Android Operating System

Android is a mobile operating system initially developed by Android Inc. Android is a software stack for mobile devices that includes an operating system, middleware and key applications. Android was bought by Google in 2005.Android is based upon a modified version of the Linux kernel.
Google and other members of the Open Handset Alliance collaborated on Android's development and release. The Android Open Source Project (AOSP) is tasked with the maintenance and further development of Android. Unit sales for Android OS smartphones ranked first among all smartphone OS handsets sold in the U.S. in the second and third quarters of 2010, with a third quarter market share of 43.6%. The Android operating system consists of 12 million lines of code including 3 million lines of XML, 2.8 million lines of C, 2.1 million lines of Java, and 1.75 million lines of C++. Android is under version 2 of the Apache Software License (ASL).

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.

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.

App -> Runtime Service -> lib :

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.

There are four building blocks for an Android application:

Activity -a single screen
Intent Receiver -to execute in reaction to an external event(Phone Ring)
Service -code that is long-lived and runs without a UI(Media Player)
Content Provider -an application's data to be shared with other applications

 Hierarchy of processes :  
Foreground Process -required for what the user is currently doing .
Visible Process -holding an Activity - visible to the user on-screen but not in the foreground(on pause). Service Process -holding a Service - not directly visible to the user- relevant tasks.
Background Process -holding an Activity - not visible to the user - can kill at any time(stopped).
Empty Process -doesn't hold any active application components(as a cache to improve start-up time) .

Android Applications & features :

Handset layouts  :- The platform is adaptable to larger, VGA, 2D graphics library, 3D graphics library based on OpenGL ES 2.0 specifications, and traditional smartphone layouts.
Storage :- SQLite, a lightweight relational database, is used for data storage purposes
Connectivity :- Android supports connectivity technologies including GSM/EDGE, IDEN, CDMA, EV-DO, UMTS, Bluetooth, Wi-Fi, LTE, and WiMAX.
Messaging :- SMS and MMS are available forms of messaging, including threaded text messaging and now Android Cloud to Device Messaging Framework (C2DM) is also a part of Android Push Messaging service. Web browser :- The web browser available in Android is based on the open-source WebKit layout engine, coupled with Chrome's V8 JavaScript engine. The browser scores a 93/100 on the Acid3 Test.
Java support :- While Android applications are written in Java, there's no Java Virtual Machine in the platform and Java byte code is not executed. Java classes get recompiled into Dalvik executable and run on Dalvik virtual machine. Dalvik is a specialized virtual machine designed specifically for Android and optimized for battery-powered mobile devices with limited memory and CPU. J2ME support can be provided via third-party-application such as the J2ME MIDP Runner.
Tethering :- Android supports tethering, which allows a phone to be used as a wireless/wired hotspot (All 2.2 Froyo phones). To allow a laptop to share the 3G connection on an Android phone software may need to be installed on both the phone and the laptop.

Advantages :

                      There are a host of advantages that Google’s Android will derive from being an open source software. Some of the advantages include:
The ability for anyone to customize the Google Android platform.
The consumer will benefit from having a wide range of mobile applications to choose from since the monopoly will be broken by Google Android.
Men will be able to customize a mobile phones using Google Android platform like never before Features like weather details, opening screen, live RSS feeds and even the icons on the opening screen will be able to be customized.
As a result of many mobile phones carrying Google Android, companies will come up with such innovative products like the location.
In addition the entertainment functionalities will be taken a notch higher by Google Android being able to offer online real time multiplayer games.

 Limitations :

Bluetooth limitations :
Android doesn't support: Bluetooth stereo, Contacts exchange, Modem pairing, Wireless keyboards,
But it'll work with Bluetooth headsets, but that's about it.
Firefox Mobile isn't coming to Android  Apps in Android Market need to be programmed with a custom form of Java     →  Mozilla won't have that.