Operating System
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
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