解决使用MediaPlayer开发时抛IllegalStateException异常的办法

今天编写自己的播放器时总是报了下面的错误，

而求错误指向mediaplayer.setDataSource()这个一行。

我反复看了很久也没觉得有什么错误，甚至利用Toast查看了SP的信息，发现确实是正确的文件路径，所以肯定不是路径的问题，后来去网上查了很多资料，各种各样但都没有效果，没办法接着分析自己写的代码逻辑，仔细看了看MainActivity中的iniMediaPlayer()方法，觉得很有问题，下面贴代码：

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    private MediaPlayer mediaPlayer = new MediaPlayer();
    public List musicList = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button play = (Button)findViewById(R.id.play);
        Button stop = (Button)findViewById(R.id.stop);
        Button pause = (Button)findViewById(R.id.pause);
        ListView listView = (ListView)findViewById(R.id.list_view);
        play.setOnClickListener(this);
        stop.setOnClickListener(this);
        pause.setOnClickListener(this);
        if (Environment.getExternalStorageDirectory().listFiles(new MusicFilter()).length > 0){
            for (File file : Environment.getExternalStorageDirectory().listFiles(new MusicFilter())) {
                musicList.add(file.getAbsolutePath().substring(20));
                }
        }
        ArrayAdapter adapter = new ArrayAdapter<>(MainActivity.this,
                android.R.layout.simple_list_item_1, musicList);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                String SD = "/storage/emulated/0/";
                String path =  musicList.get(i).toString();
                String SP = SD + path;
                Toast.makeText(MainActivity.this,SP,Toast.LENGTH_SHORT).show();
                try {

                     mediaPlayer.setDataSource(SP);
                     mediaPlayer.prepare();
                    mediaPlayer.start();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
        if (ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.WRITE_EXTERNAL_STORAGE)
                != PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(MainActivity.this,
                    new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE },1);
        }else{
            initMediaPlayer();
        }
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.play:
                if (!mediaPlayer.isPlaying()){
                    mediaPlayer.start();
                }
                break;

            case R.id.stop:
                if (mediaPlayer.isPlaying()){
                    mediaPlayer.reset();
                    initMediaPlayer();
                }
                break;
            case R.id.pause:
                if (mediaPlayer.isPlaying()){
                    mediaPlayer.pause();
                }
                break;
            default:
                break;
        }

    }
    private void initMediaPlayer(){
        try {
            File file = new File(Environment.getExternalStorageDirectory(),"m.mp3");
            mediaPlayer.setDataSource(file.getPath());
            mediaPlayer.prepare();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        switch (requestCode){
            case 1:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                    initMediaPlayer();
                }else{
                    Toast.makeText(this,"无权限",Toast.LENGTH_SHORT).show();
                    finish();
                }
                break;
            default:
        }
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (mediaPlayer != null){
            mediaPlayer.stop();
            mediaPlayer.release();
        }
    }
}

 try {
                    mediaPlayer.reset();
                     mediaPlayer.setDataSource(SP);
                     mediaPlayer.prepare();
                    mediaPlayer.start();
                } catch (IOException e) {
                    e.printStackTrace();
                }

仔细想想，每次使用MediaPlarer都是要先初始化和释放资源的，也就是要reset()的，之前在设置listview点击事件时没有注意到这点，所以一直有这个异常，而这个异常可以解释为：因为java里面的mediaplayer对象的状态和native的对象状态发生了不一致。也就是说，你在用MediaPlayer之前没有初始化或者没有回复到原来刚创建是的状态，所以只要加上这一句就可以了: