[Read note] Android Dev Guide -> What is Android?

 

http://developer.android.com/guide/basics/what-is-android.html

What is Android?

Android is a software stack for mobile devices that includes an operatingsystem, middleware and key applications. The Android SDKprovides the tools and APIs necessary to begin developing applications on theAndroid 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 operatingsystem. Each section is described in more detail below.

Applications

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

Application Framework

Developers have full access to the same framework APIs used by the coreapplications. The application architecture is designed to simplify the reuseof components; any application can publish its capabilities and any otherapplication may then make use of those capabilities (subject to securityconstraints enforced by the framework). This same mechanism allows componentsto 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 theAndroid system. These capabilities are exposed to developers through theAndroid 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 ofthe functionality available in the core libraries of the Java programminglanguage.

Every Android application runs in its own process, with its own instance ofthe Dalvik virtual machine. Dalvik has been written so that a device can runmultiple VMs efficiently. The Dalvik VM executes files in the DalvikExecutable (.dex) format which is optimized for minimal memoryfootprint. The VM is register-based, and runs classescompiled by a Java language compiler that have been transformed into the .dexformat by the included "dx" tool.

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

Linux Kernel

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