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Android applications are written in the Java programming language. The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with 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:
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 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:
That covers the basics regarding how an Android application exists within the system. The rest of this document introduces you to:
Application 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 each one exists as its own entity and plays a specific role—each one is a unique building block that helps define your application's 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 component is created and destroyed.
Here are the four types of application components:
An activity is implemented as a subclass of Activityand you can learn more about it in the Activities developer guide.
A service is implemented as a subclass of Serviceand you can learn more about it in the Services developer guide.
Content providers are also 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 ContentProviderand must implement a standard set of APIs that enable other applications to perform transactions. For more information, see the Content Providers developer guide.
A broadcast receiver is implemented as a subclass of BroadcastReceiverand each broadcast is delivered as an Intentobject. For more information, see the BroadcastReceiverclass.
A unique aspect of the Android system design is that any application can start another application’s component. For example, if you want the user to capture a photo with the device camera, there's probably another application that does that and your application can use it, instead of developing an activity to capture a photo yourself. You don'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 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 process for that application (if it's 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 application's process. Therefore, unlike applications on most other systems, Android applications don't have a single entry point (there'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 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 component for you.
Three of the four component types—activities, services, and broadcast receivers—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 other components), whether the component belongs to your application or another.
An intent is created with an Intentobject, which defines a message to activate either a specific component or a specific type of component—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 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 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 component 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 component that's performing transactions with the provider doesn't need to and instead calls methods on the ContentResolverobject. This leaves a layer of abstraction between the content provider and the component requesting information (for security).
There are separate methods for activating each type of component:
For more information about using intents, see the Intents and Intent Filters document. More information about activating specific components is also provided in the following documents: Activities, Services, BroadcastReceiverand Content Providers.
Before the Android system can start an application component, the system must know that the component exists by reading the application's AndroidManifest.xmlfile (the "manifest" file). Your application must declare all its components in this file, which must be at the root of the application project directory.
The manifest does a number of things in addition to declaring the application's components, such as:
The primary task of the manifest is to inform the system about the application's components. For example, a manifest file can declare an activity as follows:
xml version="1.0" encoding="utf-8"?>
android:icon="@drawable/app_icon.png" ... >
android:label="@string/example_label" ... >
In the element, the android:iconattribute points to resources for an icon that identifies the application.
In the element, the android:nameattribute specifies the fully qualified class name of the Activitysubclass and the android:labelattributes specifies a string to use as the user-visible label for the activity.
You must declare all application components this way:
Activities, services, and content providers that you include in your source but do not declare in the manifest are not visible to the system and, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiverobjects) and registered with the system by calling registerReceiver().
For more about how to structure the manifest file for your application, see the The AndroidManifest.xml File documentation.
As discussed above, in Activating Components, you can use an Intentto start activities, services, and broadcast receivers. You can do so by explicitly naming the target component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perform (and optionally, the data upon which you’d like to perform the action) and allow the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the user selects which one to use.
The way the system identifies the components that can respond to an intent is by comparing the intent received to the intent filters provided in the manifest file of other applications on the device.
When you declare a component in your application's manifest, you can optionally include intent filters that declare the capabilities of the component so it can respond to intents from other applications. You can declare an intent filter for your component by adding an element as a child of the component's declaration element.
For example, an email application with an activity for composing a new email might declare an intent filter in its manifest entry to respond to "send" intents (in order to send email). An activity in your application can then create an intent with the “send” action (ACTION_SEND), which the system matches to the email application’s “send” activity and launches it when you invoke the intent with startActivity().
For more about creating intent filters, see the Intents and Intent Filters document.
There are a variety of devices powered by Android and not all of them provide the same features and capabilities. In order to prevent your application from being installed on devices that lack features needed by your application, it's important that you clearly define a profile for the types of devices your application supports by declaring device and software requirements in your manifest file. Most of these declarations are informational only and the system does not read them, but external services such as Google Play do read them in order to provide filtering for users when they search for applications from their device.
For example, if your application requires a camera and uses APIs introduced in Android 2.1 (API Level 7), you should declare these as requirements in your manifest file. That way, devices that do not have a camera and have an Android version lower than 2.1 cannot install your application from Google Play.
However, you can also declare that your application uses the camera, but does not require it. In that case, your application must perform a check at runtime to determine if the device has a camera and disable any features that use the camera if one is not available.
Here are some of the important device characteristics that you should consider as you design and develop your application:
The screen sizes are: small, normal, large, and extra large.
The screen densities are: low density, medium density, high density, and extra high density.
By default, your application is compatible with all screen sizes and densities, because the Android system makes the appropriate adjustments to your UI layout and image resources. However, you should create specialized layouts for certain screen sizes and provide specialized images for certain densities, using alternative layout resources, and by declaring in your manifest exactly which screen sizes your application supports with the element.
For more information, see the Supporting Multiple Screens document.
It's important that you declare all such requirements for your application, because, when you distribute your application on Google Play, the store uses these declarations to filter which applications are available on each device. As such, your application should be available only to devices that meet all your application requirements.
For more information about how Google Play filters applications based on these (and other) requirements, see the Filters on Google Play document.
An Android application is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the application. For example, you should define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using application resources makes it easy to update various characteristics of your application without modifying code and—by providing sets of alternative resources—enables you to optimize your application for a variety of device configurations (such as different languages and screen sizes).
For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your application code or from other resources defined in XML. For example, if your application contains an image file named logo.png(saved in the res/drawable/directory), the SDK tools generate a resource ID named R.drawable.logo, which you can use to reference the image and insert it in your user interface.
One of the most important aspects of providing resources separate from your source code is the ability for you to provide alternative resources for different device configurations. For example, by defining UI strings in XML, you can translate the strings into other languages and save those strings in separate files. Then, based on a language qualifier that you append to the resource directory's name (such as res/values-fr/for French string values) and the user's language setting, the Android system applies the appropriate language strings to your UI.
Android supports many different qualifiers for your alternative resources. The qualifier is a short string that you include in the name of your resource directories in order to define the device configuration for which those resources should be used. As another example, you should often create different layouts for your activities, depending on the device's screen orientation and size. For example, when the device screen is in portrait orientation (tall), you might want a layout with buttons to be vertical, but when the screen is in landscape orientation (wide), the buttons should be aligned horizontally. To change the layout depending on the orientation, you can define two different layouts and apply the appropriate qualifier to each layout's directory name. Then, the system automatically applies the appropriate layout depending on the current device orientation.