Question

我在Application子类中得到nullpointerexception。在那里，我想要一个应该可以在整个应用程序中使用的上下文，因此我调用了getApplicationContext（），但这会导致异常以下是logcat消息：

12-22 17:15:35.639: E/AndroidRuntime(572): FATAL EXCEPTION: main
12-22 17:15:35.639: E/AndroidRuntime(572): java.lang.RuntimeException: Unable to instantiate application com.gaurav.contactmanager.model.ContactManagerApplication: java.lang.NullPointerException
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.LoadedApk.makeApplication(LoadedApk.java:466)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.ActivityThread.handleBindApplication(ActivityThread.java:3260)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.ActivityThread.access$2200(ActivityThread.java:117)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:969)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.os.Handler.dispatchMessage(Handler.java:99)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.os.Looper.loop(Looper.java:123)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.ActivityThread.main(ActivityThread.java:3683)
12-22 17:15:35.639: E/AndroidRuntime(572):  at java.lang.reflect.Method.invokeNative(Native Method)
12-22 17:15:35.639: E/AndroidRuntime(572):  at java.lang.reflect.Method.invoke(Method.java:507)
12-22 17:15:35.639: E/AndroidRuntime(572):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
12-22 17:15:35.639: E/AndroidRuntime(572):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
12-22 17:15:35.639: E/AndroidRuntime(572):  at dalvik.system.NativeStart.main(Native Method)
12-22 17:15:35.639: E/AndroidRuntime(572): Caused by: java.lang.NullPointerException
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:100)
12-22 17:15:35.639: E/AndroidRuntime(572):  at com.gaurav.contactmanager.model.ContactManagerApplication.<init>(ContactManagerApplication.java:10)
12-22 17:15:35.639: E/AndroidRuntime(572):  at java.lang.Class.newInstanceImpl(Native Method)
12-22 17:15:35.639: E/AndroidRuntime(572):  at java.lang.Class.newInstance(Class.java:1409)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.Instrumentation.newApplication(Instrumentation.java:957)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.Instrumentation.newApplication(Instrumentation.java:942)
12-22 17:15:35.639: E/AndroidRuntime(572):  at android.app.LoadedApk.makeApplication(LoadedApk.java:461)
12-22 17:15:35.639: E/AndroidRuntime(572):  ... 11 more

以下是源代码：

package com.gaurav.contactmanager.model;

import android.app.Application;
import android.content.Context;
import android.util.Log;

public class ContactManagerApplication extends Application {
    public static Context context = null;
    public ContactManagerApplication() {
        Log.d("@gaurav", getApplicationContext()+"");
        context = getApplicationContext();
    }
}

和android-manifest声明如下：

 <application
        android:name=".model.ContactManagerApplication"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

Answer 1

像这样改变

public class ContactManagerApplication extends Application {
    public static Context context = null;

     @Override
    public void onCreate() {
        super.onCreate();
        Log.d("@gaurav", getApplicationContext()+"");
        context = getApplicationContext();

    }

Answer 2

使用ContactManagerApplication.this代替getApplicationContext()。

Answer 3

您不需要构建Application类的构造函数。它是由android框架自动调用的，你必须覆盖onCreate()类的Application方法。

getApplicationContext()返回应用程序的Context，它由android框架分配，而你已经将它放在你自己的构造函数中，所以android从不调用应用程序预定义的构造函数，所以{{ 1}}当时应用程序类的对象是context，导致null而NullPointerException类正在尝试访问ContextWrapper的上下文。

Answer 4

您的Application类生命周期与Activity的生命周期非常相似。您不需要关心构造函数，而是覆盖已知的onCreate()来进行对象设置：

public class ContactManagerApplication extends Application {
    public static Context context = null;

    @Override
    public void onCreate() {
        super.onCreate();

        Log.d("@gaurav", getApplicationContext()+"");
        context = getApplicationContext();
    }
}

Answer 5

通常你会有更像这样的东西：

@Override
public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.your_layout);
        //After this is when you normally would call the getContext
        Context myContext = this.getApplicationContext();
}

但这只是在这是被调用的类时。如果这不是您正在调用的类并且在被调用的类中，那么您可以将上下文作为实例变量并从那里读取它，或者您可以将上下文作为参数发送到此类的构造函数中然后使用它。但如果是这种情况，那么如果没有直接调用，我就不会看到这个类的重点是活动的扩展。

Application Subclass中的NullPointerException？

5 个答案: