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1、 毕业设计（论文） 外文翻译题 目Android Application Fundamentals专 业 网络工程 班 级 网络 学 生 指导教师 2012 年Android Application Fundamentals Android applications are written in the Java programming language. The Android SDK tools compile the codealong with any data and resource filesinto anAndroid package, an archive file with
2、 an.apksuffix. All the code in a single.apkfile is considered to be one application and is the file that Android-powered devices use to install the application.Once installed on a device, each Android application lives in its own security sandbox:The Android operating system is a multi-user Linux sy
3、stem in which each application is a different user.By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permissions for all the files in an application so that only the user ID assigned to that a
4、pplication can access them.Each process has its own virtual machine (VM), so an applications code runs in isolation from other applications.By default, every application runs in its own Linux process. Android starts the process when any of the applications components need to be executed, then shuts
5、down the process when its no longer needed or when the system must recover memory for other applications.In this way, the Android system implements theprinciple of least privilege. That is, each application, by default, has access only to the components that it requires to do its work and no more. T
6、his creates a very secure environment in which an application cannot access parts of the system for which it is not given permission.However, there are ways for an application to share data with other applications and for an application to access system services:Its possible to arrange for two appli
7、cations to share the same Linux user ID, in which case they are able to access each others files. To conserve system resources, applications with the same user ID can also arrange to run in the same Linux process and share the same VM (the applications must also be signed with the same certificate).
8、An application can request permission to access device data such as the users contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.That covers the basics regarding how an Android application exis
9、ts within the system. The rest of this document introduces you to:The core framework components that define your application.The manifest file in which you declare components and required device features for your application.Resources that are separate from the application code and allow your applic
10、ation to gracefully optimize its behavior for a variety of device configurations. Application ComponentsApplication components are the essential building blocks of an Android application. Each component is a different point through which the system can enter your application. Not all components are
11、actual entry points for the user and some depend on each other, but each one exists as its own entity and plays a specific roleeach one is a unique building block that helps define your applications overall behavior.There are four different types of application components. Each type serves a distinc
12、t purpose and has a distinct lifecycle that defines how the component is created and destroyed.Here are the four types of application components:ActivitiesAnactivityrepresents a single screen with a user interface. For example, an email application might have one activity that shows a list of new em
13、ails, another activity to compose an email, and another activity for reading emails. Although the activities work together to form a cohesive user experience in the email application, each one is independent of the others. As such, a different application can start any one of these activities (if th
14、e email application allows it). For example, a camera application can start the activity in the email application that composes new mail, in order for the user to share a picture.An activity is implemented as a subclass ofActivityand you can learn more about it in theActivitiesdeveloper guide.Servic
15、esAserviceis a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking user interaction with an activity. Another component, such as an activity, can start the service and let it run or bind to it in order to interact with it.A service is implemented as a subclass ofServiceand you can lear
