Android is a software stack for mobile devices that includes an operating
system, middleware and key applications. The Android SDK
provides the tools and APIs necessary to begin developing applications on the
Android platform using the Java programming language.
Features
- 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 Architecture
The following diagram shows the major components of the Android operating
system. Each section is described in more detail below.
Applications
Android will ship with a set of core applications including an email
client, SMS program, calendar, maps, browser, contacts, and
others. All applications are written using the Java programming language.
Application Framework
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 backstack
For more details and a walkthrough of an application, see the Notepad Tutorial.
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
Android Runtime
Android includes a set of core libraries that provides most of
the functionality available in the core libraries of the Java programming
language.
Every Android application runs in 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.
Linux Kernel
Android relies on Linux version 2.6 for core system services such as
security, memory management, process management, network stack, and driver
model. The kernel also acts as an abstraction layer between the hardware and
the rest of the software stack.
