ExceptionInInitializerError - static final variables

Question

The following code sometimes produces ExceptionInInitializerError . Apparently the static variable res is null although it is used after it should have been initialized. What could be the reason?

public class GameActivity extends Activity {
    private static final Resources res = App.getAppContext().getResources();
    public static final String foo = res.getString(R.string.foo); //NullPointer here
}

public class App extends Application {
    private static Context context;
    public void onCreate() {
        super.onCreate();
        context = getApplicationContext();
    }   
    public static Context getAppContext() {
        return context;
    }
}

Answer 1

You can not access resources before app initialize. static fields are generated in .dex file with static definition, mean they should be static like "foo" . But you can not use resources in static initialization with app resources.

Change your activity class by this.

public class GameActivity extends Activity {
    private static Resources res;
    public static String foo;

    @Override
    protected void onCreate(@android.support.annotation.Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        res = App.getAppContext().getResources();
        foo = res.getString(R.string.foo);
    }
}

By the way why you need to create Resources object when you have directly access to getString method in Activity class

You can just do this.

public class GameActivity extends Activity {
    public static String foo;

    @Override
    protected void onCreate(@android.support.annotation.Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        foo = getString(R.string.foo);
    }
}

Answer 2

First Declare variables then after get a value in onCreate(), you missed onCreate() method.

 public class GameActivity extends Activity {
        Context context;
 private static Resources res;
    public static String foo;

        @Override
        protected void onCreate(@Nullable Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_add_address);
            context = this;
            init();
        }

        private void init() {
            res = App.getAppContext().getResources();
            foo = res.getString(R.string.foo); //NullPointer here
        }

        public class App extends Application {
            private static Context context;

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

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

Directly you can get a lause of string from string.xml file like this

public class GameActivity extends Activity {
    Context context;
    public String foo;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_address);
        context = this;
        init();
    }

    private void init() {
        foo = getString(R.string.foo);
    }
}

Answer 3

代替使用res = App.getAppContext().getResources()只需调用res = getApplicationContext().getResources() 。