Android is a flexible, open source, mobile operating system derived from Linux Kernel version 2.6 for core system services. The code base was developed and is maintained by Google. The most recent Amazon Kindle device, Kindle Fire, is powered by Amazon’s forked version of Android 2.3.2 (Gingerbread) for tablets. This is a primary example of the availability and flexibility of the OS.
The Android software stack includes an operating system, middleware and applications. The SDK gives developers access to all tools and APIs required to develop applications for the platform. Development on the Android platform requires the use of Java.
- Application framework enabling reuse and replacement of components
- Dalvik virtual machine optimized for mobile devices
- Integrated browser based on the open source WebKit engine
- Optimized graphics powered by a custom 2D graphics library; 3D graphics based on the OpenGL ES 1.0 specification (hardware acceleration optional)
- SQLite for structured data storage
- Media support for common audio, video and still image formats (MPEG4, H.264, MP3, AAC, AMR, JPG, PNG, GIF)
- GSM Telephony (hardware dependent)
- Bluetooth, EDGE, 3G, and WiFi (hardware dependent)
- Camera, GPS, compass and accelerometer (hardware dependent)
- Rich development environment including a device emulator, tools for debugging, memory and performance profiling and a plugin for the Eclipse IDE
Android ships with a set of core applications including an email client, SMS program, calendar, maps, browser and contacts. All applications are written using the Java programming language.
By providing an open development platform, Android offers the ability to build extremely rich and innovative applications. Developers are free to take advantage of the device hardware, access location information, run background services, set alarms, add notifications to the status bar and much, much more.
Developers have full access to the same framework APIs used by the core applications. The application architecture is designed to simplify the reuse of components; any application can publish its capabilities and any other application may then make use of those capabilities (subject to security constraints enforced by the framework). This same mechanism allows components to be replaced by the user.
Underlying all applications is a set of services and systems, including:
- A rich and extensible set of Views that can be used to build an application, including lists, grids, text boxes, buttons and even an embeddable web browser
- Content Providers that enable applications to access data from other applications (such as Contacts), or to share their own data
- A Resource Manager providing access to non-code resources such as localized strings, graphics and layout files
- A Notification Manager that enables all applications to display custom alerts in the status bar
- An Activity Manager that manages the lifecycle of applications and provides a common navigation
For more, take a look at this tutorial on writing a notepad application.
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:
- 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
Android includes a set of core libraries that provides most of the functionality available in the core libraries of Java.
Every Android application runs its own process, with its own instance of the Dalvik virtual machine. Dalvik has been written so that a device can run multiple VMs efficiently. The Dalvik VM executes files in the Dalvik Executable (.dex) format, which is optimized for minimal memory footprint. The VM is register-based and runs classes compiled by a Java language compiler that have been transformed into the .dex format by the included “dx” tool.
The Dalvik VM relies on the Linux kernel for underlying functionality such as threading and low-level memory management.
Read the full overview (complete with workflow diagrams) at developer.android.com.