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Android OS is a software stack of different layers, in which each layer is a group of some program components. It includes your operating system, middle-ware and important applications. Each layer in the architecture provides different services to the layer just above it. which is roughly divided into five sections and four main layers as

Applications :

The applications are at the topmost layer of the Android stack. All the applications will be installed in this layer such as contacts,Apps and Games from google Play Store etc.

The application layer runs within the Android run time using the classes and services made available from the application framework.

Application Framework:

Applications directly interact with these blocks of the Android architecture. These programs manage the basic functions of phone like telephony service, location services, notification manager, NFC service, view system, etc.  It provides many higher-level services to applications in the form of Java classes. Application developers are allowed to make use of these services in their applications.

The Android framework includes the following key services −

Activity Manager − Controls all aspects of the application life-cycle and activity stack.

Content Providers − Allows applications to publish and share data with other applications.

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

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

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

Run time :

This is the third section of the architecture and available on the second layer from the bottom.Runtime provides a key component called Dalvik Virtual Machine (DVM). Dalvik is an open-source software and Java Virtual Machine specially designed and optimized for Android.

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

The Android runtime also provides a set of core libraries which enable Android application developers to write Android applications using standard Java programming language.

Libraries :

The next layer in the Android architecture includes libraries. Libraries carry a set of instructions to guide the device to handle different types of data.

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

android.app − Provides access to the application model and is the cornerstone of all Android applications.

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

android.content − Facilitates content access, publishing and messaging between applications and application components.

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

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

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

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

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

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

Linux Kernel Layer:

Linux kernel is the bottom most layer in the architecture of android. It never really interacts with the users and developers, but is at the heart of the whole system. Its importance stems from the fact that it provides the functions in the Android system such as hardware abstraction, memory management programs, security settings, power management software and it contains all the essential hardware drivers like camera, keypad, display, network stack 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.

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