Android 应用上下文 null 在 Activity.onCreate()

Question

This code in Activity

public void onCreate(Bundle bundle)
{
    super.onCreate(bundle);
    DisplayMetrics metrics = App.getAppContext().getResources().getDisplayMetrics(); //crash
}

sometimes produces a NullPointerException .

This is the custom App class:

public class App extends Application
{
    private static Context context;

    public void onCreate() {
        super.onCreate();
        context = getApplicationContext();
    }

    public static Context getAppContext()
    {
        return context;
    }
}

How can the NullPointerException be fixed?

Answer 1

Replace:

DisplayMetrics metrics = App.getAppContext().getResources().getDisplayMetrics();

with:

DisplayMetrics metrics = getResources().getDisplayMetrics();

Not only do you not need the Application here, anything GUI-related should be using an Activity anyway.

Answer 2

You don't need App.getAppContext()

just

 DisplayMetrics metrics = getResources().getDisplayMetrics();

or

DisplayMetrics metrics = this.getResources().getDisplayMetrics();

or

DisplayMetrics metrics = getApplicationContext().getResources().getDisplayMetrics();