mpPause(); // Pause the player.
            super.onPause();
        }

        @Override
        public void onPlay() {
            Log.d(TAG, "onPlay called (media button pressed)");
            mpStart(); // Start player/playback.
            super.onPlay();
        }

        @Override
        public void onStop() {
            Log.d(TAG, "onStop called (media button pressed)");
            mpReset(); // Stop and/or reset the player.
            super.onStop();
        }
    });

    s_mediaSession.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS | MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS);
    s_mediaSession.setActive(true);
}

The following method sends meta data (stored in a HashMap) to the device using A2DP:

void sendMetaData(@NonNull final HashMap<String, String> hm) {
    // Return if Bluetooth A2DP is not in use.
    if (!((AudioManager) getSystemService(Context.AUDIO_SERVICE)).isBluetoothA2dpOn()) return;

    MediaMetadata metadata = new MediaMetadata.Builder()
            .putString(MediaMetadata.METADATA_KEY_TITLE, hm.get("Title"))
            .putString(MediaMetadata.METADATA_KEY_ALBUM, hm.get("Album"))
            .putString(MediaMetadata.METADATA_KEY_ARTIST, hm.get("Artist"))
            .putString(MediaMetadata.METADATA_KEY_AUTHOR, hm.get("Author"))
            .putString(MediaMetadata.METADATA_KEY_COMPOSER, hm.get("Composer"))
            .putString(MediaMetadata.METADATA_KEY_WRITER, hm.get("Writer"))
            .putString(MediaMetadata.METADATA_KEY_DATE, hm.get("Date"))
            .putString(MediaMetadata.METADATA_KEY_GENRE, hm.get("Genre"))
            .putLong(MediaMetadata.METADATA_KEY_YEAR, tryParse(hm.get("Year")))
            .putLong(MediaMetadata.METADATA_KEY_DURATION, tryParse(hm.get("Raw Duration")))
            .putLong(MediaMetadata.METADATA_KEY_TRACK_NUMBER, tryParse(hm.get("Track Number")))
            .build();

    s_mediaSession.setMetadata(metadata);
}

The following method sets the PlaybackState. It also sets which button actions the MediaSession will respond to:

private void setPlaybackState(@NonNull final int stateValue) {
    PlaybackState state = new PlaybackState.Builder()
            .setActions(PlaybackState.ACTION_PLAY | PlaybackState.ACTION_SKIP_TO_NEXT
                    | PlaybackState.ACTION_PAUSE | PlaybackState.ACTION_SKIP_TO_PREVIOUS
                    | PlaybackState.ACTION_STOP | PlaybackState.ACTION_PLAY_PAUSE)
            .setState(stateValue, PlaybackState.PLAYBACK_POSITION_UNKNOWN, 0)
            .build();

    s_mediaSession.setPlaybackState(state);
}