guide/components/fundamentals.jd

page.title=Application Fundamentals
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>In this document</h2>
<ol>
<li><a href="#Components">App Components</a>
  <ol>
    <li><a href="#ActivatingComponents">Activating components</a></li>
  </ol>
</li>
<li><a href="#Manifest">The Manifest File</a>
  <ol>
    <li><a href="#DeclaringComponents">Declaring components</a></li>
    <li><a href="#DeclaringRequirements">Declaring app requirements</a></li>
  </ol>
</li>
<li><a href="#Resources">App Resources</a></li>
</ol>
</div>
</div>

<p>Android apps are written in the Java programming language. The Android SDK tools compile
your code&mdash;along with any data and resource files&mdash;into an APK: an <i>Android package</i>,
which is an archive file with an {@code .apk} suffix. One APK file contains all the contents
of an Android app and is the file that Android-powered devices use to install the app.</p>

<p>Once installed on a device, each Android app lives in its own security sandbox: </p>

<ul>
 <li>The Android operating system is a multi-user Linux system in which each app is a
different user.</li>

<li>By default, the system assigns each app a unique Linux user ID (the ID is used only by
the system and is unknown to the app). The system sets permissions for all the files in an
app so that only the user ID assigned to that app can access them. </li>

<li>Each process has its own virtual machine (VM), so an app's code runs in isolation from
other apps.</li>

<li>By default, every app runs in its own Linux process. Android starts the process when any
of the app'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 apps.</li>
</ul>

<p>In this way, the Android system implements the <em>principle of least privilege</em>. That is,
each app, 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 app cannot access parts of
the system for which it is not given permission.</p>

<p>However, there are ways for an app to share data with other apps and for an
app to access system services:</p>

<ul>
  <li>It's possible to arrange for two apps to share the same Linux user ID, in which case
they are able to access each other's files.  To conserve system resources, apps with the
same user ID can also arrange to run in the same Linux process and share the same VM (the
apps must also be signed with the same certificate).</li>
  <li>An app 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
app permissions must be granted by the user at install time.</li>
</ul>

<p>That covers the basics regarding how an Android app exists within the system. The rest of
this document introduces you to:</p>
<ul>
  <li>The core framework components that define your app.</li>
  <li>The manifest file in which you declare components and required device features for your
app.</li>
  <li>Resources that are separate from the app code and allow your app to
gracefully optimize its behavior for a variety of device configurations.</li>
</ul>


<h2 id="Components">App Components</h2>

<p>App components are the essential building blocks of an Android app. Each
component is a different point through which the system can enter your app. 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&mdash;each one is a unique building block that
helps define your app's overall behavior.</p>

<p>There are four different types of app components. Each type serves a distinct purpose
and has a distinct lifecycle that defines how the component is created and destroyed.</p>

<p>Here are the four types of app components:</p>

<dl>

<dt><b>Activities</b></dt>

<dd>An <i>activity</i> represents a single screen with a user interface. For example,
an email app 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 app, each one
is independent of the others. As such, a different app can start any one of these
activities (if the email app allows it). For example, a camera app can start the
activity in the email app that composes new mail, in order for the user to share a picture.

<p>An activity is implemented as a subclass of {@link android.app.Activity} and you can learn more
about it in the <a href="{@docRoot}guide/components/activities.html">Activities</a>
developer guide.</p>
</dd>


<dt><b>Services</b></dt>

<dd>A <i>service</i> 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 app, 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.

<p>A service is implemented as a subclass of {@link android.app.Service} and you can learn more
about it in the <a href="{@docRoot}guide/components/services.html">Services</a> developer
guide.</p>
</dd>


<dt><b>Content providers</b></dt>

<dd>A <i>content provider</i> manages a shared set of app data. You can store the data in
the file system, an SQLite database, on the web, or any other persistent storage location your
app can access. Through the content provider, other apps 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 app with the proper
permissions can query part of the content provider (such as {@link
android.provider.ContactsContract.Data}) to read and write information about a particular person.

<p>Content providers are also useful for reading and writing data that is private to your
app and not shared. For example, the <a
href="{@docRoot}resources/samples/NotePad/index.html">Note Pad</a> sample app uses a
content provider to save notes.</p>

<p>A content provider is implemented as a subclass of {@link android.content.ContentProvider}
and must implement a standard set of APIs that enable other apps to perform
transactions. For more information, see the <a
href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a> developer
guide.</p>
</dd>


<dt><b>Broadcast receivers</b></dt>

<dd>A <i>broadcast receiver</i> is a component that responds to system-wide broadcast
announcements.  Many broadcasts originate from the system&mdash;for example, a broadcast announcing
that the screen has turned off, the battery is low, or a picture was captured.
Apps can also initiate broadcasts&mdash;for example, to let other apps 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 <a
href="{@docRoot}guide/topics/ui/notifiers/notifications.html">create a status bar notification</a>
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.

<p>A broadcast receiver is implemented as a subclass of {@link android.content.BroadcastReceiver}
and each broadcast is delivered as an {@link android.content.Intent} object. For more information,
see the {@link android.content.BroadcastReceiver} class.</p>
</dd>

</dl>


<p>A unique aspect of the Android system design is that any app can start another
apps component. For example, if you want the user to capture a
photo with the device camera, there's probably another app that does that and your
app 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 app.
Instead, you can simply start the activity in the camera app that captures a
photo. When complete, the photo is even returned to your app so you can use it. To the user,
it seems as if the camera is actually a part of your app.</p>

<p>When the system starts a component, it starts the process for that app (if it's not
already running) and instantiates the classes needed for the component. For example, if your
app starts the activity in the camera app that captures a photo, that activity
runs in the process that belongs to the camera app, not in your app's process.
Therefore, unlike apps on most other systems, Android apps don't have a single entry
point (there's no {@code main()} function, for example).</p>

<p>Because the system runs each app in a separate process with file permissions that
restrict access to other apps, your app cannot directly activate a component from
another app. The Android system, however, can. So, to activate a component in
another app, you must deliver a message to the system that specifies your <em>intent</em> to
start a particular component. The system then activates the component for you.</p>


<h3 id="ActivatingComponents">Activating Components</h3>

<p>Three of the four component types&mdash;activities, services, and
broadcast receivers&mdash;are activated by an asynchronous message called an <em>intent</em>.
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 app or another.</p>

<p>An intent is created with an {@link android.content.Intent} object, which defines a message to
activate either a specific component or a specific <em>type</em> of component&mdash;an intent
can be either explicit or implicit, respectively.</p>

<p>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 {@link android.content.Intent} (for example, you can issue an intent to let
the user pick a personal contact and have it returned to you&mdash;the return intent includes a
URI pointing to the chosen contact).</p>

<p>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").</p>

<p>The other component type, content provider, is not activated by intents. Rather, it is
activated when targeted by a request from a {@link android.content.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 {@link
android.content.ContentResolver} object. This leaves a layer of abstraction between the content
provider and the component requesting information (for security).</p>

<p>There are separate methods for activating each type of component:</p>
<ul>
  <li>You can start an activity (or give it something new to do) by
passing an {@link android.content.Intent} to {@link android.content.Context#startActivity
startActivity()} or {@link android.app.Activity#startActivityForResult startActivityForResult()}
(when you want the activity to return a result).</li>
  <li>You can start a service (or give new instructions to an ongoing service) by
passing an {@link android.content.Intent} to {@link android.content.Context#startService
startService()}. Or you can bind to the service by passing an {@link android.content.Intent} to
{@link android.content.Context#bindService bindService()}.</li>
  <li>You can initiate a broadcast by passing an {@link android.content.Intent} to methods like
{@link android.content.Context#sendBroadcast(Intent) sendBroadcast()}, {@link
android.content.Context#sendOrderedBroadcast(Intent, String) sendOrderedBroadcast()}, or {@link
android.content.Context#sendStickyBroadcast sendStickyBroadcast()}.</li>
  <li>You can perform a query to a content provider by calling {@link
android.content.ContentProvider#query query()} on a {@link android.content.ContentResolver}.</li>
</ul>

<p>For more information about using intents, see the <a
href="{@docRoot}guide/components/intents-filters.html">Intents and
Intent Filters</a> document. More information about activating specific components is also provided
in the following documents: <a
href="{@docRoot}guide/components/activities.html">Activities</a>, <a
href="{@docRoot}guide/components/services.html">Services</a>, {@link
android.content.BroadcastReceiver} and <a
href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a>.</p>


<h2 id="Manifest">The Manifest File</h2>

<p>Before the Android system can start an app component, the system must know that the
component exists by reading the app's {@code AndroidManifest.xml} file (the "manifest"
file). Your app must declare all its components in this file, which must be at the root of
the app project directory.</p>

<p>The manifest does a number of things in addition to declaring the app's components,
such as:</p>
<ul>
  <li>Identify any user permissions the app requires, such as Internet access or
read-access to the user's contacts.</li>
  <li>Declare the minimum <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">API Level</a>
required by the app, based on which APIs the app uses.</li>
  <li>Declare hardware and software features used or required by the app, such as a camera,
bluetooth services, or a multitouch screen.</li>
  <li>API libraries the app needs to be linked against (other than the Android framework
APIs), such as the <a
href="http://code.google.com/android/add-ons/google-apis/maps-overview.html">Google Maps
library</a>.</li>
  <li>And more</li>
</ul>


<h3 id="DeclaringComponents">Declaring components</h3>

<p>The primary task of the manifest is to inform the system about the app's components. For
example, a manifest file can declare an activity as follows: </p>

<pre>
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;manifest ... &gt;
    &lt;application android:icon="@drawable/app_icon.png" ... &gt;
        &lt;activity android:name="com.example.project.ExampleActivity"
                  android:label="@string/example_label" ... &gt;
        &lt;/activity&gt;
        ...
    &lt;/application&gt;
&lt;/manifest&gt;</pre>

<p>In the <code><a
href="{@docRoot}guide/topics/manifest/application-element.html">&lt;application&gt;</a></code>
element, the {@code android:icon} attribute points to resources for an icon that identifies the
app.</p>

<p>In the <code><a
href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a></code> element,
the {@code android:name} attribute specifies the fully qualified class name of the {@link
android.app.Activity} subclass and the {@code android:label} attributes specifies a string
to use as the user-visible label for the activity.</p>

<p>You must declare all app components this way:</p>
<ul>
  <li><code><a
href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a></code> elements
for activities</li>
  <li><code><a
href="{@docRoot}guide/topics/manifest/service-element.html">&lt;service&gt;</a></code> elements for
services</li>
  <li><code><a
href="{@docRoot}guide/topics/manifest/receiver-element.html">&lt;receiver&gt;</a></code> elements
for broadcast receivers</li>
  <li><code><a
href="{@docRoot}guide/topics/manifest/provider-element.html">&lt;provider&gt;</a></code> elements
for content providers</li>
</ul>

<p>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
{@link android.content.BroadcastReceiver} objects) and registered with the system by calling
{@link android.content.Context#registerReceiver registerReceiver()}.</p>

<p>For more about how to structure the manifest file for your app, see <a
href="{@docRoot}guide/topics/manifest/manifest-intro.html">The AndroidManifest.xml File</a>
documentation. </p>


<h3 id="DeclaringComponentCapabilities">Declaring component capabilities</h3>

<p>As discussed above, in <a href="#ActivatingComponents">Activating Components</a>, you can use an
{@link android.content.Intent} to 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 <em>implicit intents</em>. An implicit intent
simply describes the type of action to perform (and, optionally, the data upon which youd like to
perform the action) and allows 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.</p>

<p>The way the system identifies the components that can respond to an intent is by comparing the
intent received to the <i>intent filters</i> provided in the manifest file of other apps on
the device.</p>

<p>When you declare an activity in your app's manifest, you can optionally include
intent filters that declare the capabilities of the activity so it can respond to intents
from other apps. You can declare an intent filter for your component by
adding an <a href="{@docRoot}guide/topics/manifest/intent-filter-element.html">{@code
&lt;intent-filter&gt;}</a> element as a child of the component's declaration element.</p>

<p>For example, if you've built an email app with an activity for composing a new email, you can
declare an intent filter to respond to "send" intents (in order to send a new email) like this:</p>
<pre>
&lt;manifest ... >
    ...
    &lt;application ... &gt;
        &lt;activity android:name="com.example.project.ComposeEmailActivity">
            &lt;intent-filter>
                &lt;action android:name="android.intent.action.SEND" />
                &lt;data android:type="*/*" />
                &lt;category android:name="android.intent.category.DEFAULT" />
            &lt;/intent-filter>
        &lt;/activity>
    &lt;/application&gt;
&lt;/manifest>
</pre>

<p>Then, if another app creates an intent with the {@link
android.content.Intent#ACTION_SEND} action and pass it to {@link android.app.Activity#startActivity
startActivity()}, the system may start your activity so the user can draft and send an
email.</p>

<p>For more about creating intent filters, see the <a
href="{@docRoot}guide/components/intents-filters.html">Intents and Intent Filters</a> document.
</p>


<h3 id="DeclaringRequirements">Declaring app requirements</h3>

<p>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 app from being installed on devices
that lack features needed by your app, it's important that you clearly define a profile for
the types of devices your app 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 apps from their device.</p>

<p>For example, if your app requires a camera and uses APIs introduced in Android 2.1 (<a
href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">API Level</a> 7),
you should declare these as requirements in your manifest file like this:</p>

<pre>
&lt;manifest ... >
    &lt;uses-feature android:name="android.hardware.camera.any"
                  android:required="true" />
    &lt;uses-sdk android:minSdkVersion="7" android:targetSdkVersion="19" />
    ...
&lt;/manifest>
</pre>

<p>Now, devices that do <em>not</em> have a camera and have an
Android version <em>lower</em> than 2.1 cannot install your app from Google Play.</p>

<p>However, you can also declare that your app uses the camera, but does not
<em>require</em> it. In that case, your app must set the <a href=
"{@docRoot}guide/topics/manifest/uses-feature-element.html#required">{@code required}</a>
attribute to {@code "false"} and check at runtime whether
the device has a camera and disable any camera features as appropriate.</p>

<p>More information about how you can manage your app's compatibility with different devices
is provided in the <a href="{@docRoot}guide/practices/compatibility.html">Device Compatibility</a>
document.</p>


<h2 id="Resources">App Resources</h2>

<p>An Android app is composed of more than just code&mdash;it requires resources that are
separate from the source code, such as images, audio files, and anything relating to the visual
presentation of the app. For example, you should define animations, menus, styles, colors,
and the layout of activity user interfaces with XML files. Using app resources makes it easy
to update various characteristics of your app without modifying code and&mdash;by providing
sets of alternative resources&mdash;enables you to optimize your app for a  variety of
device configurations (such as different languages and screen sizes).</p>

<p>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 app code or from
other resources defined in XML. For example, if your app contains an image file named {@code
logo.png} (saved in the {@code res/drawable/} directory), the SDK tools generate a resource ID
named {@code R.drawable.logo}, which you can use to reference the image and insert it in your
user interface.</p>

<p>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 <em>qualifier</em>
that you append to the resource directory's name (such as {@code res/values-fr/} for French string
values) and the user's language setting, the Android system applies the appropriate language strings
to your UI.</p>

<p>Android supports many different <em>qualifiers</em> 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.</p>

<p>For more about the different kinds of resources you can include in your application and how to
create alternative resources for different device configurations, read <a href=
"{@docRoot}guide/topics/resources/providing-resources.html">Providing Resources</a>.</p>


<div class="next-docs">
<div class="col-6">
  <h2 class="norule">Continue reading about:</h2>
  <dl>
    <dt><a href="{@docRoot}guide/components/intents-filters.html">Intents and Intent Filters</a>
    </dt>
    <dd>Information about how to use the {@link android.content.Intent} APIs to
    activate app components, such as activities and services, and how to make your app components
    available for use by other apps.</dd>
    <dt><a href="{@docRoot}guide/components/activities.html">Activities</a></dt>
    <dd>Information about how to create an instance of the {@link android.app.Activity} class,
    which provides a distinct screen in your application with a user interface.</dd>
    <dt><a
href="{@docRoot}guide/topics/resources/providing-resources.html">Providing Resources</a></dt>
    <dd>Information about how Android apps are structured to separate app resources from the
   app code, including how you can provide alternative resources for specific device
   configurations.
    </dd>
  </dl>
</div>
<div class="col-6">
  <h2 class="norule">You might also be interested in:</h2>
  <dl>
    <dt><a href="{@docRoot}guide/practices/compatibility.html"
        >Device Compatibility</a></dt>
    <dd>Information about Android works on different types of devices and an introduction
    to how you can optimize your app for each device or restrict your app's availability
    to different devices.</dd>
    <dt><a href="{@docRoot}guide/topics/security/permissions.html"
        >System Permissions</a></dt>
    <dd>Information about how Android restricts app access to certain APIs with a permission
    system that requires the user's consent for your app to use those APIs.</dd>
  </dl>
</div>
</div>