Why getApplicationContext() in constructor of Activity throws null pointer exception?

问题

After some time spent bug hunting it turns out that :

public class MainActivity extends BaseActivity { // BaseActivity extends Activity

    public MainActivity() {
        super();
        getApplicationContext(); // NPE here
    }
}

Why ? Where is this documented ?
Froyo

回答1:

Wait for the end of onCreate to call this method.

public class MainActivity extends BaseActivity { 

    public onCreate(Bundle savedInstanceState) {
        super(savedInstanceState);
        getApplicationContext(); //activity has a context now
    }
}

回答2:

Just to get a feeling of what's going on. Activity extends ContextThemeWrapper which extends ContextWrapper from whom Activity inherits getApplicationContext(). ContextWrapper implements it as :

@Override
public Context  getApplicationContext() {
    return mBase.getApplicationContext(); // mBase is a Context
}

The only public constructor of ContextWrapper is :

 public  ContextWrapper(Context base) {
     mBase = base;
 } 

in ContextThemeWrapper we have :

 public  ContextThemeWrapper() {
     super(null);
 }

and since Activity does not define an explicit constructor the constructor above is called - mBase == null in Activity's constructor - boom.

_{Links from 4.2.2_r1}

来源：https://stackoverflow.com/questions/16004891/why-getapplicationcontext-in-constructor-of-activity-throws-null-pointer-excep