what Is Android?

what Is Android?

Android is a complete software stack for mobile devices such as cell phones, PDAs and high end MP3 players. The software stack is split into four layers:

* The application layer
* The application framework
* The libraries and runtime
* The kernel

Cell phone users obviously work with applications in the application layer. Android developers write those applications using the application framework. Unlike many embedded operating environments, Android applications are all equal -- that is, the applications that come with the phone are no different than those that any developer writes. In fact, using the IntentFilter API, any application can handle any event that the user or system can generate. This sounds a bit scary at first, but Android has a well thought-out security model based on Unix file system permissions that assure applications have only those abilities that cell phone owner gave them at install time. The framework is supported by numerous open source libraries such as openssl, sqlite and libc. It is also supported by the Android core libraries -- more on that in a second. At the bottom of the stack sits the Linux 2.6 kernel, providing the low level hardware interfaces that we all expect from a kernel. This is a Unix based system -- that is, the Unix C APIs are available -- but don't expect to drop to a shell and start executing shell scripts with your old friends grep and awk. Most of the Unix utilities are simply not there. Instead Android supplies a well thought out API for writing applications -- in Java using the Android core libraries.

That's right, Android applications are almost exclusively written in Java. The Android core library is a big .jar file that is supported by the Dalvik Virtual Machine -- a fast and efficient JVM work-alike that enables java-coded applications to work on the Android cell phone. This is similar to, but not the same as using Sun's JVM directly.

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