媒体播放器为什么会导致我的应用程序与空指针异常崩溃？

Question

我得到以下logcat的输出：

11-22 20:57:01.394: E/MediaPlayer(1647): error (1, 0) 
11-22 20:57:01.404: D/MediaPlayer(1647): create failed: 
11-22 20:57:01.404: D/MediaPlayer(1647): java.io.IOException: Prepare failed.: status=0x1 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.media.MediaPlayer.prepare(Native Method) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.media.MediaPlayer.create(MediaPlayer.java:668) 
11-22 20:57:01.404: D/MediaPlayer(1647): at com.jesse.abc.Apple.onCreate(Apple.java:25) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.app.ActivityThread.access$1500(ActivityThread.java:117) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.os.Handler.dispatchMessage(Handler.java:99) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.os.Looper.loop(Looper.java:123) 
11-22 20:57:01.404: D/MediaPlayer(1647): at android.app.ActivityThread.main(ActivityThread.java:3683) 
11-22 20:57:01.404: D/MediaPlayer(1647): at java.lang.reflect.Method.invokeNative(Native Method) 
11-22 20:57:01.404: D/MediaPlayer(1647): at java.lang.reflect.Method.invoke(Method.java:507) 
11-22 20:57:01.404: D/MediaPlayer(1647): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 
11-22 20:57:01.404: D/MediaPlayer(1647): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 
11-22 20:57:01.404: D/MediaPlayer(1647): at dalvik.system.NativeStart.main(Native Method) 
11-22 20:57:01.414: D/AndroidRuntime(1647): Shutting down VM 
11-22 20:57:01.414: W/dalvikvm(1647): threadid=1: thread exiting with uncaught exception (group=0x40015560) 
11-22 20:57:01.414: E/AndroidRuntime(1647): FATAL EXCEPTION: main 
11-22 20:57:01.414: E/AndroidRuntime(1647): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.jesse.abc/com.jesse.abc.Apple}: java.lang.NullPointerException 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.ActivityThread.access$1500(ActivityThread.java:117) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.os.Handler.dispatchMessage(Handler.java:99) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.os.Looper.loop(Looper.java:123) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.ActivityThread.main(ActivityThread.java:3683) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at java.lang.reflect.Method.invokeNative(Native Method) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at java.lang.reflect.Method.invoke(Method.java:507) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at dalvik.system.NativeStart.main(Native Method) 
11-22 20:57:01.414: E/AndroidRuntime(1647): Caused by: java.lang.NullPointerException 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at com.jesse.abc.Apple.onCreate(Apple.java:32) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611) 
11-22 20:57:01.414: E/AndroidRuntime(1647):  ... 11 more 
11-22 20:57:02.934: I/Process(1647): Sending signal. PID: 1647 SIG: 9 

这是我的代码：

我敢肯定，我的清单和布局文件都OK。错误指向第32行，即myPlayer.start();。当我在测试过程中重复地在各个活动之间来回切换时，才会发生崩溃。任何帮助将不胜感激。

package com.jesse.abc; 

import android.app.Activity; 
import android.content.Context; 
import android.content.Intent; 
import android.media.MediaPlayer; 
import android.os.Bundle; 
import android.view.KeyEvent; 
import android.view.View; 
import android.widget.Button; 
import android.widget.ImageButton; 

public class Car extends Activity{ 

MediaPlayer mPlayer; 
MediaPlayer myPlayer; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    // TODO Auto-generated method stub 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.cletter); 

    mPlayer = MediaPlayer.create(this, R.raw.button_sound1); 
    myPlayer = MediaPlayer.create(this, R.raw.carsound); 
    final Button bButton = (Button)findViewById(R.id.cbackbutton); 
    final Button nButton = (Button)findViewById(R.id.cnextbutton); 
    final ImageButton cButton = (ImageButton)findViewById(R.id.cButton); 
    final Context ctx = this; 

    myPlayer.start(); 



    bButton.setOnClickListener(new View.OnClickListener() { 

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

      mPlayer.start(); 
      Intent myIntent = new Intent(v.getContext(), Ball.class); 
      ctx.startActivity(myIntent); 
      myPlayer.release(); 

     } 
    }); 

    nButton.setOnClickListener(new View.OnClickListener() { 

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

      mPlayer.start(); 
      Intent myIntent2 = new Intent(v.getContext(), Duck.class); 
      ctx.startActivity(myIntent2); 
      myPlayer.release(); 

     } 
    }); 

    cButton.setOnClickListener(new View.OnClickListener() { 

     public void onClick(View v) { 
      // TODO Auto-generated method stub 
      myPlayer.start(); 

     } 
    }); 


} 
@Override 
public boolean onKeyDown(int keyCode, KeyEvent event) { 
    // TODO Auto-generated method stub 

    if(keyCode==KeyEvent.KEYCODE_BACK){ 

     this.startActivity(new Intent(Car.this, StartingPoint.class)); 
     myPlayer.release(); 
     finish(); 

    } 
    return true; 
} 



} 

Answer 1

是的，它肯定会崩溃。

 myPlayer.release(); 
     myPlayer=null; 
     myPlayer.start(); 

@Override 
protected void onPause() { 
    // TODO Auto-generated method stub 
    super.onPause(); 
    myPlayer.release(); 
    myPlayer=null; 
    finish(); 
} 

你的对象myPlayer是null，所以它崩溃:)

Answer 2

你刚才已经回答你自己的问题。 myPlayer为null。调试并一直到该行，将鼠标悬停在myPlayer上，它是否为空？你说在切换进出你的应用程序时会发生这种情况，你可能不得不切换，然后返回，然后调试到该行。