【正文】 en shuts down the process when it39。 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).s 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: The manifest file in which you declare ponents and required device features for your application.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:ActivitiesAn activity represents 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 of Activity and you can learn more about it in the Activities developer guide.ServicesA service is 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 of Service and you can learn more about it in the Services developer guide.Content providersA content provider manages 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。t display a user interface, they may create a status bar notification to alert the user when a broadcast event occurs. More monly, though, a broadcast receiver is just a gateway to other ponents and is intended to do a very minimal amount 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 of BroadcastReceiver and each broadcast is delivered as an Intent object. For more information, see the BroadcastReceiver class.A unique aspect of the Android system design is that any application can start another application’s ponent. For example, if you want the user to capture a photo with the device camera, there39。t need to incorporate or even link to the code from the camera application. Instead, you can simply start the activity in the camera application that captures a photo. When plete, the photo is even returned to your application so you can use it. To the user, it seems as if the camera is actually a part of your application.When the system starts a ponent, it starts the process for that application (if it39。s process. Therefore, unlike applications on most other systems, Android applications don39。s no main() function, for example).Because the system runs each application in a separate process with file permissions that restrict access to other applications, your application cannot directly activate a ponent from another application. The Android system, however, can. So, to activate a ponent in another application, you must deliver a message to the system that specifies your intent to start a particular ponent. The system then activates the ponent for you.Activating ComponentsThree of the four ponent types—activities, services, and broadcast receivers—are activated by an asynchronous message called an intent. Intents bind individual ponents to each other at runtime (you can think of them as the messengers that request an action from other ponents), whether the ponent belongs to your application or another.An intent is created with an Intent object, which defines a message to activate either a specific ponent or a specific type of ponent—an intent can be either explicit or implicit, respectively.For activities and services, an intent defines the action to perform (for example, to view or send something) and may specify the URI of the data to act on (among other things that the ponent being started might need to know). For example, an intent might convey a request for an activity to show an image or to open a web page. In some cases, you can start an activity to receive a result, in which case, the activity also returns the result in an Intent (for example, you can issue an intent to let the user pick a personal contact and have it returned to you—the return intent includes a URI pointing to the chosen contact).For broadcast receivers, the intent simply defines the announcement being broadcast (for example, a broadcast to indicate the device battery is low includes only a known action string that indicates battery is low).The other ponent type, content provider, is not activated by intents. Rather, it is activated when targeted by a request from a ContentResolver. The content resolver handles all direct transactions with the content provider so that the ponent that39