The Android software stack is composed of the elements shown in Figure 1-1 and described in further detail after it. Put simply, a Linux kernel and a collection of C/C++ libraries are exposed through an application framework that provides services for, and management of, the run time and applications.
Linux kernel Core services (including hardware drivers, process and memory management, security, network, and power management) are handled by a Linux 2.6 kernel. The kernel also provides an abstraction layer between the hardware and the remainder of the stack.
Libraries Running on top of the kernel, Android includes various C/C++ core libraries such as libc and SSL, as well as:
A media library for playback of audio and video media
A surface manager to provide display management
Graphics libraries that include SGL and OpenGL for 2D and 3D graphics
SQLite for native database support
SSL andWebKit for integrated web browser and Internet security
Android run time What makes an Android phone an Android phone rather than a mobile Linux implementation is the Android run time. Including the core libraries and the Dalvik virtual machine, the Android run time is the engine that powers your applications and, along with the libraries, forms the basis for the application framework.
Core libraries While Android development is done in Java, Dalvik is not a Java VM. The core Android libraries provide most of the functionality available in the core Java libraries as well as the Android-specific libraries.
Dalvik virtual machine Dalvik is a register-based virtual machine that’s been optimized to ensure that a device can run multiple instances efficiently. It relies on the Linux kernel for threading and low-level memory management.
Application framework The application framework provides the classes used to create Android applications. It also provides a generic abstraction for hardware access and manages the user interface and application resources.
Application layer All applications, both native and third-party, are built on the application layer by means of the same API libraries. The application layer runs within the Android run time, using the classes and services made available from the application framework.
The Dalvik Virtual Machine
One of the key elements of Android is the Dalvik virtual machine. Rather than use a traditional Java virtual machine (VM) such as Java ME (Java Mobile Edition), Android uses its own custom VM designed to ensure that multiple instances run efficiently on a single device.
The Dalvik VM uses the device’s underlying Linux kernel to handle low-level functionality including security, threading, and process and memory management. It’s also possible to write C/C++ applications that run directly on the underlying Linux OS. While you can do this, in most cases there’s no reason you should need to.
If the speed and efficiency of C/C++ is required for your application, Android now provides a Native Development Kit (NDK). The NDK is designed to enable you to create C++ libraries using the libc and libm libraries, along with native access to OpenGL.
All Android hardware and system service access is managed using Dalvik as a middle tier. By using a VM to host application execution, developers have an abstraction layer that ensures they never have to worry about a particular hardware implementation.
The Dalvik VM executes Dalvik executable files, a format optimized to ensure minimal memory footprint. You create.dex executables by transforming Java language compiled classes using the tools supplied within the SDK.
All Android hardware and system service access is managed using Dalvik as a middle tier. By using a VM to host application execution, developers have an abstraction layer that ensures they never have to worry about a particular hardware implementation.
