Android 媒体播放器 Null 点异常（Java）

Question

我正在尝试使用给定的 uri 创建 Mediaplayer session。 但它会导致NullpointerException 。

    Uri uri = Uri.parse(path);

    // Creating MediaPlayer with given song's URI
    if (mediaPlayer != null) {
        mediaPlayer.stop();
        mediaPlayer.release();
    }
    mediaPlayer = MediaPlayer.create(this, uri);

    try {
        // Setting the MediaPlayer Listener
        mediaPlayer.setOnPreparedListener(new MediaPlayer.OnPreparedListener() {
            @Override
            public void onPrepared(MediaPlayer mp) {
                seekBar.setMax(mp.getDuration());
                mediaPlayer.start();
                changeSeekbar();
            }
        });
    } catch (Exception e) {
        Log.e("ERROR", e.toString());
    }

给定Logcat：

2020-04-07 22:21:05.289 12237-12237/com.example.musicappresearch2 E/Path: /storage/emulated/0/Music/Alone - Viren.mp3
2020-04-07 22:21:05.289 12237-12237/com.example.musicappresearch2 E/ERROR: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.setOnPreparedListener(android.media.MediaPlayer$OnPreparedListener)' on a null object reference

你能告诉我我做错了什么吗？ 谢谢。

Answer 1

这段代码有两种写法，都在设备上测试过

首先，确保您正确处理android.permission.READ_EXTERNAL_STORAGE并且您确实拥有正确的 Uri。
MediaPlayer.create(this, uri); 如果上下文或 uri 无效，则将失败。

MediaPlayer.create(this, uri); 它本身已经准备好播放器，因此在这种情况下您不需要.prepareAsync() 。 并且您的代码对 go 很好。

另一种方式：

    mediaPlayer = new MediaPlayer(); // hence, we don't use .create, manually instantiate
    try {
        mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
        mediaPlayer.setDataSource(this, uri);
        mediaPlayer.setOnPreparedListener(mp -> {
            mediaPlayer.start();
        });

        /* use async, if you don't want to block UI thread
         keep in mind, this should be called after setting listener  
       because it might prepare even until the listener has been set */
        mediaPlayer.prepareAsync(); 
    } catch (Exception e) {
        Log.e("ERROR", e.toString());
    }

Answer 2

尝试这个：

            Uri uri = Uri.parse(path);

        mediaPlayer = new MediaPlayer();
        try {
           // mediaPlayer.setDataSource(String.valueOf(uri));
           mediaPlayer.setDataSource(MainActivity.this,uri);
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            mediaPlayer.prepare();
        } catch (IOException e) {
            e.printStackTrace();
        }
        mediaPlayer.start();