Singletons vs. Application Context in Android?

Question

Recalling this post enumerating several problems of using singletons and having seen several examples of Android applications using singleton pattern, I wonder if it's a good idea to use Singletons instead of single instances shared through global application state (subclassing android.os.Application and obtaining it through context.getApplication()).

What advantages/drawbacks would both mechanisms have?

To be honest, I expect the same answer in this post Singleton pattern with Web application, Not a good idea! but applied to Android. Am I correct? What's different in DalvikVM otherwise?

EDIT: I would like to have opinions on several aspects involved:

• Synchronization
• Reusability
• Testing

Answer 1

I very much disagree with Dianne Hackborn's response. We are bit by bit removing all singletons from our project in favor of lightweight, task scoped objects which can easiliy be re-created when you actually need them.

Singletons are a nightmare for testing and, if lazily initialized, will introduce "state indeterminism" with subtle side effects (which may suddenly surface when moving calls to getInstance() from one scope to another). Visibility has been mentioned as another problem, and since singletons imply "global" (= random) access to shared state, subtle bugs may arise when not properly synchronized in concurrent applications.

I consider it an anti-pattern, it's a bad object-oriented style that essentially amounts to maintaining global state.

To come back to your question:

Although the app context can be considered a singleton itself, it is framework-managed and has a well defined life-cycle, scope, and access path. Hence I believe that if you do need to manage app-global state, it should go here, nowhere else. For anything else, rethink if you really need a singleton object, or if it would also be possible to rewrite your singleton class to instead instantiate small, short-lived objects that perform the task at hand.

Answer 2

I very much recommend singletons. If you have a singleton that needs a context, have:

MySingleton.getInstance(Context c) {
    //
    // ... needing to create ...
    sInstance = new MySingleton(c.getApplicationContext());
}

I prefer singletons over Application because it helps keep an app much more organized and modular -- instead of having one place where all of your global state across the app needs to be maintained, each separate piece can take care of itself. Also the fact that singletons lazily initialize (at request) instead of leading you down the path of doing all initialization up-front in Application.onCreate() is good.

There is nothing intrinsically wrong with using singletons. Just use them correctly, when it makes sense. The Android framework actually has a lot of them, for it to maintain per-process caches of loaded resources and other such things.

Also for simple applications multithreading doesn't become an issue with singletons, because by design all standard callbacks to the app are dispatched on the main thread of the process so you won't have multi-threading happening unless you introduce it explicitly through threads or implicitly by publishing a content provider or service IBinder to other processes.

Just be thoughtful about what you are doing. :)

Answer 3

From: Developer > reference - Application

There is normally no need to subclass Application. In most situation, static singletons can provide the same functionality in a more modular way. If your singleton needs a global context (for example to register broadcast receivers), the function to retrieve it can be given a Context which internally uses Context.getApplicationContext() when first constructing the singleton.

Answer 4

Application is not the same as the Singleton.The reasons are:

1. Application's method(such as onCreate) is called in the ui thread;
2. singleton's method can be called in any thread;
3. In the method "onCreate" of Application,you can instantiate Handler;
4. If the singleton is executed in none-ui thread,you could not instantiate Handler;
5. Application has the ability to manage the life cycle of the activities in the app.It has the method "registerActivityLifecycleCallbacks".But the singletons has not the ability.

Answer 5

I had the same problem: Singleton or make a subclass android.os.Application?

First I tried with the Singleton but my app at some point makes a call to the browser

Intent myIntent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.google.com"));

and the problem is that, if the handset doesn't have enough memory, most of your classes (even Singletons) are cleaned to get some memory so, when returning from the browser to my app, it crashed everytime.

Solution: put needed data inside a subclass of Application class.

Answer 6

Consider both at the same time:

• having singleton objects as static instances inside the classes.
• having a common class (Context) that returns the singleton instances for all the singelton objects in your application, which has the advantage that the method names in Context will be meaningful for example: context.getLoggedinUser() instead of User.getInstance().

Furthermore, I suggest that you expand your Context to include not only access to singleton objects but some functionalities that need to be accessed globally, like for example: context.logOffUser(), context.readSavedData(), etc. Probably renaming the Context to Facade would make sense then.

Answer 7

They're actually the same. There's one difference I can see. With Application class you can initialize your variables in Application.onCreate() and destroy them in Application.onTerminate(). With singleton you have to rely VM initializing and destroying statics.

Answer 8

From the proverbial horse's mouth...

When developing your app, you may find it necessary to share data, context or services globally across your app. For example, if your app has session data, such as the currently logged-in user, you will likely want to expose this information. In Android, the pattern for solving this problem is to have your android.app.Application instance own all global data, and then treat your Application instance as a singleton with static accessors to the various data and services.

When writing an Android app, you're guaranteed to only have one instance of the android.app.Application class, and so it's safe (and recommended by Google Android team) to treat it as a singleton. That is, you can safely add a static getInstance() method to your Application implementation. Like so:

public class AndroidApplication extends Application {

    private static AndroidApplication sInstance;

    public static AndroidApplication getInstance(){
        return sInstance;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        sInstance = this;
    }
}

Answer 9

My 2 cents:

I did notice that some singleton / static fields were reseted when my activity was destroyed. I noticed this on some low end 2.3 devices.

My case was very simple : I just have a private filed "init_done" and a static method "init" that I called from activity.onCreate(). I notice that the method init was re-executing itself on some re-creation of the activity.

While I cannot prove my affirmation, It may be related to WHEN the singleton/class was created/used first. When the activity get destroyed/recycled, it seem that all class that only this activity refer are recycled too.

I moved my instance of singleton to a sub class of Application. I acces them from the application instance. and, since then, did not notice the problem again.

I hope this can help someone.

Answer 10

My activity calls finish() (which doesn't make it finish immediately, but will do eventually) and calls Google Street Viewer. When I debug it on Eclipse, my connection to the app breaks when Street Viewer is called, which I understand as the (whole) application being closed, supposedly to free up memory (as a single activity being finished shouldn't cause this behavior). Nevertheless, I'm able to save state in a Bundle via onSaveInstanceState() and restore it in the onCreate() method of the next activity in the stack. Either by using a static singleton or subclassing Application I face the application closing and losing state (unless I save it in a Bundle). So from my experience they are the same with regards to state preservation. I noticed that the connection is lost in Android 4.1.2 and 4.2.2 but not on 4.0.7 or 3.2.4, which in my understanding suggests that the memory recovery mechanism has changed at some point.