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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

ID is used only by the system and is unknown to the application)。 The system sets

permissions for all the files in an application so that only the user ID

assigned to that application can access them。

Each process has its own virtual machine (VM), so an application's code

runs in 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

It's possible to arrange for two applications to share the same Linux user

ID, in which case they are able to access each other's files. To conserve

system resources， applications with the same user ID can also arrange to run in

the same Linux process and share the same VM (the applications must also be

signed with the same certificate）.

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

configurations.

Application Components

Application components are the essential building blocks of an Android

application. Each component is a different point through which the system can

enter your application. Not all components are actual entry points for the

user and some depend on each other, but each one exists as its own entity and

plays a specific role—each one is a unique building block that helps define

your application's overall behavior。

is created and destroyed.

Here are the four types of application components：

different application can start any one of these activities （ if the email

application allows it). For example， a camera application can start the activity

in the email application that composes new mail, in order for the user to

share a picture。

An activity is implemented as a subclass of Activity and you can learn

more about it in the Activities developer guide.

Services

A

long-running operations or to perform work for remote processes。 A service does

not provide a user interface. For example, a service might play music in the

Content providers

A

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 can query or even modify the data （if the content provider allows

it）. For example， the Android system provides a content provider that manages

the user's contact information。 As such, any application with the proper permissions

can query part of the content provider （such as ContactsContract。Data) to read and

write information about a particular person.

Content providers are also useful for reading and writing data that is

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 captured。 Applications can also initiate broadcasts—for

example, to let other applications know that some data has been downloaded to

the device and is available for them to use。 Although broadcast receivers don't

display a user interface, they may create a status bar notification to alert the

user when a broadcast event occurs。 More commonly, though， a broadcast receiver

is just a "gateway” to other components and is intended to do a very minimal