java - 当我在 Application 的构造函数中调用 android.content.Context.getApplicationContext() 时，为什么会出现 NPE？

Question

我的应用程序类看起来与此类似，

public class MyApplication extends Application {

    private static final String TAG = MyApplication.class.getSimpleName();
    public static Context mContext;

    public MyApplication() {
        Log.i(TAG, "Inside my application.. " + this);
        Log.i(TAG, "Value of application context is  : " + this.getApplicationContext());
        mContext = this;
    }
} 

当我执行上述操作时，我收到 NPE 并且我的应用程序崩溃了，

12-15 12:03:57.886  8174  8174 D AndroidRuntime: Shutting down VM
12-15 12:03:57.890  8174  8174 E AndroidRuntime: FATAL EXCEPTION: main
12-15 12:03:57.890  8174  8174 E AndroidRuntime: Process: com.example.gm, PID: 8174
12-15 12:03:57.890  8174  8174 E AndroidRuntime: java.lang.RuntimeException: Unable to instantiate application com.example.gm.MyApplication: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.LoadedApk.makeApplication(LoadedApk.java:1087)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5905)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.ActivityThread.access$1100(ActivityThread.java:207)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1660)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.os.Handler.dispatchMessage(Handler.java:106)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.os.Looper.loop(Looper.java:193)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.ActivityThread.main(ActivityThread.java:6734)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at java.lang.reflect.Method.invoke(Native Method)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
12-15 12:03:57.890  8174  8174 E AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:116)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at com.example.gm.MyApplication.<init>(MyApplication.java:12)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at java.lang.Class.newInstance(Native Method)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.AppComponentFactory.instantiateApplication(AppComponentFactory.java:50)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at androidx.core.app.CoreComponentFactory.instantiateApplication(CoreComponentFactory.java:49)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.Instrumentation.newApplication(Instrumentation.java:1120)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    at android.app.LoadedApk.makeApplication(LoadedApk.java:1079)
12-15 12:03:57.890  8174  8174 E AndroidRuntime:    ... 9 more

而在一个类的纯java代码中，看起来像这样，

public class Student {
    int age;
    String name;
    String badgeid;

    public Student(int age, String name) {
        this.age = age;
        this.name = name;
        this.badgeid = this.age + this.name;
    }
}

当我执行 this.age 和 this.name 时，不会导致 NPE。上面的应用程序 fragment 与下面的有何不同？

Answer 1

How do the above application snippet is something different than the below?

Student 没有父类(super class)(默认情况下 Object 除外)。您正在引用您自己在 Student 上定义的字段。

Application 具有父类(super class)，如 the documentation 中所示。 。因此，让我们为您的 Student 添加一个父类(super class):

public class Base {
  String something;

  public void onCreate() {
    something = "a value";
  }
}

public class Student extends Base {
    int age;
    String name;
    String badgeid;
    int value;

    public Student(int age, String name) {
        this.age = age;
        this.name = name;
        this.badgeid = this.age + this.name;
        this.value = something.length;
    }
}

在这里，您将因 NullPointerException 崩溃，因为在您尝试调用 something.length 时，something 为 null 。在调用 onCreate() 之前，父类(super class)不会将 something 设置为值。您太早引用了某些内容。

同样，在适当的时间之前，您无法调用 Context 公开的方法。对于 Application、Activity 和 Service 而言，即在 onCreate() 调用 之后super.onCreate().