Dev-Guide_Android Basics_What is Android?

leixiaofeng

我将从这里开始记录一些技术备份，笔记。先从android的docs资料开始。目前主要是翻译，为了怕误人子弟，附带英文原文。如果若有不幸浏览到的童鞋，欢迎你提出意见。

What is Android?

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.

什么是Android？

Android系统是为移动设备定制的软件集合，它包括了操作系统，中间件和一些基本的应用程序。Android SDK为用java语言在Android平台开发应用程序提供了工具和必要的应用程序接口。

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
  

特性

应用程序框架允许重用和替换组件

为移动设备优化过的Dalvik虚拟机

集成的基于开源的Webkit引擎的浏览器

使用自定义的2D图形库优化过的图形处理 ；基于OpenGL ES 1.0规范的的3D图形处理（硬件加速可选）

SQLIte结构化的数据存储

多媒体支持，常规的声音，视频，和图像格式（MPEG4,H.264,MP3,AAC,AMR,JPG,PNG,GIF）

GSM技术（硬件支持）

蓝牙，EDGE，3G，和WIFI（硬件支持）

丰富的开发环境，包括设备模拟器，调试，内存查看，性能分析和Eclips IDE下的插件

Android Architecture

The following diagram shows the major components of the Android operating system. Each section is described in more detail below.

图（略）

Android 体系结构

下图展示了Android操作系统的主要组件，每个部分讲在后面详细描述。

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.

应用程序

Android 装载了一系列的核心应用程序包括邮件客户端，SMS程序，日历，地图，浏览器，联系人等。所有的应用程序都用java程序语言写的。

Application Framework

By providing an open development platform, Android offers developers 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 [url=]Views[/url] that can be used to build an application, including lists, grids, text boxes, buttons, and even an embeddable web browser
• [url=]Content Providers[/url] that enable applications to access data from other applications (such as Contacts), or to share their own data
• A [url=]Resource Manager[/url], providing access to non-code resources such as localized strings, graphics, and layout files
• A [url=]Notification Manager[/url] that enables all applications to display custom alerts in the status bar
• An [url=]Activity Manager[/url] that manages the lifecycle of applications and provides a common navigation backstack
  

For more details and a walkthrough of an application, see the [url=]Notepad Tutorial[/url].

应用程序框架

通过提供了一个开放的开发平台，Android提供给了开发者创建非常丰富和富有创意的应用程序的能力。开发者可以自由的利用设备硬件，访问位置信息，运行后台服务，设置闹钟，在状态栏添加提醒，等等等等。

开发者可以完全访问核心应用程序所使用到的相同的框架API。应用程序的架构被设计为可以非常简单的重用组件，一个应用程序可以发布它的功能并且另外一些应用程序可以使用这些功能（受限于框架的强制安全约束）。这个原则允许系统中的组件被用户程序所替代。

所有基础的应用程序是一系列的服务和系统，包括：

一套丰富的可扩展的View（视图）集合可以用来构建应用程序，包括列表，表格，文本框，按钮，甚至一个内嵌的web浏览器

内容提供者允许应用程序访问其他程序的数据，或者共享自己的数据，比如电话簿

资源管理器提供了不需要编码就能访问资源的能力，如固定字符串，图形，和布局文件

提醒管理器允许用户在状态栏显示自定义提示

活动管理器管理应用程序的生命周期和提供常用的导航回退功能

更详细的说明和走读一个应用程序代码，请查看记事本程序教程

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:

Android包括一组被各种Android系统组件使用到的C/C++库，这些功能通过Android程序框架暴露给开发者使用。部分核心库列表如下：

• 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
  

系统c库--一个基于嵌入式linux设备的由BSD派生的标准c系统库

媒体库--基于 PacketVideo的 OpenCORE；这个库支持回放，以及许多流行的音视频和图像的格式，包括MPEG4, H.264, MP3, AAC, AMR, JPG, and PNG

表层管理器--管理访问显示子系统和多应用程序的2D，3D图像层的无缝合成

web核心库--一个流行的同时驱动Android浏览器和嵌入式web视图的引擎

SGL--基础2D图像引擎

3D库--一个基于OpenGL ES 1.0 APIs的实现；这个库用了硬件3D加速或者是包括了高优化的软件光栅

FreeType--位图和矢量字体渲染

SQLite--为所有应用程序提供的一个强大的轻量级的关系数据库引擎

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.

Android运行时

Android包含一组java语言的核心库用来提供大多数核心库的功能。

每一个Android应用程序运行在它自己的进程中，拥有自己的虚拟机实例。Dalvik被编写为可以在一个设备上有效地运行多个虚拟机实例，Dalvik执行文件是一个为小内存环境优化过的Dalvik（.dex）可执行文件格式。虚拟机是基于寄存器的，运行的class是经过java编译器编译过并且由“dx”工具转换为.dex格式。

Dalvik虚拟机依靠Linux内核提供基础功能，如线程和低级内存管理。

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.

Android依赖Linux 2.6版本来提供系统服务，如安全管理、内存管理、进程管理、网络协议栈和驱动模块。内核也扮演一个硬件与其他软件之间的一个抽象层的角色