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Android Application Fundamentals

Android applications are written in the Java programming language. The Android SDK

tools compile the code—along with any data and resource files—into anAndroid package, an

archive file with an.apksuffix. All the code in a single.apkfile is considered to be one

application and is the file that Android-powered devices use to install the application.

Once installed on a device, each Android application lives in its own security sandbox:

 The Android operating system is a multi-user Linux system in which each

isolation from other applications.

 By default, every application runs in its own Linux process. Android starts the

process when any of the application's components need to be executed, then shuts

down the process when it's no longer needed or when the system must recover

memory for other applications.

In this way, the Android system implements theprinciple of least privilege. That is, each

application, by default, has access only to the components that it requires to do its work and

no more. This creates a very secure environment in which an application cannot access parts

of the system for which it is not given permission.

However, there are ways for an application to share data with other applications and for

 An application can request permission to access device data such as the user's

contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and

more. All application permissions must be granted by the user at install time.

That covers the basics regarding how an Android application exists within the system. The

rest of this document introduces you to:

 The core framework components that define your application.

 The manifest file in which you declare components and required device features for

your application.

 Resources that are separate from the application code and allow your application to

gracefully optimize its behavior for a variety of device configurations.

There are four different types of application components. Each type serves a distinct purpose

and has a distinct lifecycle that defines how the component is created and destroyed.

Here are the four types of application components:

Activities

Anactivityrepresents a single screen with a user interface. For example, an email

application might have one activity that shows a list of new emails, another activity to

compose an email, and another activity for reading emails. Although the activities

work together to form a cohesive user experience in the email application, each one is

independent of the others. As such, a different application can start any one of these

activities (if the email application allows it). For example, a camera application can

interface. For example, a service might play music in the background while the user is

in a different application, or it might fetch data over the network without blocking user

interaction with an activity. Another component, such as an activity, can start the

service and let it run or bind to it in order to interact with it.

A service is implemented as a subclass ofServiceand you can learn more about it in

theServicesdeveloper guide.

Content providers

Acontent providermanages a shared set of application data. You can store the data in

the file system, an SQLite database, on the web, or any other persistent storage

location your application can access. Through the content provider, other applications

application and not shared. For example, theNote Padsample application uses a

content provider to save notes.

A content provider is implemented as a subclass ofContentProviderand must

implement a standard set of APIs that enable other applications to perform

transactions. For more information, see theContent Providersdeveloper guide.

Broadcast receivers

Abroadcast receiveris a component that responds to system-wide broadcast

announcements. Many broadcasts originate from the system—for example, a

broadcast announcing that the screen has turned off, the battery is low, or a picture was

for them to use. Although broadcast receivers don't display a user interface, they

theBroadcastReceiverclass.

A unique aspect of the Android system design is that any application can start another

application’s component. For example, if you want the user to capture a photo with the device

camera, there's probably another application that does that and your application can use it,

instead of developing an activity to capture a photo yourself. You don't need to incorporate or

even link to the code from the camera application. Instead, you can simply start the activity in

the camera application that captures a photo. When complete, the photo is even returned to

your application so you can use it. To the user, it seems as if the camera is actually a part of

your application.

When the system starts a component, it starts the process for that application (if it's not

already running) and instantiates the classes needed for the component. For example, if your

application starts the activity in the camera application that captures a photo, that activity runs

another application, you must deliver a message to the system that specifies yourintentto

start a particular component. The system then activates the component for you.

Activating Components

each other at runtime (you can think of them as the messengers that request an action from

An intent is created with anIntentobject, which defines a message to activate either a

specific component or a specifictypeof component—an intent can be either explicit or

implicit, respectively.

For activities and services, an intent defines the action to perform (for example, to "view"

or "send" something) and may specify the URI of the data to act on (among other things that

the component being started might need to know). For example, an intent might convey a

request for an activity to show an image or to open a web page. In some cases, you can start

an activity to receive a result, in which case, the activity also returns the result in

anIntent(for example, you can issue an intent to let the user pick a personal contact and have

it returned to you—the return intent includes a URI pointing to the chosen contact).

For broadcast receivers, the intent simply defines the announcement being broadcast (for

example, a broadcast to indicate the device battery is low includes only a known action string

that indicates "battery is low").

The other component type, content provider, is not activated by intents. Rather, it is

activated when targeted by a request from aContentResolver. The content resolver handles all

direct transactions with the content provider so that the component that's performing

transactions with the provider doesn't need to and instead calls methods on

 You can start a service (or give new instructions to an ongoing service) by passing

anIntenttostartService(). Or you can bind to the service by passing

anIntenttobindService().