空指针异常

Question

I released a free app image slider with spoken words for small kids. Weeks ago, I got the first crash report. This seems to happen only rarely (once a week).

The report states: NullPointerException in Splash.onCreate()

Stack tace shows this messages:

java.lang.RuntimeException: Unable to start activity 
java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
    at android.app.ActivityThread.access$600(ActivityThread.java:130)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4745)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at co.uk.musenuovo.michal.Splash.onCreate(Splash.java:18)
    at android.app.Activity.performCreate(Activity.java:5008)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)

My code in spash activity (intial screen displaying image and playing tune for 5 seconds):

import android.app.Activity;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;

public class Splash extends Activity {

    MediaPlayer ourSong;

    @Override
    protected void onCreate(Bundle ShowSplashPage) {
        // TODO Auto-generated method stub
        super.onCreate(ShowSplashPage);
        setContentView(R.layout.splash);
        ourSong = MediaPlayer.create(Splash.this, R.raw.xylophone_open);
        ourSong.start();
        Thread timer = new Thread() {

            public void run() {
                try{
                    sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    Intent openStartingPoint = new Intent("co.uk.musenuovo.michal.STARTINGPOINT");
                    startActivity(openStartingPoint);

                }
            }

        };
        timer.start();

    }

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

}

The error clearly points to onstart in spash activity. Is this because I forgot to import some library? or did I forgot to assign something? I am very new to java and do not know how to debug and find out what is the problem. I read a lot about NullPointerException but this seems to be so vague of an error. Thank you for all your help.

Answer 1

May be the ourSong variable is null..you need to check before using it

ourSong  = MediaPlayer.create(Splash.this, R.raw.xylophone_open);

    if(ourSong  == null) {
        Log.v("LOG", "Create() on MediaPlayer failed.");
    }

Answer 2

您应该始终对来自包的数据进行空检查，包括包本身。

Answer 3

One value is clearly null, it could be the uninitiated variable 'ourSong' or the bundle that you run the method with. Add a line such as System.out.println(ourSong) and/or another for the bundle, and discover which variable is null.

Answer 4

Looks like ourSong is null.

Use simple check:

if(ourSong != null) {
   ourSong.start();
}

And next time when the ourSong will be null it just won't start() instead of crash your app.

Answer 5

Since this is an run time error you won't be probably getting an error while debbugging on your system. Look for every period (.) or open bracket ([) on the line of code that caused the error. Something immediately to the left of one of these periods/brackets must have been the null value. The error would have been generated by

ourSong = MediaPlayer.create(Splash.this, R.raw.xylophone_open);
ourSong.start();

I would suggest you to print value of ourSong before starting the service.