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.

1. What is Android?

1.1. Android Operation System

Android is an operating system based on Linux with a Java programming interface. It provides tools, e.g. a compiler, debugger and a device emulator as well as its own Java Virtual machine (Dalvik Virtual Machine - DVM).

Android is officially guided by the Open Handset Alliance but in reality Google leads the project.

Android supports 2-D and 3-D graphics using the OpenGL libraries and supports data storage in a SQLite database.

Every Android applications runs in its own process and under its own user id which is generated automatically by the Android system during deployment. Therefore the application is isolated from other running applications and a misbehaving application cannot easily harm other Android applications.

1.2. Important Android components

An Android application consists out of the following parts:

• Activity - represents the presentation layer of an Android application, e.g. a screen which the user sees. An Android application can have several activities and it can be switched between them during runtime of the application.

• Views - the User interface of an Activities is built with widget classes which inherit from android.view.View. The layout of the views is managed by android.view.ViewGroups. Views often have attributes which can be used to change their appearance and behavior.

• Services - perform background tasks without providing an UI. They can notify the user via the notification framework in Android.

• ContentProvider - provides data to applications, via a content provider your application can share data with other applications. Android contains a SQLite DB which can serve as data provider

• Intents - are asynchronous messages which allow the application to request functionality from other services or activities. An application can call directly a service or activity (explicit intent) or ask the Android system for registered services and applications for an intent (implicit intents). For example the application could ask via an intent for a contact application. Applications register themselves to an intent via an IntentFilter. Intents are a powerful concept as they allow the creation of loosely coupled applications.

• BroadcastReceiver - receives system messages and implicit intents, can be used to react to changed conditions in the system. An application can register as a BroadcastReceiver for certain events and can be started if such an event occurs.

• Widgets - interactive components primary used on the Android homescreen to display certain data and to allow the user to have quick access the the information

Other Android components are Live Folders and Android Live Wallpapers. Live Folders display data on the homescreen without launching the corresponding application.

1.3. Dalvik Virtual Machine

Android uses a special virtual machine, e.g. the Dalvik Virtual Machine. Dalvik uses special bytecode. Therefore you cannot run standard Java bytecode on Android. Android provides a tool dx which allows to convert Java Class files into dex (Dalvik Executable) files. Android applications are packed into an .apk (Android Package) file by the program aapt (Android Asset Packaging Tool) To simplify development Google provides the Android Development Tools (ADT) for Eclipse. The ADT performs automatically the conversion from class to dex files and creates the apk during deployment.

1.4. Security and permissions

Android defines certain permissions for certain tasks. For example if the application wants to access the Internet it must define in its configuration file that it would like to use the related permission. During the installation of an Android application the user receives a screen in which he needs to confirm the required permissions of the application.