public abstract class MyRoomDatabase extends RoomDatabase {

  private static MyRoomDatabase sInstance;

  public static MyRoomDatabase getInstance(Context context) {
    if (sInstance == null) {
      sInstance = Room.databaseBuilder(context.getApplicationContext(), MyRoomDatabase.class, "MyDb.db").build(); // یا هر Initilization ی دیگه ای
    }
    return sInstance;
  }
}

public class MyApp extends Application {

  private static MyApp sInstance;

  @Override
  public void onCreate() {
    super.onCreate();
    sInstance = this;
  }
	
  public static MyApp getInstance() {
    return sInstance;
  }
}