Android Application Framework

Do all the Activities and Services of an application run in a single process?

All Activities and Services in an application run in a single process by default. The android:process attribute can be used to explicitly place a component (Activity/Service) in another process.

Do all Activities run in the main thread of an application process?

By default, all of the application code in a single process runs in the main UI thread. This is the same thread that also handles UI events. The only exception is the code that handles IPC calls coming in from other processes. The system maintains a separate pool of transaction threads in each process to dispatch all incoming IPC calls. The developer should create separate threads for any long-running code, to avoid blocking the main UI thread.

How do I pass data between Activities/Services within a single application?

It depends on the type of data that you want to share:

Primitive Data Types

To share primitive data between Activities/Services in an application, use Intent.putExtras(). For passing primitive data that needs to persist use the Application Preferences.

Non-Persistent Objects

For sharing complex non-persistent user-defined objects for short duration, the following approaches are recommended:

The class

The is a base class for those who need to maintain global application state. It can be accessed via getApplication() from any Activity or Service. It has a couple of life-cycle methods and will be instantiated by Android automatically if your register it in AndroidManifest.xml.

A public static field/method

An alternate way to make data accessible across Activities/Services is to use public static fields and/or methods. You can access these static fields from any other class in your application. To share an object, the activity which creates your object sets a static field to point to this object and any other activity that wants to use this object just accesses this static field.

A HashMap of WeakReferences to Objects

You can also use a HashMap of WeakReferences to Objects with Long keys. When an activity wants to pass an object to another activity, it simply puts the object in the map and sends the key (which is a unique Long based on a counter or time stamp) to the recipient activity via intent extras. The recipient activity retrieves the object using this key.

A Singleton class

There are advantages to using a static Singleton, such as you can refer to them without casting getApplication() to an application-specific class, or going to the trouble of hanging an interface on all your Application subclasses so that your various modules can refer to that interface instead.

But, the life cycle of a static is not well under your control; so to abide by the life-cycle model, the application class should initiate and tear down these static objects in the onCreate() and onTerminate() methods of the Application Class

Persistent Objects

Even while an application appears to continue running, the system may choose to kill its process and restart it later. If you have data that you need to persist from one activity invocation to the next, you need to represent that data as state that gets saved by an activity when it is informed that it might go away.

For sharing complex persistent user-defined objects, the following approaches are recommended:

  • Application Preferences
  • Files
  • contentProviders
  • SQLite DB

If the shared data needs to be retained across points where the application process can be killed, then place that data in persistent storage like Application Preferences, SQLite DB, Files or ContentProviders. Please refer to the Storing, Retrieving and Exposing Data for further details on how to use these components.

How can I check if an Activity is already running before starting it?

The general mechanism to start a new activity if its not running— or to bring the activity stack to the front if is already running in the background— is the to use the NEW_TASK_LAUNCH flag in the startActivity() call.

If an Activity starts a remote service, is there any way for the Service to pass a message back to the Activity?

The remote service can define a callback interface and register it with the clients to callback into the clients. The RemoteCallbackList provides methods to register and unregister clients with the service, and send and receive messages.

The sample code for remote service callbacks is given in ApiDemos/RemoteService

How to avoid getting the Application not responding dialog?

Please check the Application Responsiveness section to design your application for better responsiveness:

How does an application know if a package is added or removed?

Whenever a package is added, an intent with PACKAGE_ADDED action is broadcast by the system. Similarly when a package is removed, an intent with PACKAGE_REMOVED action is broadcast. To receive these intents, you should write something like this:

       <receiver android:name ="">
             <action android:name="android.intent.action.PACKAGE_ADDED"/>
              <action android:name="android.intent.action.PACKAGE_REMOVED"/>            
              <data android:scheme="package" />

Here PackageReceiver is a BroadcastReceiver class.Its onReceive() method is invoked, every time an application package is installed or removed.