An extended Application class can declare global variables. Are there other reasons?
Introduction:
If we consider an apk file in our mobile, it is comprised of
multiple useful blocks such as, Activitys, Services and
others.
These components do not communicate with each other regularly and
not forget they have their own life cycle. which indicate that
they may be active at one time and inactive the other moment.
Requirements:
Sometimes we may require a scenario where we need to access a
variable and its states across the entire Application regardless of
the Activity the user is using,
An example is that a user might need to access a variable that holds his
personnel information (e.g. name) that has to be accessed across the
Application,
We can use SQLite but creating a Cursor and closing it again and
again is not good on performance,
We could use Intents to pass the data but it's clumsy and activity
itself may not exist at a certain scenario depending on the memory-availability.
Uses of Application Class:
Access to variables across the Application,
You can use the Application to start certain things like analytics
etc. since the application class is started before Activitys or
Servicess are being run,
There is an overridden method called onConfigurationChanged() that is
triggered when the application configuration is changed (horizontal
to vertical & vice-versa),
There is also an event called onLowMemory() that is triggered when
the Android device is low on memory.
Application class is the object that has the full lifecycle of your application. It is your highest layer as an application. example possible usages:
You can add what you need when the application is started by overriding onCreate in the Application class.
store global variables that jump from Activity to Activity. Like Asynctask.
etc
Sometimes you want to store data, like global variables which need to be accessed from multiple Activities - sometimes everywhere within the application. In this case, the Application object will help you.
For example, if you want to get the basic authentication data for each http request, you can implement the methods for authentication data in the application object.
After this,you can get the username and password in any of the activities like this:
MyApplication mApplication = (MyApplication)getApplicationContext();
String username = mApplication.getUsername();
String password = mApplication.getPassword();
And finally, do remember to use the Application object as a singleton object:
public class MyApplication extends Application {
private static MyApplication singleton;
public MyApplication getInstance(){
return singleton;
}
#Override
public void onCreate() {
super.onCreate();
singleton = this;
}
}
For more information, please Click Application Class
Offhand, I can't think of a real scenario in which extending Application is either preferable to another approach or necessary to accomplish something. If you have an expensive, frequently used object you can initialize it in an IntentService when you detect that the object isn't currently present. Application itself runs on the UI thread, while IntentService runs on its own thread.
I prefer to pass data from Activity to Activity with explicit Intents, or use SharedPreferences. There are also ways to pass data from a Fragment to its parent Activity using interfaces.
The Application class is a singleton that you can access from any activity or anywhere else you have a Context object.
You also get a little bit of lifecycle.
You could use the Application's onCreate method to instantiate expensive, but frequently used objects like an analytics helper. Then you can access and use those objects everywhere.
Best use of application class.
Example: Suppose you need to restart your alarm manager on boot completed.
public class BaseJuiceApplication extends Application implements BootListener {
public static BaseJuiceApplication instance = null;
public static Context getInstance() {
if (null == instance) {
instance = new BaseJuiceApplication();
}
return instance;
}
#Override
public void onCreate() {
super.onCreate();
}
#Override
public void onBootCompleted(Context context, Intent intent) {
new PushService().scheduleService(getInstance());
//startToNotify(context);
}
Not an answer but an observation: keep in mind that the data in the extended application object should not be tied to an instance of an activity, as it is possible that you have two instances of the same activity running at the same time (one in the foreground and one not being visible).
For example, you start your activity normally through the launcher, then "minimize" it. You then start another app (ie Tasker) which starts another instance of your activitiy, for example in order to create a shortcut, because your app supports android.intent.action.CREATE_SHORTCUT. If the shortcut is then created and this shortcut-creating invocation of the activity modified the data the application object, then the activity running in the background will start to use this modified application object once it is brought back to the foreground.
I see that this question is missing an answer. I extend Application because I use Bill Pugh Singleton implementation (see reference) and some of my singletons need context. The Application class looks like this:
public class MyApplication extends Application {
private static final String TAG = MyApplication.class.getSimpleName();
private static MyApplication sInstance;
#Contract(pure = true)
#Nullable
public static Context getAppContext() {
return sInstance;
}
#Override
public void onCreate() {
super.onCreate();
Log.d(TAG, "onCreate() called");
sInstance = this;
}
}
And the singletons look like this:
public class DataManager {
private static final String TAG = DataManager.class.getSimpleName();
#Contract(pure = true)
public static DataManager getInstance() {
return InstanceHolder.INSTANCE;
}
private DataManager() {
doStuffRequiringContext(MyApplication.getAppContext());
}
private static final class InstanceHolder {
#SuppressLint("StaticFieldLeak")
private static final DataManager INSTANCE = new DataManager();
}
}
This way I don't need to have a context every time I'm using a singleton and get lazy synchronized initialization with minimal amount of code.
Tip: updating Android Studio singleton template saves a lot of time.
I think you can use the Application class for many things, but they are all tied to your need to do some stuff BEFORE any of your Activities or Services are started.
For instance, in my application I use custom fonts. Instead of calling
Typeface.createFromAsset()
from every Activity to get references for my fonts from the Assets folder (this is bad because it will result in memory leak as you are keeping a reference to assets every time you call that method), I do this from the onCreate() method in my Application class:
private App appInstance;
Typeface quickSandRegular;
...
public void onCreate() {
super.onCreate();
appInstance = this;
quicksandRegular = Typeface.createFromAsset(getApplicationContext().getAssets(),
"fonts/Quicksand-Regular.otf");
...
}
Now, I also have a method defined like this:
public static App getAppInstance() {
return appInstance;
}
and this:
public Typeface getQuickSandRegular() {
return quicksandRegular;
}
So, from anywhere in my application, all I have to do is:
App.getAppInstance().getQuickSandRegular()
Another use for the Application class for me is to check if the device is connected to the Internet BEFORE activities and services that require a connection actually start and take necessary action.
Source: https://github.com/codepath/android_guides/wiki/Understanding-the-Android-Application-Class
In many apps, there's no need to work with an application class directly. However, there are a few acceptable uses of a custom application class:
Specialized tasks that need to run before the creation of your first activity
Global initialization that needs to be shared across all components (crash reporting, persistence)
Static methods for easy access to static immutable data such as a shared network client object
You should never store mutable instance data inside the Application object because if you assume that your data will stay there, your application will inevitably crash at some point with a NullPointerException. The application object is not guaranteed to stay in memory forever, it will get killed. Contrary to popular belief, the app won’t be restarted from scratch. Android will create a new Application object and start the activity where the user was before to give the illusion that the application was never killed in the first place.
To add onto the other answers that state that you might wish store variables in the application scope, for any long-running threads or other objects that need binding to your application where you are NOT using an activity (application is not an activity).. such as not being able to request a binded service.. then binding to the application instance is preferred. The only obvious warning with this approach is that the objects live for as long as the application is alive, so more implicit control over memory is required else you'll encounter memory-related problems like leaks.
Something else you may find useful is that in the order of operations, the application starts first before any activities. In this timeframe, you can prepare any necessary housekeeping that would occur before your first activity if you so desired.
2018-10-19 11:31:55.246 8643-8643/: application created
2018-10-19 11:31:55.630 8643-8643/: activity created
You can access variables to any class without creating objects, if its extended by Application. They can be called globally and their state is maintained till application is not killed.
The use of extending application just make your application sure for any kind of operation that you want throughout your application running period. Now it may be any kind of variables and suppose if you want to fetch some data from server then you can put your asynctask in application so it will fetch each time and continuously, so that you will get a updated data automatically.. Use this link for more knowledge....
http://www.intridea.com/blog/2011/5/24/how-to-use-application-object-of-android
I'm utilizing Firebase in my application and I figure it'd be better practice if I constantly observed the AuthState once the user has signed up or logged in and updated one class that can be accessed globally throughout my application (but that can only be instantiated once). Do I use a Singleton design pattern for this? How do I accomplish something like this in Android?
A common pattern is to create your own subclass of Android's Application class (specified in AndroidManifest.xml as shown below for example) and store variable there....effectively a singleton.
<application
android:name=".MyApplication"
In my app I use a static way for doing something similar.. maybe it helps to find solution for your situation:
I created a class which can not be instantiated because the constructor is set private and the class is used as some "variable-holder" accessible from all app statically. Here is the class:
VarHolder.java
package your.package.name;
public class VarHolder {
private VarHolder() {}
static int myInt;
static String myString;
}
than in app I use it this way:
VarHolder.myInt = 1;
VarHolder.myString = "Hello";
on another place in app :
textView.setText(VarHolder.myString);
This is the Nth question about how to store #Singleton scoped Dagger 2 Components whose lifetime should equal the application's lifetime.
In Android apps using Dagger 2 there is usually at least one Component which is #Singleton scoped and should last for all the application's lifetime: because of these requirements it is usually initialised and stored inside a custom Application class.
Since the instance of this Component must be reachable in all parts of our Application I've seen code like this:
1. Store the component in a public static variable inside the application class.
public class App extends Application {
public static AppComponent appComponent;
#Override
public void onCreate() {
super.onCreate();
appComponent = DaggerAppComponent.builder()
.appModule(new AppModule(this)).build();
}
}
This way it can be accessed anywhere else with:
App.appComponent.inject(this);
2. Store the component in a private variable inside the application instance and create a static accessor for it.
public class App extends Application {
private static AppComponent appComponent;
#Override
public void onCreate() {
super.onCreate();
appComponent = DaggerAppComponent.builder()
.appModule(new AppModule(this)).build();
}
public static AppComponent getAppComponent() {
return appComponent;
}
}
This way it can be accessed anywhere else with:
App.getAppComponent().inject(this);
3. Store the component in a private variable inside the application instance and create a non static accessor for it.
public class App extends Application {
private AppComponent appComponent;
#Override
public void onCreate() {
super.onCreate();
appComponent = DaggerAppComponent.builder()
.appModule(new AppModule(this)).build();
}
public AppComponent getAppComponent() {
return appComponent;
}
}
This way it can be accessed only from class instances which hold a reference to a Context:
// From within an Activity.
((App) getApplication()).getAppComponent().inject(this);
// From within a Fragment.
((App) getActivity().getApplication()).getAppComponent().inject(this);
// From within any other class which holds a reference to a Context.
((App) context.getApplicationContext()).getAppComponent().inject(this);
This last way makes it pretty much compulsory to pass a Context reference to any class willing to access the Component (even if that Context isn't needed by that class for any other purposes).
IMHO having to "manually inject" a Context instance only to access the injector itself sounds a bit counter intuitive.
On the other side many advise against using static variables but: why? If an object must stay in memory for the application's lifetime (which means for the whole lifetime of the JVM instance) what's the problem if it's stored in a static variable?
Others say that static stuff can't be mocked in tests and it's true, though I'm not sure I totally get this because it is the DI pattern which enables easy mocking/testing and not the injector itself, so why would we want to mock the injector itself?
What are the pros and cons of these alternatives? Are there any other possible alternatives besides the ones already mentioned here?
With 1 and 2 you are using static references. This is a good thread about why to avoid them
Why are static variables considered evil?
So the only option left is the 3rd. That is what I am using on my projects.
About if you should pass the context as argument or not, depends on the architecture of your project and how you designed the Dagger dependencies. Personally I don't have that problem because I am only injecting in Activities/Fragments. Can you give me an example where you need to pass the context to inject dependencies?
I use method #2. The main problem with method #1 is that you're exposing a mutable field. If your module doesn't require a Context to construct, you could make the field final. But as a matter of style, I still prefer not to expose fields.
You should normally avoid global state, especially in Android because of the complex and sometimes unintuitive lifecycles of components and the VM itself. But Application is the exception to this rule. Exactly one instance of it exists per VM, and its onCreate() method is called exactly once, before any other component is created. This makes it an acceptable place to create and store a static singleton.
I have found one answer that appears to say I should create a separate class and make a static MyApplication object and make a get method. Then any class can call MyApplication.get() to retrieve the context.
Is there any other cleaner way? This is my situation:
I have a class A and a class B. Class A contains an object from class B (let's call the object b). In class A I call, "b.play()". However, I get a null pointer exception because class B needs to pass a context to the MediaPlayer.create() method.
Until now I threw together a hack and from class A I called.... "b.play(this)" and simply passed the context to B. However that is pretty ugly and looks like a bad use of OOP.
Any thoughts?
This problem seem to arise a lot in Android development. One solution to obtaining a reference to a specific Context is subclassing the Application and grab a reference to the Context which you want.
public class MyApplication extends Application {
private Context context;
#Override
public onCreate() {
super.onCreate();
this.context = getApplicationContext() // Grab the Context you want.
}
public static Context getApplicationContext() { return this.context; }
}
This solution however requires that you specify the name of your subclass in your manifest.
<application
android:name=".MyApplication"
</application>
You can then use this anywhere in your application like this in non-activity classes.
MyApplication.getContext(); // Do something with the context! :)
If class B requires a Context to operate, then I don't see any problem having class A provide that to it (through a parameter on the play method, a parameter in a constructor, etc).
I don't think you are doing any poor OOP by providing class B the dependencies that it needs to do it's job.
Passing this around is a viable way of doing things, especially if this is the activity that creates the object in need of a Context. Sometimes, I'll put the Context into the constructor (like public MyObject(Context context){this.context = context;}), so that you don't need to send it every time. However, if your object is shared across multiple Activities, you should probably update the context it is looking at with the new Activity, though I haven't tested what happens if you use the old activity.
I've answered also here.
You can do that using ContextWrapper, as described here.
For example:
public class MyContextWrapper extends ContextWrapper {
public MyContextWrapper(Context base) {
super(base);
}
public void someMethod() {
// MediaPlayer.create(this, ...)
}
}
regarding the Android Assets manager, is it possible to get a file from the assets without context? I read that I can initialize a File object with "file:///android_assets" kind of path, and then get the file from there, but it says that the file does not exist (Honeycomb).
What is the general workaround around this issue? There is no any way to get the context on the place I need the files from the assets.
Thanks
To get access to the Context where it's not present you can extend Application class and create the static reference to the Context. This way you can access it from anywhere within the application:
public class MyApp extends Application {
public static Context context;
#Override
public void onCreate() {
super.onCreate();
context = getApplicationContext();
}
}