Android
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[edit] Contents
1 Introduction
1.1 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 the APIs necessary to begin developing applications on the Android platform. Applications are written using the Java programming language and run on Dalvik, a custom virtual machine specifically designed for embedded use. Dalvik runs on top of a stripped down Linux kernel. Android was originally developed by Google and unveiled to the developer community on 5th November 2007. Android was designed to be distributed based on an open source license and to compete against other mobile platforms, including those from Microsoft, Research In Motion and Apple Inc. Google also announced the founding of the Open Handset Alliance (OHA), a consortium of 47 technology companies devoted to advancing open standards for mobile devices. Android isn’t currently completely open source but a large portion of it is. Google and the OHA is committed to eventually making Android 100% open source. The first commercially available mobile device running android is T-Mobile’s G1 smart phone. The G1 became available on the 22nd of October 2008. OHA includes members such as Sony Ericsson, Motorola, LG, Samsung and HTC who between them account for close to 50% of all handsets being manufactured. The future for Android definitely looks very bright.
1.2 Architectural Overview
The Android environment is built on top of a Linux kernel and includes a set of C/C++ libraries, the Android Runtime environment, an Application Framework and a set of core applications as described in the diagram below.
Linux Kernel At the base of the Android environment is a stripped down Linux Kernel. The current release of the environment uses Linux kernel version 2.6. Linux is used to communicate with the mobile phone’s hardware and Android supports multiple phone processors. Much of the Android work though happens in the layers above the OS.
Android Runtime The Android runtime environment consists of a set of core libraries and virtual machine. The core libraries provide most of the functionality available in the core libraries of the Java programming language.
On conventional computing devices software runs directly on the operating system kernel but an Android application runs in its own process, with its own instance of the Dalvik virtual machine. Dalvik has been written so that a mobile device can run multiple instances of it in an efficient manner. Dalvik is not your typical JVM though it’s very similar. 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.
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
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 • An Activity Manager that manages the life cycle of applications and provides a common navigation back-stack
Applications Android ships with a set of core applications including an email client, SMS program, calendar, maps, browser and others. All applications are written using the Java programming language syntax. A java application written for Android is not compatible with Java programs written for the Java SE and Java ME platforms.
