Well, to understand any operating system its very important to understand its architecture. I know most of you might not really understand what exactly does a “operating system architecture” mean. Architecture is basically a design, design flow of how the machine works in order to perform any operation. What you see on screen is just the upper layer of this architecture. Any action you perform on this layer(screen) goes through some set of layers and in the end interacts with the hardware of the machine commanding it perform the action(requested by you). Android was created for small machines. So designers had to keep in mind that they would be working with limited resources yet will have to replicate the operations performed on big machines. Therefore they had to come up with a architecture specific to small devices.
Android Architecture can be classified into four layers
- Linux Kernel
- Libraries+Android Runtime
- Application Framework
- Applications
Before diving deep on its working you might want to know how Android started and who invented it. Read How Android Started to know more
Linux Kernel
This is the at the bottom of the Android architecture. Let us first understand what exactly Kernel means. Kernel is central module of a OS. It is responsible for memory management, process management and disk management. Basically kernel is the first layer of software that interacts with the device hardware.
So even Android needs a kernel and instead of writing its own they choose Linux. Why? Because Linux is open source, Google’s Android developers could modify the Linux kernel to fit their needs. Linux gives the Android developers a pre-built, already maintained operating system kernel to start with. This is the way many different devices are built — for example, the PlayStation 4 uses the open-source FreeBSD kernel, while the Xbox One uses the Windows NT kernel found in modern versions of Windows.
HAL(Hardware Abstraction Layer)
Android application/framework communicates with the underlying hardware through Java APIs not by system calls. But the Linux has the ability to handle only systems calls from application. Therefore a layer is required which acts as bridge between software and hardware. HAL acts as this layer. The hardware abstraction layer, or HAL as its popularly known defines a standard interface for hardware vendors to implement and allows Android Applications to be agnostic about lower-level driver implementations. It is a c/c++ layer which is a vendor specific implementation. HAL implementations are packaged into modules (.so) file and loaded by the Android system at the appropriate time. Android HAL allows the Android application/framework to communicate with the hardware specific device drivers.
Libraries
On the top of HAL we have a set of libraries These libraries are used to enable various features in the Android OS. All these are written in C++. I am listing some of these libraries with its function.
| SQLite |
It is used to access data published by content providers and includes SQLite database management classes |
| SSL |
It is used to provide internet security |
| OpenGL |
It is used to provide Java interface to the OpenGL/ES 3D graphics rendering API. |
| Media framework |
It is used to provides different media codecs which allow the recording and playback of different media formats |
WebKit |
It is the browser engine used to display internet content or HTML content |
Within this layer comes the most important part of Android Architecture – Android Runtime
Android Runtime basically has two components
- Dalvik Virtual Machine
- Core Libraries
Dalvik Virtual Machine
Well I consider this as the most important part of Android architecture. This is what makes Android ->”ANDROID”:P Actually everything above this level of the Android architecture is written in Java. So basically we will have a set of .class files. Running those .class files on a small mobile processor is could lead to issues. Dalvik Virtual machine converts these .class files into .dex files which makes processing much faster. These files run with minimum memory footprint. Hence you can have multiple instances of this Virtual Machine running i.e. Multitasking. Dalvik Virtual Machine is similar to JVM but only difference is that it is designed and optimized for Android.
Core Libraries
These are a set of libraries written in Java to provide APIs for Android development
Android Application Framework
This is the topmost level of Android Architecture. It is basically the environment in which your application runs. This layer provides high level APIs for your android application, enabling it to implement custom features. I have listed some components below
- 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.
- Package Manager – The system by which applications are able to find out information about other applications currently installed on the device.
- Telephony Manager – Provides information to the application about the telephony services available on the device such as status and subscriber information.
- Location Manager – Provides access to the location services allowing an application to receive updates about location changes.
Applications
This contains all the application that you see on your device. This includes system based apps like Contacts, Calender etc and all the applications you have installed from the Google playstore. All these are developed with the help of APIs from core libraries and Android framework
Now that you have understood the Android architcture click here to read about the four important components of Android.
Android Clarified 