Learn Android Application Development - Introduction & Architecture

Introduction

The Android Platform is a software stack and it was designed primarily for mobiles and tablets. This stack as several layers from low-level operating system services that manage the device to sample applications like dialler, contacts database and the web browser etc., Android also comes with a software developer kit (SDK) which is used to create Android Apps. A lot of content is available to get understanding on Android. 

Architecture

Linux Kernel Layer

The most bottom layer of software in the Android Platform. It contains the core sources that any Android device will rely on. Like general Linux kernel, this layer provides the most basic operating system services such as Permissions Architecture which can be used to restrict data access to only those processes that have authorization, Memory and Process Management, Low level details of File and Network I/O, Interaction with Device Drivers which are often coupled to Computing Devices. In addition to general Linux kernel features, it also has several Android Specific Components like Power Management Services, Memory Sharing (Android Shared Memory) and Management (Low Memory Killer) Features, Inter Process Communication Mechanism (Binder) and many more.

Libraries and Android Runtime

This includes a variety of system libraries. These libraries are typically written in C and C++ and this is why they often refer to native libraries which handle a lot of core performance-sensitive activities on device, such as quickly rendering web pages and updating them on display. Android has its own System C Library (Bionic LibC) which implements the standard OS system calls which do things like process and thread creation, mathematical computations, memory allocation and many more. Surface Manager for updating the display, Media Framework for playing Audio / Video files, Web Kit for rendering and displaying web pages, OpenGL for high performance graphics, SQLite for managing in-memory relational databases.

In addition to all these libraries it also includes Android Runtime which is for writing and running Android Applications. This includes Core Java Libraries and Dalvik Virtual Machine.

Android Applications are typically written in Java Programming Language, to make it easier to write applications Android provides a lot of reusable Java Building Blocks, for example Java (java.*) and Java X (javax.*) (Java Extension Packages) include basic software for things like common data structures like concurrency mechanisms and file I/O,  Android Packages (android.*) that have software that’s specific to the life cycle of mobile applications, ORG Packages (org.*) that support various internet or web operations, JUnit (junit.*) Packages which support the Unit Testing of applications.

Dalvik Virtual Machine is the software that executes android applications. Once an App is written in Java, the files are compiled into multiple Java Byte Code Files, which are converted by a tool called DX into a single DEX Code File which is usually called classes.dex, then the .dex file is packaged with other application resources and installed on the device. When the application is launched the DVM executes the classes.dex file. Since DVM unlike JVM was designed to run in a resource-constrained environment which is typical of mobile devices.

Application Framework

Contains the reusable software that many mobile applications are likely to need, for example the

·        Package Manager is a database that keeps track of all the applications currently installed on device. It  helps one application to interact with another like data sharing, request services.

·        Window Manager manages the windows that compromise an app.

·   View System contains the common graphical things that many applications include in their user interfaces such as icons, buttons, text entry boxes and more.

·    Resource Manager manages the non-compiled resources that make up an application like strings, graphics, and user interface layout files.

·         Activity Manager manages the lifecycle and navigation stack. At a high level Android activities often correspond to a single user interface screen. Applications are created by stringing together multiple activities through which a user can navigate. The activity manager helps to coordinate and support this navigation.

·   Content Provider(s) are essential databases that allow applications to store and share structured information. Also these are designed for cross applications.

·    Location Manager allows applications to receive location and movement information such as that generated by GPS system, and this allows applications to perform context-specific tasks.

·         Notification Manager allows applications to place information in the notification bar.

     Application Layer

Android comes with a set of default applications such as Home Screen, Contacts, Browser, Mail, Phone and more. The beauty of android is none of these apps are hard-coded into the system which means that if we’ve a better app we can substitute it in place of these standard apps.

We'll see the Android Development Environment in the next post.

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