Android Architecture (The Stack)

The Android operating system is like a cake consisting of various layers. Each layer has its own characteristics and purpose. The layers are not cleanly separated but often seep into each other.

Lets discuss about each layer from bottom to top.

• Linux Kernel

Android is built on top of a solid and proven foundation: the Linux kernel. Created by Linus Torvalds in 1991, Linux can be found today in everything from wristwatches to supercomputers. Linux provides the hardware abstraction layer for Android, allowing Android to be ported to a wide variety of platforms in the future. There are many good reasons for choosing Linux as the base of the Android stack. Some of the main ones are its portability, security, and features

• Native Libraries

The native libraries are C/C++ libraries, often taken from the open source community in order to provide necessary services to the Android application layer. Among others, they include:

Webkit            

A fast web-rendering engine used by Safari, Chrome, and other browsers

SQLite

A full-featured SQL database

Apache Harmony

An open source implementation of Java

OpenGL

3D graphics libraries

OpenSSL

The secure locket layer

Although many of these libraries are used as-is, one notable exception is Bionic, which is basically a rewritten version of the standard C library. Bionic is used for two reasons:

Technology

To make it purpose-built for tiny, battery-powered devices

License

To make it license-friendly for others who might want to adopt it and change it

Dalvik

Dalvik is a purpose-built virtual machine designed specifically for Android, developed by Dan Bornstein and his team at Google.

The Java virtual machine (VM) was designed to be a one-size-fits-all solution, and the Dalvik team felt they could do a better job by focusing strictly on mobile devices. They looked at which constraints specific to a mobile environment are least likely to change in the near future. One of these is battery life, and the other is processing power. Dalvik was built from the ground up to address those constraints.

Another side effect of replacing the Java VM with the Dalvik VM is the licensing. Whereas the Java language, Java tools, and Java libraries are free, the Java virtual machine is not. This was more of an issue back in 2005 when the work on Dalvik started. Nowadays, there are open source alternatives to Sun’s Java VM, namely the OpenJDK and Apache Harmony projects.

By developing a truly open source and license-friendly virtual machine, Android yet again provides a full-featured platform that others are encouraged to adopt for a variety of devices without having to worry about the license.

Android and Java

In Java, you write your Java source file, compile it into a Java byte code using the Java compiler, and then run this byte code on the Java VM. In Android, things are different. You still write the Java source file, and you still compile it to Java byte code using the same Java compiler. But at that point, you recompile it once again using the Dalvik compiler to Dalvik byte code. It is this Dalvik byte code that is then executed on the Dalvik VM.

You may wonder, why not compile straight from Java into the Dalvik byte code? There are a couple of good reasons for the extra steps. Back in 2005, when work on Dalvik started, the Java language was going through frequent changes, but the Java byte code was more or less set in stone. So, the Android team chose to base Dalvik on Java byte code instead of Java source code. A side effect of this is that in theory you could write Android applications in any other language that compiles down to Java byte code. For example, you could use Python or Ruby.

• Application Framework

Sitting above the native libraries and runtime, you’ll find the Application Framework layer. This layer provides the high-level building blocks you will use to create your applications. The framework comes preinstalled with Android, but you can also extend it with your own components as needed.

The most important parts of the framework are as follows:

Activity Manager: This controls the life cycle of applications 

Content providers: These objects encapsulate data that needs to be shared between applications, such as contacts.

Resource manager: Resources are anything that goes with your program that is not code.

Location manager: An Android phone always knows where it is.

Notification manager: Events such as arriving messages, appointments, proximity alerts and more can be presented in an unobtrusive fashion to the user.

• Applications

And finally, there are the applications that you and other developers create. These applications are what end users find valuable about Android. They can come preinstalled on the device or can be downloaded from one of the many Android markets.

The APK

An application is a single application package (APK) file. An APK file roughly has three main components. An API consists of the following major components:

Dalvik executable

This is all your Java source code compiled down to a Dalvik executable. This is the code that runs your application.

Resources

Resources are everything that is not code. Your application may contain a number of images and audio/video clips, as well as numerous XML files describing layouts, language packs, and so on. Collectively, these items are the resources.

Native libraries

Optionally, your application may include some native code, such as C/C++ libraries.

These libraries could be packaged together with your APK file.