What is Android?
Android is a software stack for mobile devices that includes an operating
system, middleware
and key
applications. This early look at 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 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 Writing an Android Application.
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 n 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.