1、Android Application FundamentalsAndroid applications are written in the Java programming language. The Android SDK tools compile the code一along with any data and resource files一into an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one ap
2、plication 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 system in which each application is a different user.By default, the system
3、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.Each process has its own virtual machine (VM),
4、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 down the process when its no longer needed or when the system must recover
5、 memory for other applications. In this way, the Android system implements the principle 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. This creates a very secure environment in which an application cannot acc
6、ess 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:Ifs possible to arrange for two applications to share the same Linux user ID, in which case they are able to
7、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).An application can request permission to access device data such as the
8、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 exists within the system. The rest of this document introduces you to:1、The
9、core framework components that define your application.2、The manifest file in which you declare components and required device features for your application.3、Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device
10、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 actual entry points for the user and some depend on each other, but
11、 each one exists as its own entity and plays a specific role一each 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 distinct purpose and has a distinct lifecycle that defines how the compon
12、ent is created and destroyed.Here are the four types of application components:ActivitiesAn activity represents a single screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity
13、 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 the email application allows it). For example, a camera applicatio
14、n 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 of Activity and you can learn more about it in the Activities developer guide.ServicesA service is a component that runs in the background to pe
15、rform 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 a
16、n 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 of Service and you can learn more about it in the Services developer guide.Content providersA content provider manages a shared s
17、et 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
18、, the Android system provides a content provider that manages the users contact information. As such, any application with the proper permissions can query part of the content provider (such as ContactsContract.Data) to read and write information about a particular person.Content providers are also
19、useful for reading and writing data that is private to your application and not shared. For example, the Note Pad sample application uses a content provider to save notes.A content provider is implemented as a subclass of Contentprovider and must implement a standard set of APIs that enable other ap
20、plications to perform transactions. For more information, see the Content Providers developer guide.Broadcast receiversA broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the systemfor example, a broadcast announcing that the scree
21、n has turned o氐 the battery is low, or a picture was captured. Applications can also initiate broadcasts一for example, to let other applications know that some data has been downloaded to the device and is available for them to use. Although broadcast receivers dont display a user interface, they may
22、 create a status bar notification to alert the user when a broadcast event occurs. More commonly, though, a broadcast receiver is just a gateway to other components 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.
23、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 applications component. For example
24、, if you want the user to capture a photo with the device camera, theres probably another application that does that and your application can use it, instead of developing an activity to capture a photo yourself You dont need to incorporate or even link to the code from the camera application. Inste
25、ad, you can simply start the activity in the camera application that captures a photo. When complete, 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 component, it starts the proc
26、ess for that application (if its not already running) and instantiates the classes needed for the component. For example, if your application starts the activity in the camera application that captures a photo, that activity runs in the process that belongs to the camera application, not in your app
27、lications process. Therefore, unlike applications on most other systems, Android applications dont have a single entry point (theres 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
28、application cannot directly activate a component from another application. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies your intent to start a particular component. The system then activates the componen
29、t for you.Activating ComponentsThree of the four component types一activities, services, and broadcast receivers一are activated by an asynchronous message called an intent. Intents bind individual components to each other at runtime (you can think of them as the messengers that request an action from o
30、ther components), whether the component belongs to your application or another.An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of component一an intent can be either explicit or implicit, respectively.For activities and ser
31、vices, 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 component 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
32、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 youthe 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 component type, content provider, is not activated by intents.
