Android: extending Application class. Why do we need to implement singleton pattern?

Question

I'd like to put my database connection object to Application class. I read some tutorials and everywhere I see explicit implementation of a singleton pattern. Something like this

public class MyApplication extends Application {
    private static MyApplication singleton;

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

My question is why do I need to do this explicit implementation? Nobody explains.

I'd like to add there some property, initialized in onCreate() and get it's value in activity.

myProp = (MyApplication)getApplicationContext().getMyProperty()

Is it really necessary to implement singleton pattern?

Answer 1

An Application class is an access point to application context and generally it is used as a initializator for all application-scoped dependencies like your database object. This class is initialized only once per application and persists in memory until application is no longer in memory. So it is a natively created singleton.

By having such static access point to application you may have access to the application context in any class and in general case this context is much better for retrieving resources, system services etc. when you need such dependencies in your custom classes, because it doesn't hold a link to the activity and view so is a leak-safe. Of course in your example the Activity has an access to application, but you have to cast context anyway and better use same approach for a whole app.

Despite you may use it like a global access point to all application scoped and initialized dependencies, do not overload it with huge initialization logic, better create some other singletons for this purpose and just initialize it from Application class. In a big project consider using DI framework, Dagger the best one, for providing dependencies.

Answer 2

By default the application class is singleton..You are not have allowed to create application yourself.Android System will create this when the first time app is loaded into the memory.You are using singleton pattern here just to get Application object from anywhere within the application