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: