Google Android Architecture
Android operating system is a stack of software components which is roughly divided 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 2.6 with approximately 115 patches. This provides basic system functionality like process management, memory management, device management like 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.Google usually refers to the Android OS as a software stack. Each layer of the stack groups together several programs that support specific operating system functions.
The base of the stack is the kernel. Google used the Linux version 2.6 OS to build Android's kernel, which includes Android's memory management programs, security settings, power management software and several hardware drivers. Drivers are programs that control hardware devices. For example, the Nexus One has a camera. The Android kernel includes a camera driver, which allows the user to send commands to the camera hardware.
Libraries
On top of Linux kernel there is a set of libraries including open-source Web browser engine WebKit, well known library libc, SQLite database which is a useful repository for storage and sharing of application data, libraries to play and record audio and video, SSL libraries responsible for Internet security etc.You can think of libraries as a set of instructions that tell the device how to handle different kinds of data. For example, the media framework library supports playback and recording of various audio, video and picture formats. Other libraries include a three-dimensional acceleration library (for devices with accelerometers) and a Web browser library.
Some of the important native libraries include the following:
Surface Manager: It is used for compositing window manager with off-screen buffering. Off-screen buffering means you cant directly draw into the screen, but your drawings go to the off-screen buffer. There it is combined with other drawings and form the final screen the user will see. This off screen buffer is the reason behind the transparency of windows.
Media framework: Media framework provides different media codecs allowing the recording and playback of different media formats
SQLite: SQLite is the database engine used in android for data storage purposes
WebKit: It is the browser engine used to display HTML content
OpenGL: Used to render 2D or 3D graphics content to the screen
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 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.
Located on the same level as the libraries layer, the Android runtime layer includes a set of core Java libraries -- Android application programmers build their apps using the Java programming language. It also includes the Dalvik Virtual Machine.
A virtual machine is a software application that behaves as if it were an independent device with its own operating system. You can run a virtual machine on a computer that operates on a completely different OS than the physical machine's OS. The Android OS uses virtual machines to run each application as its own process. That's important for a few reasons. First, no application is dependent upon another. Second, if an application crashes, it shouldn't affect any other applications running on the device. Third, it simplifies memory management.
Application Framework
The Application Framework layer 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.This includes the programs that manage the phone's basic functions like resource allocation, telephone applications, switching between processes or programs and keeping track of the phone's physical location. Application developers have full access to Android's application framework. This allows them to take advantage of Android's processing capabilities and support features when building an Android application. Think of the application framework as a set of basic tools with which a developer can build much more complex tools.
Important blocks of Application framework are:
Activity Manager: Manages the activity life cycle of applications
Content Providers: Manage the data sharing between applications
Telephony Manager: Manages all voice calls. We use telephony manager if we want to access voice calls in our application.
Location Manager: Location management, using GPS or cell tower
Resource Manager: Manage the various types of resources we use in our Application
Applications
You will find all the Android application at the top layer. You will write your application to be installed on this layer only. Examples of such applications are Contacts Books, Browser, Games etc.This is where you find the basic functions of the device such as making phone calls, accessing the Web browser and accessing your contacts list. If you're an average user, this is the layer you'll use most. You do that with the user interface. Only Google programmers, application developers and hardware manufacturers access the other layers further down the stack.
- SMS client app
- Dialer
- Web browser
- Contact manager
