Android, currently the most popular mobile operating system in the world, continues to further solidify its position as the major mobile platform in the world. In this article I will briefly cover how Android has appeared, and will explain its software structure and what has to be paid attention when developing apps for this platform.
What is Android?
Linux operating system is the main component in Android's software stack. It does not differ much from Microsoft's Windows OS or Apple's Mac OS which we use on the daily basis. Therefore, theoretically you can enjoy Android not only on mobile devices but also on other tablet devices as well just like Windows even to a point where various applications can be downloaded and used. Of course, if you want to make an Android application by yourself, you should also install ADK (Android Development Kit) to your PC and use Java programming language for development. For a lower level program, NDK (Native Development Kit) for C language shall be used.
It seemed that Android had been shunned publicly for not having as sensitive UI as iPhone. However, now it is considered as the most viable blue-chip mobile OS with advantageous openness, free, and Java-based characteristics.
Another interesting aspect is that most users mistakenly think that Android was developed by Google. In fact, it was developed in 2003 by "Android" company based in California. Back then, few people knew about it, even less developed for it. However, everything was changed in 2005 when Google acquired Android company to give birth to Android that we know now.
The History of Google Android
Since its initial release of 1.0 version in September 23rd, 2008, Android has enjoyed a remarkable growth. All versions until 3.0 (code name Honeycomb) are available under Apache free open source license. According toAndroid official blog, the source code of the Honeycomb series was not released to the public because it was specifically created for tablet computers, and Google did not want external developers to mess up the user interface by putting on smartphones a version of Android intended for tablets.
However, Google later announced that in the next 4.0 version, In year 2011 (code nameIce Cream Sandwitch), Android will combine both the smartphone version (code name Gingerbread) and the tablet version (HoneyComb).
The first version of Android, released back in 2008, had drawbacks in many aspects. Some users voiced their worries about its terrible UI and development tools, saying “who would want to use it”. However with its advantage of using Java as SDK language and its openness Android gradually solidified itself as the most rapidly developing mobile OS, and it seems as if no one dare to challenge the idea that Android will further expand and attract more users than iPhone and be loaded onto more various mobile devices.
The following are the key functions of each Android version
2.0, 2.1 (Eclair): Web browser supporting new UI and HTML5. Advanced camera app, new launcher app and weather app. Supports moving background.
2.2 (Froyo) : Improved performance by optimizing JIT. Chrome V8 Javascript engine. Support Wi-fi hot spot tethering and Flash.
2.3 (Gingerbread) : Optimized UI for minimum battery use. Enhanced soft keyboard, COPY/PASTE function, NFC (Near Field Communication, local network).
3.0 (Honeycomb) : Support tablet screen. New UI and multi-core processor.
3.2 (Honeycomb) : Compatible mode for zoom-to-fill screen. Read media file directly from SD card. Additional screen API. Controls UI more delicately.
4.0 (Ice Cream Sandwich) : Combined version of Gingerbread and Honeycomb.
4.1-4.3(Jelly Bean):Triple buffering in the graphics pipeline.Support for wireless display .OpenGL ES 3.0 support, allowing for improved game graphics.
4.4(KitKat):New framework for UI transitions.New experimental runtime virtual machine, ART (not enabled by default).
The Structure of Android
Android basically runs on DVM (Dalvik Virtual Machine) which is Android runtime loaded on Linux kernel. The figure below illustrates all the components. Further I will explain each of them from top to down.
ANDROID STRUCTURE
Applications
This Section is for applications developed with Java. Generally, email client, SMS program, calendar, browser and contacts applications are loaded.
Application Framework
Application framework provides all kinds of APIs needed to develop a full-featured application for Android platform. Using APIs in this domain allows to express buttons and text on the screen or use data of other applications such as images and character strings. Also it is possible to manage application’s lifecycle.
Libraries
This Section provides various C/C++ libraries which can be used on Android. All libraries in this domain can be used by developers via application framework. Standard C system libraries based on BSD (Berkeley Software Distribution) are revised to be suitable for Linux-based devices. Media library based on OpenCore of PacketVideo supports MPEG4, H.264, MP3, AAC, AMR, JPG and PNG files. Surface manager supports 2D and 3D graphics whileWebKit supports browser functions. SQLite is also available as a database engine which can be used by applications.
Android Runtime
Although Android is developed in Java language, DVM (Dalvik Virtual Machine) is used instead of JVM (Java Virtual Machine). Thus, a source code (Java) is compiled to a class file (.class) with Java compiler which is later transformed to Dalvik Executable by DX tools. The reason for this is that Dalvik is optimized to run on small sized devices with limited memory.
Linux Kernel
Android is developed based on Linux kernel 2.6 which provides high security, memory and process management, network stack and various drivers.
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