Let's assume we have a custom App extends Application.
I understand that wherever possible getApplicationContext() method should be used, but it's only applicable if you're within a scope that provides this method. Sometimes you need to access it outside of it and that's why I started wondering.
Here's the sample code
public class App extends Application
{
private static App instance = null;
public static App getInstance()
{
return instance;
}
public static Context getContext()
{
return instance;
}
#Override
public void onCreate()
{
instance = this;
super.onCreate();
}
}
Does App.getContext() return the same context getApplicationContext() would return in an appropriate scope? Is it safe to bind the App context like this?
Lifetime of statics is practically the same as your Application. It's safe in the sense that there won't be a memory leak and it will always refer to the correct Application.
getApplicationContext() returns the Application object which also is-a Context.
However, static data like this is a design smell you should try to avoid. It adds unnecessary coupling to your code, making components harder to test in isolation. Instead, consider passing a Context as a parameter where one is needed.
For a good article about what a particular kind of Context such as Application is good for, read http://www.doubleencore.com/2013/06/context/
Problem using Application
I'm rewriting an app (first 'version' had little-to-nothing in terms of analysis and it ended up piling a bunch of problems I wanted to get rid of) and I'm bumping my head against a problem that never showed up in the first version.
Thing is: I have a Class for geographical data. It just supplies String arrays that I can tuck into spinners adapters. Since I used a values xml file, the class needs access to Context to get the proper resources.
Since I use this geographical data in several points of the app, I thought I could create a Class to extend Application and, in onCreate, instantiate the Geography Class, I thought it would be more efficient to load it just once and use it as many times as I wanted. This worked on my first version:
This is MyApplication class
private static Context context;
public void onCreate(){
super.onCreate();
MyApplication.context = getApplicationContext();
geografiaEspana = GeographyClass.getInstance(context);
}
public static GeographyClass getGeografiaEspana() {
if(ctx==null){
Log.w("TAPABOOK", "Tapabook.context nulo");
}
if (geografiaEspana==null){
Log.w("TAPABOOK", "Tapabook.geografiaEspana nula, instanciando");
geografiaEspana = GeographyClass.getInstance(ctx);
}
Log.i("TAPABOOK", "Tapabook.geografiaEspana instanciada");
return geografiaEspana;
}
And this is my GeographyClass
private static GeographyClass instance = null;
public static GeographySpain getInstance(Context context){
if(instance== null){
instance = new GeographySpain(context);
}
return instance;
}
public GeographySpain(Context context){
Resources res = context.getResources();
// load resources data
}
This worked, as I said, ok in my first version. However, in my new version I'm getting a NullPointerException on this line "Resources res = context.getResources();" I've checked and it turns out that the context I'm supplying it's null... And I don't get to understand why or what I'm doing wrong
Ok, I solved it (I'd swear I already commented on this, but since it's gone...).
Thing is, I'm not used to use Application classes and I had forgotten to declare MyApplication in the Manifest file. Noob mistake. As soon as I declared it, the app ran OK
I have a class that needs to obtain a reference to it's application's AssetManager. This class does not extend any sort of android UI class, so it doesn't have a getContext() method, or anything similar. Is there some sort of static Context.getCurrentApplicationContext() type of method?
To clarify: my class is intended to be used like a library, for other applications. It has no associated AndroidManifest.xml or control over the context which is calling it.
Create a subclass of Application, for instance public class App extends Application {
Set the android:name attribute of your <application> tag in the AndroidManifest.xml to point to your new class, e.g. android:name=".App"
In the onCreate() method of your app instance, save your context (e.g. this) to a static field named app and create a static method that returns this field, e.g. getApp():
This is how it should look:
public class App extends Application{
private static Context mContext;
#Override
public void onCreate() {
super.onCreate();
mContext = this;
}
public static Context getContext(){
return mContext;
}
}
Now you can use: App.getContext() whenever you want to get a context, and then getAssetManager() (or App.getContext().getAssetManager()).
I am not sure of the best answer to the OP question. However, I do know that you have to be very careful when using a static context as suggested in Android developer resources:
In the onCreate() method of your app instance, save your context (e.g. this) to a static field named app and create a static method that returns this field, e.g. getApp():
Using static contexts can leak to leaked memory issues, especially if the static context is used for references to views.
When programming for Android sometimes you have to use static methods. But when you try to access you resources in a static method with getString(R.string.text) you'll get an error. Making it static doesn't work.
Does anyone knows a good way around this? The resource files in Android are very helpful for creating things in different languages or making changes to a text.
One way or another, you'll need a Context for that... For static methods this probably means you need to pass along a Context when calling them.
You could use Resources.getSystem().getStringArray(android.R.array.done);
This is how I access resources from inside static methods. Maybe not ideal, but.
First, I extend Application and set some public static field(s), and create a method to initialise them:
public class MyApp extends Application {
// static resources
public static String APP_NAME;
public static void initResources(Context context) {
APP_NAME = context.getResources().getString(R.string.app_name);
}
}
And in my manifest I register the extended Application:
<application
android:name=".MyApp"/>
In my starter activity (MainActivity), I make a call to initialise the static resources:
#Override
protected void onCreate(Bundle savedInstanceState) {
MyApp.initResources(this);
}
Then anywhere in your project, after MainActivity.onCreate(Bundle b) has run, you can call static methods that access your specified static resources:
public static void printAppName() {
Log.w("tag", "my app name: " + MyApp.APP_NAME);
}
Pass in a Context (i.e. Activity) instance as a parameter object to static method. Then invoke getString on the parameter.
The post below gives a tip for creating an Application class to save your current context. Your new Application class will then be accessible from any other static method.
How can I get a resource content from a static context?
One way is you can pass context to your static method.
check this out it definitely works
public class Sounds {
public static MediaPlayer getSoundTouch(Context context){
return MediaPlayer.create(context, R.raw.touch);
}
public static MediaPlayer getSoundLeak(Context context){
return MediaPlayer.create(context, R.raw.leak);
}
public static MediaPlayer getSoundFinish(Context context){
return MediaPlayer.create(context, R.raw.finish);
}
}
In various bits of Android code I've seen:
public class MyActivity extends Activity {
public void method() {
mContext = this; // since Activity extends Context
mContext = getApplicationContext();
mContext = getBaseContext();
}
}
However I can't find any decent explanation of which is preferable, and under what circumstances which should be used.
Pointers to documentation on this, and guidance about what might break if the wrong one is chosen, would be much appreciated.
I agree that documentation is sparse when it comes to Contexts in Android, but you can piece together a few facts from various sources.
This blog post on the official Google Android developers blog was written mostly to help address memory leaks, but provides some good information about contexts as well:
In a regular Android application, you
usually have two kinds of Context,
Activity and Application.
Reading the article a little bit further tells about the difference between the two and when you might want to consider using the application Context (Activity.getApplicationContext()) rather than using the Activity context this). Basically the Application context is associated with the Application and will always be the same throughout the life cycle of your app, where as the Activity context is associated with the activity and could possibly be destroyed many times as the activity is destroyed during screen orientation changes and such.
I couldn't find really anything about when to use getBaseContext() other than a post from Dianne Hackborn, one of the Google engineers working on the Android SDK:
Don't use getBaseContext(), just use
the Context you have.
That was from a post on the android-developers newsgroup, you may want to consider asking your question there as well, because a handful of the people working on Android actual monitor that newsgroup and answer questions.
So overall it seems preferable to use the global application context when possible.
Here's what I've found regarding the use of context:
1) . Within an Activity itself, use this for inflating layouts and menus, register context menus, instantiating widgets, start other activities, create new Intent within an Activity, instantiating preferences, or other methods available in an Activity.
Inflate layout:
View mView = this.getLayoutInflater().inflate(R.layout.myLayout, myViewGroup);
Inflate menu:
#Override
public boolean onCreateOptionsMenu(Menu menu) {
super.onCreateOptionsMenu(menu);
this.getMenuInflater().inflate(R.menu.mymenu, menu);
return true;
}
Register context menu:
this.registerForContextMenu(myView);
Instantiate widget:
TextView myTextView = (TextView) this.findViewById(R.id.myTextView);
Start an Activity:
Intent mIntent = new Intent(this, MyActivity.class);
this.startActivity(mIntent);
Instantiate preferences:
SharedPreferences mSharedPreferences = this.getPreferenceManager().getSharedPreferences();
2) . For application-wide class, use getApplicationContext() as this context exist for the lifespan of the application.
Retrieve the name of the current Android package:
public class MyApplication extends Application {
public static String getPackageName() {
String packageName = null;
try {
PackageInfo mPackageInfo = getApplicationContext().getPackageManager().getPackageInfo(getApplicationContext().getPackageName(), 0);
packageName = mPackageInfo.packageName;
} catch (NameNotFoundException e) {
// Log error here.
}
return packageName;
}
}
Bind an application-wide class:
Intent mIntent = new Intent(this, MyPersistent.class);
MyServiceConnection mServiceConnection = new MyServiceConnection();
if (mServiceConnection != null) {
getApplicationContext().bindService(mIntent, mServiceConnection, Context.BIND_AUTO_CREATE);
}
3) . For Listeners and other type of Android classes (e.g. ContentObserver), use a Context substitution like:
mContext = this; // Example 1
mContext = context; // Example 2
where this or context is the context of a class (Activity, etc).
Activity context substitution:
public class MyActivity extends Activity {
private Context mContext;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mContext = this;
}
}
Listener context substitution:
public class MyLocationListener implements LocationListener {
private Context mContext;
public MyLocationListener(Context context) {
mContext = context;
}
}
ContentObserver context substitution:
public class MyContentObserver extends ContentObserver {
private Context mContext;
public MyContentObserver(Handler handler, Context context) {
super(handler);
mContext = context;
}
}
4) . For BroadcastReceiver (including inlined/embedded receiver), use the receiver's own context.
External BroadcastReceiver:
public class MyBroadcastReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
final String action = intent.getAction();
if (action.equals(Intent.ACTION_SCREEN_OFF)) {
sendReceiverAction(context, true);
}
private static void sendReceiverAction(Context context, boolean state) {
Intent mIntent = new Intent(context.getClass().getName() + "." + context.getString(R.string.receiver_action));
mIntent.putExtra("extra", state);
context.sendBroadcast(mIntent, null);
}
}
}
Inlined/Embedded BroadcastReceiver:
public class MyActivity extends Activity {
private BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
final boolean connected = intent.getBooleanExtra(context.getString(R.string.connected), false);
if (connected) {
// Do something.
}
}
};
}
5) . For Services, use the service's own context.
public class MyService extends Service {
private BroadcastReceiver mBroadcastReceiver;
#Override
public void onCreate() {
super.onCreate();
registerReceiver();
}
private void registerReceiver() {
IntentFilter mIntentFilter = new IntentFilter();
mIntentFilter.addAction(Intent.ACTION_SCREEN_OFF);
this.mBroadcastReceiver = new MyBroadcastReceiver();
this.registerReceiver(this.mBroadcastReceiver, mIntentFilter);
}
}
6) . For Toasts, generally use getApplicationContext(), but where possible, use the context passed from an Activity, Service, etc.
Use context of the application:
Toast mToast = Toast.makeText(getApplicationContext(), message, Toast.LENGTH_LONG);
mToast.show();
Use context passed from a source:
public static void showLongToast(Context context, String message) {
if (context != null && message != null) {
Toast mToast = Toast.makeText(context, message, Toast.LENGTH_LONG);
mToast.show();
}
}
And last, don't use getBaseContext() as advised by Android's framework developers.
UPDATE: Add examples of Context usage.
I read this thread a few days ago, asking myself the same question. My decision after reading this was simple: always use applicationContext.
However, I encountered a problem with this, I spent a few hours to find it, and a few seconds to solve it... (changing one word...)
I am using a LayoutInflater to inflate a view containing a Spinner.
So here are two possibilities:
1)
LayoutInflater layoutInflater = LayoutInflater.from(this.getApplicationContext());
2)
LayoutInflater layoutInflater = LayoutInflater.from(this.getBaseContext());
Then, I am doing something like this:
// managing views part
View view = ContactViewer.mLayoutInflater.inflate(R.layout.aViewContainingASpinner, theParentView, false);
Spinner spinner = (Spinner) view.findViewById(R.id.theSpinnerId);
String[] myStringArray = new String[] {"sweet","love"};
// managing adapter part
// The context used here don't have any importance -- both work.
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this.getApplicationContext(), myStringArray, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);
theParentView.addView(view);
What I noticed: If you instantiated your linearLayout with the applicationContext, then when you click on the spinner in your activity, you will have an uncaught exception, coming from the dalvik virtual machine (not from your code, that's why I have spent a lot of time to find where was my mistake...).
If you use the baseContext, then that's all right, the context menu will open and you will be able to choose among your choices.
So here is my conclusion: I suppose (I have not tested it further) than the baseContext is required when dealing with contextMenu in your Activity...
The test has been done coding with API 8, and tested on an HTC Desire, android 2.3.3.
I hope my comment have not bored you so far, and wish you all the best. Happy coding ;-)
First, I agree that we should use appcontext whenever possible. then "this" in activity. i've never had a need for basecontext.
In my tests, in most cases they can be interchanged. In most cases, the reason you want to get a hold of a context is to access files, preferences, database etc. These data is eventually reflected as files in your app's private data folder (/data/data/). No matter which context you use, they'll be mapped to the same folder/files so you are OK.
That's what I observed. Maybe there are cases you should distinguish them.
In some cases you may use Activity context over application context when running something in a thread. When thread completes execution and you need to return the result back to the caller activity, you need that context with a handler.
((YourActivity) context).yourCallbackMethod(yourResultFromThread, ...);
In simple words
getApplicationContext() as the method name suggest will make your app aware of application wide details which you can access from anywhere in the app. So you can make use of this in service binding, broadcast registration etc. Application context will be alive till the app exits.
getActivity() or this will make your app aware of the current screen which is visible also the app level details provided by application context. So whatever you want to know about the current screen like Window ActionBar Fragementmanger and so are available with this context. Basically and Activity extend Context. This context will alive till the current component(activity) is alive
The confusion stems from the fact that there are numerous ways to
access Context, with (on the surface) no discernible differences.
Below are four of the most common ways you may be able to access
Context in an Activity.
getContext()
getBaseContext()
getApplicationContext()
getActionBar().getThemedContext() //new
What is a Context?
I personally like to think of Context as the state of your application at any given time. The application Context represents a global or base configuration of your application and an Activity or Service can build upon it and represents a configuration instance of your Application or a transitive state for it.
If you look at the source for android.content.Context, you see that Context is an abstract class and the comments on the class are as follows:
Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It
allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.
What I take away from this is that Context provides a common implementation to access application level as well as system level resources. Application level resources may be accessing things like String resources [getResources()] or assets [getAssets()] and system-level resource is anything that you access with Context.getSystemService().
As a matter of fact, take a look at the comments on the methods and they seem to reinforce this notion:
getSystemService(): Return the handle to a system-level service by name. The class of the returned object varies by the requested name.
getResources(): Return a Resources instance for your application’s package.
getAssets(): Return a Resources instance for your application’s package.
It may be worth pointing out that in the Context abstract class, all of the above methods are abstract! Only one instance of getSystemService(Class) has an implementation and that invokes an abstract method. This means, the implementation for these should be provided mostly by the implementing classes, which include:
ContextWrapper
Application
Activity
Service
IntentService
Looking at the API documentation, the hierarchy of the classes looks like this:
Context
| — ContextWrapper
|— — Application
| — — ContextThemeWrapper
|— — — — Activity
| — — Service
|— — — IntentService
Since we know that Context itself is not providing any insight, we move down the tree and take a look at the ContextWrapper and realize that there isn't much there either. Since Application extends ContextWrapper, there isn't much to look at over there either since it doesn't override the implementation provided by ContextWrapper. This means that the implementation for Context is provided by the OS and is hidden from the API. You can take a look at the concrete implementation for Context by looking at the source for the ContextImpl class.
I've only used this and getBaseContext when toasting from an onClick (very green noob to both Java and android). I use this when my clicker is directly in the activity and have to use getBaseContext in an anonymous inner clicker. I'm guessing that is pretty much the trick with getBaseContext, it is perhaps returning the context of the activity in which the inner class is hiding.