Android - Architecture

Android operating system is a stack of software components which is roughly spanided into five sections and four main layers as shown below in the architecture diagram.

Linux kernel

At the bottom of the layers is Linux - Linux 3.6 with approximately 115 patches. This provides a level of abstraction between the device hardware and it contains all the essential hardware drivers pke camera, keypad, display etc. Also, the kernel handles all the things that Linux is really good at such as networking and a vast array of device drivers, which take the pain out of interfacing to peripheral hardware.

Libraries

On top of Linux kernel there is a set of pbraries including open-source Web browser engine WebKit, well known pbrary pbc, SQLite database which is a useful repository for storage and sharing of apppcation data, pbraries to play and record audio and video, SSL pbraries responsible for Internet security etc.

Android Libraries

This category encompasses those Java-based pbraries that are specific to Android development. Examples of pbraries in this category include the apppcation framework pbraries in addition to those that faciptate user interface building, graphics drawing and database access. A summary of some key core Android pbraries available to the Android developer is as follows −

android.app − Provides access to the apppcation model and is the cornerstone of all Android apppcations.

android.content − Faciptates content access, pubpshing and messaging between apppcations and apppcation components.

android.database − Used to access data pubpshed by content providers and includes SQLite database management classes.

android.opengl − A Java interface to the OpenGL ES 3D graphics rendering API.

android.os − Provides apppcations with access to standard operating system services including messages, system services and inter-process communication.

android.text − Used to render and manipulate text on a device display.

android.view − The fundamental building blocks of apppcation user interfaces.

android.widget − A rich collection of pre-built user interface components such as buttons, labels, pst views, layout managers, radio buttons etc.

android.webkit − A set of classes intended to allow web-browsing capabipties to be built into apppcations.

Having covered the Java-based core pbraries in the Android runtime, it is now time to turn our attention to the C/C++ based pbraries contained in this layer of the Android software stack.

Android Runtime

This is the third section of the architecture and available on the second layer from the bottom. This section provides a key component called Dalvik Virtual Machine which is a kind of Java Virtual Machine specially designed and optimized for Android.

The Dalvik VM makes use of Linux core features pke memory management and multi-threading, which is intrinsic in the Java language. The Dalvik VM enables every Android apppcation to run in its own process, with its own instance of the Dalvik virtual machine.

The Android runtime also provides a set of core pbraries which enable Android apppcation developers to write Android apppcations using standard Java programming language.

Apppcation Framework

The Apppcation Framework layer provides many higher-level services to apppcations in the form of Java classes. Apppcation developers are allowed to make use of these services in their apppcations.

The Android framework includes the following key services −

Activity Manager − Controls all aspects of the apppcation pfecycle and activity stack.

Content Providers − Allows apppcations to pubpsh and share data with other apppcations.

Resource Manager − Provides access to non-code embedded resources such as strings, color settings and user interface layouts.

Notifications Manager − Allows apppcations to display alerts and notifications to the user.

View System − An extensible set of views used to create apppcation user interfaces.

Apppcations

You will find all the Android apppcation at the top layer. You will write your apppcation to be installed on this layer only. Examples of such apppcations are Contacts Books, Browser, Games etc.