java - 当您在没有点击播放的那一刻按暂停或停止时，Android 广播流媒体应用程序崩溃

Question

当我启动我的应用程序并按下停止或暂停时，Android 应用程序将崩溃。如果您先按播放然后停止或暂停，效果会很好。我在 google 和 stackoverflow 上搜索过，但找不到太多相关信息。我认为问题是因为 NullPointerException，但由于我也是 java 新手，所以它并没有告诉我很多关于这个问题的信息

代码:

     import android.app.Activity;
     import android.media.AudioManager;
     import android.media.MediaPlayer;
     import android.net.Uri;
     import android.os.Bundle;
     import android.util.Log;
     import android.view.View;
     import android.view.View.OnClickListener;
     import android.widget.Button;

     public class myMain extends Activity implements
       MediaPlayer.OnCompletionListener, MediaPlayer.OnPreparedListener,
       MediaPlayer.OnErrorListener, MediaPlayer.OnBufferingUpdateListener, OnClickListener {


      private String TAG = getClass().getSimpleName();
      private MediaPlayer mp= null;

      private Button play;
      private Button pause;
      private Button stop;




      @Override
      public void onCreate(Bundle icicle) {

       super.onCreate(icicle);
       setContentView(R.layout.main);

       play = (Button) findViewById(R.id.play);
       pause = (Button) findViewById(R.id.pause);
       stop = (Button) findViewById(R.id.stop);




       play.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view) {
         play();
        }
       });

       pause.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view){
         pause();
        }
       });

       stop.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view){
         stop();

        }
       });
      }


      private void play() {
       Uri myUri = Uri.parse("url");
       try {
            if (mp == null) {
             this.mp = new MediaPlayer();
            } else {
             mp.stop();
             mp.reset();
            }
        mp.setDataSource(this, myUri); // Go to Initialized state
        mp.setAudioStreamType(AudioManager.STREAM_MUSIC);
        mp.setOnPreparedListener(this);
        mp.setOnBufferingUpdateListener(this);

        mp.setOnErrorListener(this);
        mp.prepareAsync();

        Log.d(TAG, "LoadClip Done");
       } catch (Throwable t) {
        Log.d(TAG, t.toString());
       }
      }

      public void onPrepared(MediaPlayer mp) {
       Log.d(TAG, "Stream is prepared");
       mp.start();
      }

      private void pause() {
           mp.pause();
          }
      private void stop() {
           mp.stop();

          }

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

      }

      public void onCompletion(MediaPlayer mp) {
       stop();
      }

      public boolean onError(MediaPlayer mp, int what, int extra) {
       StringBuilder sb = new StringBuilder();
       sb.append("Media Player Error: ");
       switch (what) {
       case MediaPlayer.MEDIA_ERROR_NOT_VALID_FOR_PROGRESSIVE_PLAYBACK:
        sb.append("Not Valid for Progressive Playback");
        break;
       case MediaPlayer.MEDIA_ERROR_SERVER_DIED:
        sb.append("Server Died");
        break;
       case MediaPlayer.MEDIA_ERROR_UNKNOWN:
        sb.append("Unknown");
        break;
       default:
        sb.append(" Non standard (");
        sb.append(what);
        sb.append(")");
       }
       sb.append(" (" + what + ") ");
       sb.append(extra);
       Log.e(TAG, sb.toString());
       return true;
      }

      public void onBufferingUpdate(MediaPlayer mp, int percent) {
       Log.d(TAG, "PlayerService onBufferingUpdate : " + percent + "%");
      }

     public void onClick(View v) {
         // TODO Auto-generated method stub

     }

         }

错误:

      02-11 20:45:43.837: D/AndroidRuntime(338): Shutting down VM
      02-11 20:45:43.837: W/dalvikvm(338): threadid=1: thread exiting with uncaught exception (group=0x40015560)
      02-11 20:45:43.857: E/AndroidRuntime(338): FATAL EXCEPTION: main
      02-11 20:45:43.857: E/AndroidRuntime(338): java.lang.NullPointerException
      02-11 20:45:43.857: E/AndroidRuntime(338):    at           wadio.media.internetradio.myMain.stop(myMain.java:95)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at           wadio.media.internetradio.myMain.access$2(myMain.java:94)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at wadio.media.internetradio.myMain$3.onClick(myMain.java:55)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at android.view.View.performClick(View.java:2485)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at android.view.View$PerformClick.run(View.java:9080)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at android.os.Handler.handleCallback(Handler.java:587)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at android.os.Handler.dispatchMessage(Handler.java:92)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at android.os.Looper.loop(Looper.java:123)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at android.app.ActivityThread.main(ActivityThread.java:3683)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at java.lang.reflect.Method.invokeNative(Native Method)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at java.lang.reflect.Method.invoke(Method.java:507)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
      02-11 20:45:43.857: E/AndroidRuntime(338):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

在您的stop() 方法中，您访问了mp 变量。但是，mp 变量为空，直到您按下播放键(并且调用了 play() 方法)。因此，当您尝试访问 null 变量时，将抛出 NullPointerException。

停止 NullPointerException 的一种非常简单的方法是执行如下操作:

private void pause() {
    if(mp!=null) mp.pause();
}

private void stop() {
    if(mp!=null) mp.stop();
}

当然，此解决方案不考虑两次调用 pause 或 stop 的情况。看看 MediaPlayer documentation有关状态管理的更多信息。