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?
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.
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
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
Short version: what is the best practice way to access the same DB from both activity and from started service?
Long version: I have a case where I start persistent service from activity. Service is populating the DB with data while activity is displaying it on screen. Neither of them is running all the time - service can run when there is no activity, and activity can run when service is not started yet. But service can only be stopped from activity (it never stops itself).
I am using subclass of SQLiteOpenHelper to access the DB. The problem I tripped over is that apparently closing DB in one SQLiteOpenHelper instance closes it in another too.
I can think of these options:
singleton pattern (single SQLiteDatabase instance in a public static var - only activity can close it (because it knows if the service is running or not)) - ugly
check if service/activity is running before closing it in the other one - ugly
build acquire/release into SQLiteOpenHelper's getReadableDatabase()/close()
I would go with 3., but this must be a very common pattern, so I'm wondering if there is such a built-in mechanism in place already - I'd hate to reinvent the wheel.
In other words, what would be the proper way to do it?
The usual paradigm for any database on Android is to have one provider open the database and handle requests as intents. This has the advantage of serializing requests.
Opening databases from different code is completely non-standard.
If you really need an app to notice each time something is added, a broadcast intent from the provider might be the answer.
I have solved the problem using retain/release counter:
public class DBHandler extends SQLiteOpenHelper
{
private static int retainCount = 0;
#Override
public synchronized SQLiteDatabase getReadableDatabase()
{
DBHandler.retainCount++;
return super.getReadableDatabase();
}
#Override
public synchronized SQLiteDatabase getWritableDatabase()
{
DBHandler.retainCount++;
return super.getWritableDatabase();
}
#Override
public synchronized void close()
{
DBHandler.retainCount--;
if (DBHandler.retainCount == 0)
super.close();
}
}
}
I have an app where I store some data in a SQLIte database.
And what I'm trying to do is to get my DB opened only once at the start of my app and then use the same instance of my DB.
The idea is not to open my DB in every activity I need it.
One of the solutions I got was this-create a class that extends Application and then open the DB there:
This is how the code looks like:
public class MyApplication extends Application{
private static DBAdapter db;
public void onCreate()
{
db=new DBAdapter(getApplicationContext());
db.createDatabase();
db.openDataBase();
}
public static DBAdapter getDatabaseAdapter()
{
return db;
}
}
So in every activity of my app where I need to connect with my DB I do something like:
MyApplication myApplication = (MyApplication) this.getApplication();
DBAdapter db= myApplication.getDatabaseAdapter();
But I'm not very sure if my DB gets opened only at the beginning of my app...or it gets opened everytime I do this:
MyApplication myApplication = (MyApplication) this.getApplication();
?
Or if anyone has a better solution please tell me:).Thank u
It will be opened only once because you are calling it in onCreate method of Application class which will be called when the application is starting, before any other application objects have been created. You can look at one of my project here is link.
Hope this help.
EDIT: About Application Class : Android will automatically create an instance of that class and make it available for your entire application. You can access it from any context using the Context.getApplicationContext(). So there will be only one instance of Application which will be shared.
An alternative to your approach can be making one SingleTon Class where you can keep the reference to your open dbconnection so that it could be reused.
Since for any android application there's only single instance for applcation class spread across all the activities so your db gets open only at the start of android application.
I have followed this tutorial to use SQLite db in my android app.
Since I am a beginner I'm having problems understanding "context" parameter used in the example.
I want to call adapter and insert/update/delete records from a class that does not extend activity which in this example stands for context.
Now I don't know what to pass in the adapter as context, since I'm not calling adapter from activity.
Can someone please explain this?
Pass the ActivityName.this as class context as argument to the adapter class's constructor
the ActivityName is the name of the Activityclass in which you are calling the adapter
you could imagine that the context defines WHERE/WHEN the sqlite database exists. sqlite databases do not exist on their own, they exist within the confines of your activity, thus in your activity's context.
for the next steps you must understand that the context is a dynamic "thing" (in reallife you could imagine it as someone's HERE and NOW). the context is individual to the activity and its moment, just as your here and now are yours and yours only and change over time.
if you are calling a class from within your activity, then this does the trick (passing the activity's context from within the activity itself is OK - sorta like you saying to your buddy: this is how i am feeling NOW).
public class MyActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Context contextNew = this;
myClass(contextNew);
an easy all around solution (panacea) would be to create a static Context variable (contextVariable) inside MyActivity and access it directly from without via: MyActivity.contextVariable. but that does not work, because you get an error when you try to declare/use a static Context variable.
So, if you plan on using sqlite inside a service that is NOT called from within the main activity, like, for example, a service triggered by a broadcast receiver (neither a service nor a receiver have a context per se), then you must access the original application's context from within said receiver.
accessing the original activity's context is simple, but far from obvious.
this works for me (thanx #ZiGi and #Cristian):
import android.app.Service;
import android.content.Context;
public class BatchUploadGpsData extends Service {
public Context contextNew;
#Override
public void onCreate() {
contextNew = getApplicationContext();
this is an example from working code that uploads navigation data to a database on the web every time the android device connects to a WIFI network. i have a receiver listening to connectivity changes (existing as a separate class called directly "from" Manifest file).
i hope that this makes sense, if you want more detail on this, check out this post of mine where i have the complete (barebones) code for said receiver.
As you see in the example, there is a context passed to the ToDoAdapter. You can pass activity as a context or activity.getApplicationContext(). Read about context here.