Question

我创建了一个应用类，现在我正在将其实例用于其他类但我面临上下文无效问题。

我的申请类：

public class CommonForApp extends Application {
private static CommonForApp sInstance;
private Utilities mUtilities = null;
private static SharedPreferences mSharedPreferences;

@Override
    public void onCreate() {
        super.onCreate();
        mSharedPreferences = this.getSharedPreferences(
                "com.myandroidapp.musicplayer", Context.MODE_PRIVATE);
        mContext=this.getApplicationContext();
        mUtilities = new Utilities();
    }

    public CommonForApp() {
    }
    public static  CommonForApp getInstance() {
        if (sInstance == null)
            sInstance = new CommonForApp();

        return sInstance;
    }

}

并在AndroidManifest.xml中声明它：

 <application
        android:name="com.android.musicplayer.datastorageandarraylistsaccesser.CommonForApp"
         android:largeHeap="true"
 android:icon="@drawable/ic_launcher"
        android:label="Music" android:allowBackup="true">

现在我在其他类中使用它：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);

         mApp=CommonForApp.getInstance();
         final List<GlobalSongDetails> s=mApp.getGlobalSongsList();

}

我收到以下错误：

03-02 18:50:41.167: E/AndroidRuntime(16177): FATAL EXCEPTION: main
03-02 18:50:41.167: E/AndroidRuntime(16177): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.androidhive.musicplayer/com.android.musicplayer.Fragmentactivity}: java.lang.NullPointerException
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2306)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.ActivityThread.access$600(ActivityThread.java:156)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1340)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.os.Looper.loop(Looper.java:153)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.ActivityThread.main(ActivityThread.java:5297)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at java.lang.reflect.Method.invokeNative(Native Method)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at java.lang.reflect.Method.invoke(Method.java:511)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at dalvik.system.NativeStart.main(Native Method)
03-02 18:50:41.167: E/AndroidRuntime(16177): Caused by: java.lang.NullPointerException
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:109)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at com.android.musicplayer.datastorageandarraylistsaccesser.CommonForApp.makeGlobalSongsList(CommonForApp.java:127)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at com.android.musicplayer.datastorageandarraylistsaccesser.CommonForApp.getGlobalSongsList(CommonForApp.java:164)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at com.android.musicplayer.Fragmentactivity.onCreate(Fragmentactivity.java:156)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.Activity.performCreate(Activity.java:5122)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-02 18:50:41.167: E/AndroidRuntime(16177):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2270)
03-02 18:50:41.167: E/AndroidRuntime(16177):    ... 11 more

但如果我像这样制作实例：

mApp=(CommonForApp) this.getApplicationContext();

我没有收到任何错误。

有谁能解释我在做什么错误？

先谢谢！

Answer 1

问题是在getInstance()中使用new手动创建应用程序对象的实例。你不能这样做。只有系统实例化应用程序对象。

如果你想使用应用程序的单例，你必须这样使用它：

public class CommonForApp extends Application {

    private static CommonForApp sInstance;
    …

    @Override
    public void onCreate() {
        super.onCreate();
        sInstance = this;
        …
    }

    public static CommonForApp getInstance() {
        return sInstance;
    }
}

请注意，在极少数情况下（例如在内容提供商中），它可以为null。

应用程序类错误

1 个答案: