Android 设置通知栏的操作按钮

我知道这是一个重复的问题，但我完全不知道出了什么问题。 实际上，我正在尝试创建一个媒体播放器，它应该由通知栏控制（播放、暂停、恢复）。播放器运行良好，并且显示通知栏，但是当我点击暂停按钮时，它抛出一个空指针异常。我猜错误在于按钮本身的动作，因为它无法启动服务来执行功能。 此外，通知栏没有得到很好的更新，即使在被终止后，服务也会不断创建自己

这是我的密码：

MusicService.java

// play the music in the Service class, but control it from the Activity class,

public class MusicService extends Service implements
    MediaPlayer.OnPreparedListener, MediaPlayer.OnErrorListener,
    MediaPlayer.OnCompletionListener {

private String songTitle="";


//media player
private MediaPlayer player;
//song list
private ArrayList<Music> songs;
//current position
private int songPosn;
private int length=0;



private static final String LOG_TAG = "ForegroundService";

@Override
public void onCreate() {
    super.onCreate();
    player =new MediaPlayer();
    songPosn=0;
    Toast.makeText(this, "Service was Created", Toast.LENGTH_LONG).show();
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {


     songs = (ArrayList<Music>)intent.getSerializableExtra("array");
     songPosn=intent.getIntExtra("id",0);


    playSong();
    Notify();

            if(intent.getAction().equals(Constants.ACTION.STARTFOREGROUND_ACTION)) {
            Log.i(LOG_TAG, "Received Start Foreground Intent ");}
        else if (intent.getAction().equals(Constants.ACTION.RESUME_ACTION))     {
            Log.i(LOG_TAG, "Clicked Resume");
        } else if (intent.getAction().equals(Constants.ACTION.PLAY_ACTION)) {
            Log.i(LOG_TAG, "Clicked Play");
        } else if (intent.getAction().equals(Constants.ACTION.PAUSE_ACTION)) {
            Log.i(LOG_TAG, "Clicked pause");
        } else if (intent.getAction().equals(
                Constants.ACTION.STOPFOREGROUND_ACTION)) {
            Log.i(LOG_TAG, "Received Stop Foreground Intent");
            stopForeground(true);
            stopSelf();
        }



    return START_REDELIVER_INTENT;
}



@Override
public void onDestroy() {
    super.onDestroy();

    if (player.isPlaying()) {
        player.stop();
    }
    player.release();
    Log.i(LOG_TAG, "In onDestroy");
    stopForeground(true);
}



public void pauseMusic()
{
    if(player.isPlaying())
    {
        player.pause();
        length=player.getCurrentPosition();

    }
}

public void resumeMusic()
{
    if(player.isPlaying()==false)
    {
        player.seekTo(length);
        player.start();
    }
}


@Override
public void onCompletion(MediaPlayer mediaPlayer) {
    songPosn=songPosn+1;
    playSong();
    mediaPlayer.start();

}

@Override
public boolean onError(MediaPlayer mediaPlayer, int i, int i1) {
    return false;
}

@Override
public void onPrepared(MediaPlayer mediaPlayer) {
    player.start();

}

@Nullable
@Override
public IBinder onBind(Intent intent) {
    return null;
}





public void playSong() {


   player.reset();
    //get song

    Music playSong = songs.get(songPosn);
    songTitle=playSong.getMusicTitle();
    //get id
  long currSong = playSong.getMusicId();
  //  set uri
    Uri trackUri = ContentUris.withAppendedId(
            android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,
            currSong);



    player.setAudioStreamType(AudioManager.STREAM_MUSIC);
    player.setOnPreparedListener(this); // listner  when the MediaPlayer instance is prepared
    player.setOnErrorListener(this); //  an error is thrown
    player.setOnCompletionListener(this);

    try{

        player.setDataSource(getApplicationContext(), trackUri);
    }
    catch(Exception e){
        Log.e("MUSIC SERVICE", "Error setting data source", e);
    }


    player.prepareAsync();

    Toast.makeText(this, "play music",
            Toast.LENGTH_LONG).show();

}




public void Notify()
{

    Intent notificationIntent = new Intent(this, MainActivity.class);
    notificationIntent.setAction(Constants.ACTION.MAIN_ACTION);

    notificationIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK
            | Intent.FLAG_ACTIVITY_CLEAR_TASK);
    PendingIntent pendingIntent = PendingIntent.getActivity(this, 0,
            notificationIntent, 0);

    Intent resumeIntent = new Intent(this, MusicService.class);

    resumeIntent.setAction(Constants.ACTION.RESUME_ACTION);
    PendingIntent rresumeIntent = PendingIntent.getService(this, 0,
            resumeIntent, PendingIntent.FLAG_UPDATE_CURRENT);



    Intent playIntent = new Intent(this, MusicService.class);

    playIntent.setAction(Constants.ACTION.PLAY_ACTION);

    PendingIntent pplayIntent = PendingIntent.getService(this, 0,
            playIntent, PendingIntent.FLAG_UPDATE_CURRENT);





    Intent pauseIntent = new Intent(this, MusicService.class);
    pauseIntent.setAction(Constants.ACTION.PAUSE_ACTION);

    PendingIntent ppauseIntent = PendingIntent.getService(this, 0,
            pauseIntent, PendingIntent.FLAG_UPDATE_CURRENT);




    Bitmap icon = BitmapFactory.decodeResource(getResources(),
            R.drawable.pic);

    NotificationCompat.Builder notification = new NotificationCompat.Builder(this)
            .setContentTitle(songTitle)
            .setTicker(songTitle)
            .setContentText("My Music")
            .setSmallIcon(R.drawable.pic)
            .setLargeIcon(
                    Bitmap.createScaledBitmap(icon, 128, 128, false))
            .setContentIntent(pendingIntent)
            .setPriority(Notification.PRIORITY_MAX)
            .addAction(android.R.drawable.ic_media_ff,
                    "Resume", rresumeIntent)
            .addAction(android.R.drawable.ic_media_play, "Play",
                    pplayIntent)
            .addAction(android.R.drawable.ic_media_pause, "Pause",
                    ppauseIntent);





    NotificationManager mNotifyMgr = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
    mNotifyMgr.notify(Constants.NOTIFICATION_ID.FOREGROUND_SERVICE,
            notification.build());
    notification.setContentTitle(songTitle);
    mNotifyMgr.notify(Constants.NOTIFICATION_ID.FOREGROUND_SERVICE,notification.build());


}


}

public class MusicFragment extends android.support.v4.app.Fragment {

private final String LOG_TAG = MusicFragment.class.getSimpleName();



private ArrayList<Music> songList;
private ListView songView;

private MusicService musicSrv = new MusicService() ;
private Intent playIntent;
private boolean musicBound=false;
private LinearLayout songsLayout;
private MusicController controller;
SongAdapter songAdt;


public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

    View myView = inflater.inflate(R.layout.activity_music_fragment, container, false);
    songView = (ListView)myView.findViewById(R.id.song_list);
    songsLayout =(LinearLayout)myView.findViewById(R.id.song_layout);

    setHasOptionsMenu(true);
    songList = new ArrayList<Music>();
    getSongList();

    // sort songs alphabitcally
    Collections.sort(songList, new Comparator<Music>(){
        public int compare(Music a, Music b){
            return a.getMusicTitle().compareTo(b.getMusicTitle());
        }
    });



    songAdt = new SongAdapter(getActivity(), songList);
    songView.setAdapter(songAdt);




    songView.setOnItemClickListener(new ListClickHandler());


    return myView;
}





@Override
public void onCreateOptionsMenu (Menu menu, MenuInflater inflater) {
    super.onCreateOptionsMenu(menu, inflater);
    inflater.inflate(R.menu.service_menu, menu);
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {

        case R.id.action_end:
           getActivity().stopService(playIntent);
            musicSrv=null;
            System.exit(0);
            break;
    }
    return super.onOptionsItemSelected(item);
}



public void getSongList() {
    ContentResolver musicResolver = getContext().getContentResolver();
    Uri musicUri = android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
    Cursor musicCursor = musicResolver.query(musicUri, null, null, null, null);
    if(musicCursor!=null && musicCursor.moveToFirst()){
        //get columns
        int titleColumn = musicCursor.getColumnIndex
                (android.provider.MediaStore.Audio.Media.TITLE);
        int idColumn = musicCursor.getColumnIndex
                (android.provider.MediaStore.Audio.Media._ID);
        int artistColumn = musicCursor.getColumnIndex
                (android.provider.MediaStore.Audio.Media.ARTIST);
        //add songs to list
        do {
            long thisId = musicCursor.getLong(idColumn);
            String thisTitle = musicCursor.getString(titleColumn);
            String thisArtist = musicCursor.getString(artistColumn);
            songList.add(new Music(thisId, thisTitle, thisArtist));
        }
        while (musicCursor.moveToNext());
    }
}

public class ListClickHandler implements AdapterView.OnItemClickListener {

    @Override
    public void onItemClick(AdapterView<?> adapter, View view, int position, long arg3) {
        // TODO Auto-generated method stub

        Intent startIntent = new Intent(getActivity(), MusicService.class);
        startIntent.setAction(Constants.ACTION.STARTFOREGROUND_ACTION);
        startIntent.putExtra("array",songList);
        startIntent.putExtra("id",Integer.parseInt(view.getTag().toString()));
        getActivity().startService(startIntent);




    }}


@Override
public void onDestroy() {
    super.onDestroy();
    musicSrv.stopForeground(true);
}

}

E/AndroidRuntime: FATAL EXCEPTION: main java.lang.RuntimeException: Unable to start service .MusicService@42bb1fa0 with Intent { act=.MusicService.action.pause flg=0x10000000 cmp=/.MusicService bnds=[518,1047][720,1143] }: java.lang.NullPointerException
                                                                                          at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2850)
                                                                                          at android.app.ActivityThread.access$2000(ActivityThread.java:159)
                                                                                          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1419)
                                                                                          at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                                          at android.os.Looper.loop(Looper.java:176)
                                                                                          at android.app.ActivityThread.main(ActivityThread.java:5419)
                                                                                          at java.lang.reflect.Method.invokeNative(Native Method)
                                                                                          at java.lang.reflect.Method.invoke(Method.java:525)
                                                                                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
                                                                                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
                                                                                          at dalvik.system.NativeStart.main(Native Method)