Application Fundamentals
Once installed on a device, each Android application lives in its own security sandbox:
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The Android operating system is a multi-user Linux system in which each application is a different user.
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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.
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Each process has its own virtual machine (VM), so an application's code runs in isolation from other applications.
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By default, every application runs in its own Linux process. Android starts the process when any of the application's components need to be executed, then shuts down the process when it's no longer needed or when the system must recover memory for other applications.
However, there are ways for an application to share data with other applications and for an application to access system services:
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It's possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each other'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).
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An application can request permission to access device data such as the user'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.
Application Components
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. They are:
Activities
An 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 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 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 of Activity.
Services
A service is 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 of Class Service.
Content providers
A 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 user's 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 useful for reading and writing data that is private to your application and not shared.
A content provider is implemented as a subclass of ContentProvider and must implement a standard set of APIs that enable other applications to perform transactions.
Broadcast receivers
A broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the system—for example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. Applications can also initiate broadcasts—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 don't display a user interface, they may 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.
A broadcast receiver is implemented as a subclass of BroadcastReceiver and each broadcast is delivered as an Intent object.
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, whether the component belongs to your application or another. 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. 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"). However, 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 ContentResolver object. This leaves a layer of abstraction between the content provider and the component requesting information (for security).
Manifest File
The literal meaning of manifest is a document listing the cargo, passengers, and crew of a ship, aircraft, or vehicle for the use of customs and other officials. Android introduces this concept to the software world. So before the Android system can start an application component, the system must know that the component exists by reading the application's AndroidManifest.xml file (the "manifest" file). Your application must declare all its components in this file, which must be at the root of the application project directory.
Declaring components
The manifest does a number of things in addition to declaring the application's components, such as:
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Identify any user permissions the application requires, such as Internet access or read-access to the user's contacts.
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Declare the minimum API Level required by the application, based on which APIs the application uses.
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Declare hardware and software features used or required by the application, such as a camera, bluetooth services, or a multitouch screen.
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API libraries the application needs to be linked against (other than the Android framework APIs), such as the Google Maps library.
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And more
Declaring component capabilities
Declaring application requirements
Here are some of the important device characteristics that you should consider as you design and develop your application:
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Screen size and density
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Input configurations
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Device features
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Platform Version
Application Resources
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.