【正文】 of work. For instance, it might initiate a service to perform some work based on the event.A broadcast receiver is implemented as a subclass oft display a user interface, they maybroadcast receiverContent ProvidersContentProviderNote Pads contact information. As such, any application with the proper permissions can query part of the content provider (such ascontent providerServicesServiceserviceActivitiesActivityactivitys 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 exists within the system. The rest of this document introduces you to:l The core framework ponents that define your application.l The manifest file in which you declare ponents and required device features for your application.l Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations. Application Components Application ponents are the essential building blocks of an Android application. Each ponent is a different point through which the system can enter your application. Not all ponents are actual entry points for the user and some depend on each other, but each one exists as its own entity and plays a specific role—each one is a unique building block that helps define your application39。s possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other39。s no longer needed or when the system must recover memory for other applications.In this way, the Android system implements thes code runs in isolation from other applications.l By default, every application runs in its own Linux process. Android starts the process when any of the application39。.apk.apk.. .. .. ..英語原文Android Application Fundamentals Android applications are written in the Java programming language. The Android SDK tools pile the code—along with any data and resource files—into anAndroid package, an archive file with ansuffix. All the code in a singlefile is considered to be one application and is the file that Androidpowered devices use to install the application. Once installed on a device, each Android application lives in its own security sandbox:l The Android operating system is a multiuser Linux system in which each application is a different user.l 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 application can access them.l Each process has its own virtual machine (VM), so an application39。s ponents need to be executed, then shuts down the process when it39。principle of least privilege. That is, each application, by default, has access only to the ponents that it requires to do its work and no more. This 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:l It39。s 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).l An application can request permission to access device data such as the user39。s overall behavior.There are four different types of application ponents. Each type serves a distinct purpose and has a distinct lifecycle that defines how the ponent is created and destroyed.Here are the four types of application ponents:ActivitiesAnrepresents a single screen with a user interface. For example, an application might have one activity that shows a list of new s, another activity to pose an , and another activity for reading s. Although the activities work together to form a cohesive user experience in the application, each one is independent of the others. As such, a different application can start any one of these activities (if the application allows it). For example, a camera application can start the activity in the application that poses new mail, in order for the user to share a picture.An activity is implemented as a subclass ofand you can learn more about it in thedeveloper guide.ServicesAis a ponent that runs in the background to perform longrunning 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 ponent, 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 ofand you can learn more about it in thedeveloper guide.Content providersAmanages a shared set of application data. You can store the data in the file system, an SQLite database, on the web, or any other persistent storage location your application can access. Through the content provider, other applications can query or even modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the user39。) to read and write information about a particular person.Content providers are also useful for reading and writing data that is private to your application and not shared. For example, thesample application uses a content provider to save notes.A content provider is implemented as