2023年6月27日发(作者：)

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

application is a different user.

 By default, the system assigns each application a unique Linux user ID (the 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 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 to access system services:

 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 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.

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.

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 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

service is a component that runs in the background to perform 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 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 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 ) to read and write information about a particular person.

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

your 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 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 amount of

work. For instance, it might initiate a service to perform some work based on the

event.

A broadcast receiver is implemented as a subclass of BroadcastReceiver and each

broadcast is delivered as an Intent object. For more information, see

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 in the process that belongs to the camera application, not in your application's

process. Therefore, unlike applications on most other systems, Android applications don't

have a single entry point (there's no main() function, for example).

Because the system runs each application in a separate process with file permissions

that restrict access to other applications, your application cannot directly activate a

component from another application. The Android system, however, can. So, to activate a component in 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

Three of the four component types—activities, services, and broadcast receivers—are

activated by an asynchronous message called an

intent. Intents bind individual

components to each other at runtime (you can think of them as the messengers that

request an action from other components), whether the component belongs to your

application or another.

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

the ContentResolver object. This leaves a layer of abstraction between the content

provider and the component requesting information (for security).

There are separate methods for activating each type of component:







You can start an activity (or give it something new to do) by passing

an Intent to startActivity() or startActivityForResult() (when you want the activity

to return a result).

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().

You can initiate a broadcast by passing an Intent to methods

like sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast(). You can perform a query to a content provider by calling query() on

a ContentResolver.

For more information about using intents, see the Intents and Intent Filters document.

More information about activating specific components is also provided in the following

documents: Activities, Services, BroadcastReceiver and Content Providers.



Declaring components

The primary task of the manifest is to inform the system about the application's

components. For example, a manifest file can declare an activity as follows:

android:label="@string/example_label" ... >

...

In the element, the android:icon attribute points to resources for an

icon that identifies the application.

In the element, the android:name attribute specifies the fully qualified

class name of the Activity subclass and the android:label attributes specifies a string to

use as the user-visible label for the activity.

You must declare all application components this way:



elements for activities



elements for services



elements for broadcast receivers



elements for content providers

Activities, services, and content providers that you include in your source but do not

declare in the manifest are not visible to the system and, consequently, can never run.

However, broadcast receivers can be either declared in the manifest or created

dynamically in code (as BroadcastReceiver objects) and registered with the system by

calling registerReceiver().

Declaring component capabilities

As discussed above, in Activating Components, you can use an Intent to start

activities, services, and broadcast receivers. You can do so by explicitly naming the target

component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe the

type of action you want to perform (and optionally, the data upon which you’d like to

perform the action) and allow the system to find a component on the device that can

perform the action and start it. If there are multiple components that can perform the

action described by the intent, then the user selects which one to use.

The way the system identifies the components that can respond to an intent is by

comparing the intent received to the

intent filters provided in the manifest file of other

applications on the device.

When you declare a component in your application's manifest, you can optionally

include intent filters that declare the capabilities of the component so it can respond to

intents from other applications. You can declare an intent filter for your component by

adding an element as a child of the component's declaration element.

For example, an email application with an activity for composing a new email might

declare an intent filter in its manifest entry to respond to "send" intents (in order to send

email). An activity in your application can then create an intent with the “send” action

(ACTION_SEND), which the system matches to the email application’s “send” activity and

launches it when you invoke the intent with startActivity().

For more about creating intent filters, see the Intents and Intent Filters document.

Declaring application requirements

There are a variety of devices powered by Android and not all of them provide the

same features and capabilities. In order to prevent your application from being installed on

devices that lack features needed by your application, it's important that you clearly define

a profile for the types of devices your application supports by declaring device and

software requirements in your manifest file. Most of these declarations are informational

only and the system does not read them, but external services such as Google Play do read

them in order to provide filtering for users when they search for applications from their

device.

For example, if your application requires a camera and uses APIs introduced in

Android 2.1 (API Level 7), you should declare these as requirements in your manifest file.

That way, devices that do

not have a camera and have an Android version

lower than 2.1

cannot install your application from Google Play.

However, you can also declare that your application uses the camera, but does

not

require it. In that case, your application must perform a check at runtime to determine

if the device has a camera and disable any features that use the camera if one is not

available.

Here are some of the important device characteristics that you should consider as you

design and develop your application: Screen size and density

In order to categorize devices by their screen type, Android defines two

characteristics for each device: screen size (the physical dimensions of the screen)

and screen density (the physical density of the pixels on the screen, or dpi—dots

per inch). To simplify all the different types of screen configurations, the Android

system generalizes them into select groups that make them easier to target.

The screen sizes are: small, normal, large, and extra large.

The screen densities are: low density, medium density, high density, and extra

high density.

By default, your application is compatible with all screen sizes and densities,

because the Android system makes the appropriate adjustments to your UI layout

and image resources. However, you should create specialized layouts for certain

screen sizes and provide specialized images for certain densities, using alternative

layout resources, and by declaring in your manifest exactly which screen sizes your

application supports with the element.

For more information, see the Supporting Multiple Screens document.

Input configurations

Many devices provide a different type of user input mechanism, such as a

hardware keyboard, a trackball, or a five-way navigation pad. If your application

requires a particular kind of input hardware, then you should declare it in your

manifest with the element. However, it is rare that an

application should require a certain input configuration.

Device features

There are many hardware and software features that may or may not exist on a

given Android-powered device, such as a camera, a light sensor, bluetooth, a

certain version of OpenGL, or the fidelity of the touchscreen. You should never

assume that a certain feature is available on all Android-powered devices (other

than the availability of the standard Android library), so you should declare any

features used by your application with the element.

Platform Version

Different Android-powered devices often run different versions of the Android

platform, such as Android 1.6 or Android 2.3. Each successive version often

includes additional APIs not available in the previous version. In order to indicate

which set of APIs are available, each platform version specifies an API Level (for

example, Android 1.0 is API Level 1 and Android 2.3 is API Level 9). If you use any

APIs that were added to the platform after version 1.0, you should declare the

minimum API Level in which those APIs were introduced using

the element.

It's important that you declare all such requirements for your application, because,

when you distribute your application on Google Play, the store uses these declarations to filter which applications are available on each device. As such, your application should be

available only to devices that meet all your application requirements.

For more information about how Google Play filters applications based on these (and

other) requirements, see the Filters on Google Play document.

Application Resources

An Android application is composed of more than just code—it requires resources that

are separate from the source code, such as images, audio files, and anything relating to

the visual presentation of the application. For example, you should define animations,

menus, styles, colors, and the layout of activity user interfaces with XML files. Using

application resources makes it easy to update various characteristics of your application

without modifying code and—by providing sets of alternative resources—enables you to

optimize your application for a variety of device configurations (such as different languages

and screen sizes).

For every resource that you include in your Android project, the SDK build tools define

a unique integer ID, which you can use to reference the resource from your application

code or from other resources defined in XML. For example, if your application contains an

image file named (saved in the res/drawable/ directory), the SDK tools generate

a resource ID named , which you can use to reference the image and

insert it in your user interface.

One of the most important aspects of providing resources separate from your source

code is the ability for you to provide alternative resources for different device

configurations. For example, by defining UI strings in XML, you can translate the strings

into other languages and save those strings in separate files. Then, based on a

language

qualifier that you append to the resource directory's name (such

as res/values-fr/ for French string values) and the user's language setting, the Android

system applies the appropriate language strings to your UI.

Android supports many different

qualifiers for your alternative resources. The qualifier

is a short string that you include in the name of your resource directories in order to define

the device configuration for which those resources should be used. As another example,

you should often create different layouts for your activities, depending on the device's

screen orientation and size. For example, when the device screen is in portrait orientation

(tall), you might want a layout with buttons to be vertical, but when the screen is in

landscape orientation (wide), the buttons should be aligned horizontally. To change the

layout depending on the orientation, you can define two different layouts and apply the

appropriate qualifier to each layout's directory name. Then, the system automatically

applies the appropriate layout depending on the current device orientation.