01 what is android

What is Android?
Android is a software stack for mobile devices that includes an operating system, middleware and key
applications. The Android SDK provides the tools and APIs necessary to begin developing applications
on the Android platform using the Java programming language.

Features

 Application framework enabling reuse and replacement of components
 Dalvik virtual machine optimized for mobile devices
 Integrated browser based on the open source WebKit engine
 Optimized graphics powered by a custom 2D graphics library; 3D graphics based on the
OpenGL ES 1.0 specification (hardware acceleration optional)
 SQLite for structured data storage
 Media support for common audio, video, and still image formats
(MPEG4,H.264,MP3,AAC,AMR,JPG,PNG,GIF)
 GSM Telephony (hardware dependent)
 Bluetooth, EDGE, 3G, and WiFi (hardware dependent)
 Camera, GPS, compass, and accelerometer (hardware dependent)
 Rich development environment including a device emulator, tools for debugging, memory and
performance profiling, and a plugin for the Eclipse IDE

Android Architecture

The following diagram shows the major components of the Android operating system. Each section is
described in more detail below.

Applications

Android will ship with a set of core applications including an email client, SMS program, calendar,
maps, browser, contacts, and others. All applications are written using the Java programming
language.

Application Framework

By providing an open development platform, Android offers developers the ability to build extremely
rich and innovative applications. Developers are free to take advantage of the device hardware,
access location information, run background services, set alarms, add notifications to the status bar,
and much, much more.

Developers have full access to the same framework APIs used by the core applications. The
application architecture is designed to simplify the reuse of components; any application can publish its
capabilities and any other application may then make use of those capabilities (subject to security
constraints enforced by the framework). This same mechanism allows components to be replaced by
the user.

Underlying all applications is a set of services and systems, including:

 A rich and extensible set of Views that can be used to build an application, including lists, grids,
text boxes, buttons, and even an embeddable web browser
 Content Providers that enable applications to access data from other applications (such as
Contacts), or to share their own data
 A Resource Manager, providing access to non-code resources such as localized strings,
graphics, and layout files
 A Notification Manager that enables all applications to display custom alerts in the status bar
 An Activity Manager that manages the lifecycle of applications and provides a common
navigation backstack

For more details and a walkthrough of an application, see the Notepad Tutorial.

Libraries

Android includes a set of C/C++ libraries used by various components of the Android system. These
capabilities are exposed to developers through the Android application framework. Some of the core
libraries are listed below:

 System C library - a BSD-derived implementation of the standard C system library (libc), tuned
for embedded Linux-based devices
 Media Libraries - based on PacketVideo's OpenCORE; the libraries support playback and
recording of many popular audio and video formats, as well as static image files, including
MPEG4, H.264, MP3, AAC, AMR, JPG, and PNG

 Surface Manager - manages access to the display subsystem and seamlessly composites 2D
and 3D graphic layers from multiple applications
 LibWebCore - a modern web browser engine which powers both the Android browser and an
embeddable web view
 SGL - the underlying 2D graphics engine
 3D libraries - an implementation based on OpenGL ES 1.0 APIs; the libraries use either
hardware 3D acceleration (where available) or the included, highly optimized 3D software
rasterizer
 FreeType - bitmap and vector font rendering
 SQLite - a powerful and lightweight relational database engine available to all applications

Android Runtime

Android includes a set of core libraries that provides most of the functionality available in the core
libraries of the Java programming language.

Every Android application runs in its own process, with its own instance of the Dalvik virtual machine.
Dalvik has been written so that a device can run multiple VMs efficiently. The Dalvik VM executes files
in the Dalvik Executable (.dex) format which is optimized for minimal memory footprint. The VM is
register-based, and runs classes compiled by a Java language compiler that have been transformed
into the .dex format by the included "dx" tool.

The Dalvik VM relies on the Linux kernel for underlying functionality such as threading and low-level
memory management.

Linux Kernel

Android relies on Linux version 2.6 for core system services such as security, memory management,
process management, network stack, and driver model. The kernel also acts as an abstraction layer
between the hardware and the rest of the software stack.

Application Fundamentals
Quickview

 Android applications are composed of one or more application components (activities, services,
content providers, and broadcast receivers)
 Each component performs a different role in the overall application behavior, and each one can
be activated individually (even by other applications)
 The manifest file must declare all components in the application and should also declare all
application requirements, such as the minimum version of Android required and any hardware
configurations required
 Non-code application resources (images, strings, layout files, etc.) should include alternatives
for different device configurations (such as different strings for different languages and different
layouts for different screen sizes)

In this document

1. Application Components
1. Activating components
2. The Manifest File
1. Declaring components
2. Declaring application requirements
3. Application Resources

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 ContactsContract.Data) 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 the BroadcastReceiver 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 activiting 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 to bindService().
 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.

The Manifest File

Before the Android system can start an application component, the system must know that the
component exists by reading the application's AndroidManifest.xml file (the "manifest" file). Your
application must declare all its components in this file, which must be at the root of the application
project directory.

The manifest does a number of things in addition to declaring the application's components, such as:

 Identify any user permissions the application requires, such as Internet access or read-access
to the user's contacts.
 Declare the minimum API Level required by the application, based on which APIs the
application uses.
 Declare hardware and software features used or required by the application, such as a camera,
bluetooth services, or a multitouch screen.
 API libraries the application needs to be linked against (other than the Android framework APIs),
such as the Google Maps library.
 And more

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:

<?xml version="1.0" encoding="utf-8"?>
<manifest ... >
<application android:icon="@drawable/app_icon.png" ... >
<activity android:name="com.example.project.ExampleActivity"
android:label="@string/example_label" ... >
</activity>
...
</application>
</manifest>

In the <application> element, the android:icon attribute points to resources for an icon that identifies
the application.

In the <activity> 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:

 <activity> elements for activities
 <service> elements for services
 <receiver> elements for broadcast receivers
 <provider> 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().

For more about how to structure the manifest file for your application, see the The AndroidManifest.xml
File documentation.

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 <intent-filter> 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
Android Market 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 Android
Market.

However, you can also declare that your applicaiton 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 <supports-screens>
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 <uses-configuration> 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 <uses-feature> 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 <uses-sdk>
element.

It's important that you declare all such requirements for your application, because, when you distribute
your application on Android Market, Market 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 Android Market filters applications based on these (and other)
requirements, see the Market Filters 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 logo.png (saved in the
res/drawable/ directory), the SDK tools generate a resource ID named R.drawable.logo, 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.

For more about the different kinds of resources you can include in your application and how to create
alternative resources for various device configurations, see the Application Resources developer
guide.

01 what is android

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01 what is android