Android-The-Developers-Guide.pdf

What is Android? | Android Developers

29.04.09 0:33

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.

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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

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

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What is Android? | Android Developers

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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.

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Application Fundamentals | Android Developers

29.04.09 0:33

Application Fundamentals

Android applications are written in the Java programming language. The compiled Java code — along with any data

and resource files required by the application — is bundled by the aapt tool into an Android package , an archive file

marked by an .apk suffix. This file is the vehicle for distributing the application and installing it on mobile devices; it's

the file users download to their devices. All the code in a single .apk file is considered to be one application .

In many ways, each Android application lives in its own world:

By default, every application runs in its own Linux process. Android starts the process when any of the

application's code needs to be executed, and shuts down the process when it's no longer needed and system

resources are required by other applications.

Each process has its own Java virtual machine (VM), so application code runs in isolation from the code of all

other applications.

By default, each application is assigned a unique Linux user ID. Permissions are set so that the application's files

are visible only that user, only to the application itself — although there are ways to export them to other

applications as well.

It's possible to arrange for two applications to share the same user ID, in which case they will be able to see each

other's files. To conserve system resources, applications with the same ID can also arrange to run in the same Linux

process, sharing the same VM.

Application Components

A central feature of Android is that one application can make use of elements of other applications (provided those

applications permit it). For example, if your application needs to display a scrolling list of images and another

application has developed a suitable scroller and made it available to others, you can call upon that scroller to do the

work, rather than develop your own. Your application doesn't incorporate the code of the other application or link to it.

Rather, it simply starts up that piece of the other application when the need arises.

For this to work, the system must be able to start an application process when any part of it is needed, and instantiate

the Java objects for that part. Therefore, unlike applications on most other systems, Android applications don't have a

single entry point for everything in the application (no main() function, for example). Rather, they have essential

components that the system can instantiate and run as needed. There are four types of components:

Activities

An activity presents a visual user interface for one focused endeavor the user can undertake. For example, an

activity might present a list of menu items users can choose from or it might display photographs along with their

captions. A text messaging application might have one activity that shows a list of contacts to send messages to,

a second activity to write the message to the chosen contact, and other activities to review old messages or

change settings. Though they work together to form a cohesive user interface, each activity is independent of the

others. Each one is implemented as a subclass of the Activity base class.

An application might consist of just one activity or, like the text messaging application just mentioned, it may

contain several. What the activities are, and how many there are depends, of course, on the application and its

design. Typically, one of the activities is marked as the first one that should be presented to the user when the

application is launched. Moving from one activity to another is accomplished by having the current activity start the

next one.

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Each activity is given a default window to draw in. Typically, the window fills the screen, but it might be smaller

than the screen and float on top of other windows. An activity can also make use of additional windows — for

example, a pop-up dialog that calls for a user response in the midst of the activity, or a window that presents

users with vital information when they select a particular item on-screen.

The visual content of the window is provided by a hierarchy of views — objects derived from the base View class.

Each view controls a particular rectangular space within the window. Parent views contain and organize the layout

of their children. Leaf views (those at the bottom of the hierarchy) draw in the rectangles they control and respond

to user actions directed at that space. Thus, views are where the activity's interaction with the user takes place.

For example, a view might display a small image and initiate an action when the user taps that image. Android

has a number of ready-made views that you can use — including buttons, text fields, scroll bars, menu items,

check boxes, and more.

A view hierarchy is placed within an activity's window by the Activity.setContentView() method. The

content view is the View object at the root of the hierarchy. (See the separate User Interface document for more

information on views and the hierarchy.)

Services

A service doesn't have a visual user interface, but rather runs in the background for an indefinite period of time.

For example, a service might play background music as the user attends to other matters, or it might fetch data

over the network or calculate something and provide the result to activities that need it. Each service extends the

Service base class.

A prime example is a media player playing songs from a play list. The player application would probably have one

or more activities that allow the user to choose songs and start playing them. However, the music playback itself

would not be handled by an activity because users will expect the music to keep playing even after they leave the

player and begin something different. To keep the music going, the media player activity could start a service to

run in the background. The system would then keep the music playback service running even after the activity

that started it leaves the screen.

It's possible to connect to (bind to) an ongoing service (and start the service if it's not already running). While

connected, you can communicate with the service through an interface that the service exposes. For the music

service, this interface might allow users to pause, rewind, stop, and restart the playback.

Like activities and the other components, services run in the main thread of the application process. So that they

won't block other components or the user interface, they often spawn another thread for time-consuming tasks

(like music playback). See Processes and Threads , later.

Broadcast receivers

A broadcast receiver is a component that does nothing but receive and react to broadcast announcements. Many

broadcasts originate in system code — for example, announcements that the timezone has changed, that the

battery is low, that a picture has been taken, or that the user changed a language preference. 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.

An application can have any number of broadcast receivers to respond to any announcements it considers

important. All receivers extend the BroadcastReceiver base class.

Broadcast receivers do not display a user interface. However, they may start an activity in response to the

information they receive, or they may use the NotificationManager to alert the user. Notifications can get the user's

attention in various ways — flashing the backlight, vibrating the device, playing a sound, and so on. They typically

place a persistent icon in the status bar, which users can open to get the message.

Content providers

A content provider makes a specific set of the application's data available to other applications. The data can be

stored in the file system, in an SQLite database, or in any other manner that makes sense. The content provider

extends the ContentProvider base class to implement a standard set of methods that enable other applications to

retrieve and store data of the type it controls. However, applications do not call these methods directly. Rather

they use a ContentResolver object and call its methods instead. A ContentResolver can talk to any content

provider; it cooperates with the provider to manage any interprocess communication that's involved.

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