android播放sd卡音乐失败，怎么解决

照着第一行代码上做的，真机上无法播放
public class MainActivity extends Activity implements View.OnClickListener{
private Button play;
private Button pause;
private Button stop;
private MediaPlayer mediaPlayer = new MediaPlayer();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
play = (Button) findViewById(R.id.play);
stop = (Button) findViewById(R.id.stop);
pause = (Button) findViewById(R.id.pause);
play.setOnClickListener(this);
stop.setOnClickListener(this);
pause.setOnClickListener(this);
initMediaPlayer();
}

private void initMediaPlayer() {
    try {
        mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
        File file = new File(Environment.getExternalStorageDirectory(), "music.mp3");
        mediaPlayer.setDataSource(file.getPath());
        mediaPlayer.prepare();

    } catch (Exception e) {
        e.printStackTrace();
    }
}
@Override
public void onClick(View view) {
    switch (view.getId()) {
        case R.id.play:
            if (!mediaPlayer.isPlaying()) {
                mediaPlayer.start();
            }
            break;
        case R.id.pause:
            if (mediaPlayer.isPlaying()) {
                mediaPlayer.pause();
            }
            break;
        case R.id.stop:
            if (mediaPlayer.isPlaying()) {
                mediaPlayer.reset();
                initMediaPlayer();
            }
            break;
        default:
            break;
    }
}

@Override
protected void onDestroy() {
    super.onDestroy();
    if (mediaPlayer != null) {
        mediaPlayer.stop();
        mediaPlayer.release();
    }
}

}

自己琢磨琢磨吧好哦了

之前也写过一个SD卡音乐播放器，把manifest里的
uses-sdk 删了就变好了，也不知道为什么。共同切磋，一起进步