Android - Skeleton

Some parts of Android is familiar to all of us, such as the Linux Kernel, OpenGL, and the SQL database. Others may be completely foreign, such as Android’s idea of the application life cycle.

Starting at the bottom is the Linux Kernel. Android uses Linux for its device drivers, memory management, process management, and networking. However you will never be programming to this layer directly.

The next level up contains the Android native libraries. They are all written in C/C++ internally, but you’ll be calling them through Java interfaces. In this layer you can find the Surface Manager (for compositing windows), 2D and 3D graphics, Media codecs (MPEG-4, H.264, MP3, etc.), the SQL database (SQLite), and a native web browser engine (WebKit).

Next is the Android runtime, including the Dalvik Virtual Machine. Dalvik runs dex files, which are converted at compile time from standard class and jar files. Dex files are more compact and efficient than class files, an important consideration for the limited memory and battery powered devices that Android targets.

The core Java libraries are also part of the Android runtime. They are written in Java, as is everything above this layer. Here, Android provides a substantial subset of the Java 5 Standard Edition packages, including Collections, I/O, and so forth.

The next level up is the Application Framework layer. Parts of this toolkit are provided by Google, and parts are extensions or services that you write. The most important component of the framework is the Activity Manager, which manages the life cycle of applications and a common “back-stack” for user navigation.

Finally, the top layer is the Applications layer. Most of your code will live here, along side built-in applications such as the Phone and Web Browser.

One of the unique and powerful qualities of Android is that all applications have a level playing field. What I mean is that the applications Google writes have to go through the same public API that you use. You can even tell Android to make your application replace the standard applications if you like.

Libraries

Android includes a set of C/C++ libraries used by various components of the Android system. These capabilities are exposed to developers through the Android application framework. Some of the core libraries are listed below:

• System C library - a BSD-derived implementation of the standard C system library (libc), tuned for embedded Linux-based devices
• Media Libraries - based on PacketVideo's OpenCORE; the libraries support playback and recording of many popular audio and video formats, as well as static image files, including MPEG4, H.264, MP3, AAC, AMR, JPG, and PNG
• Surface Manager - manages access to the display subsystem and seamlessly composites 2D and 3D graphic layers from multiple applications
• LibWebCore - a modern web browser engine which powers both the Android browser and an embeddable web view
• SGL - the underlying 2D graphics engine
• 3D libraries - an implementation based on OpenGL ES 1.0 APIs; the libraries use either hardware 3D acceleration (where available) or the included, highly optimized 3D software rasterizer
• FreeType - bitmap and vector font rendering
• SQLite - a powerful and lightweight relational database engine available to all applications

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