M7350/base/docs/html/guide/practices/compatibility.jd

page.title=Android Compatibility
@jd:body

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<h2>See also</h2>
 <ol>
<li><a
href="{@docRoot}guide/appendix/market-filters.html">Market Filters</a></li>
<li><a
href="{@docRoot}guide/topics/resources/providing-resources.html#AlternativeResources">Providing Alternative Resources</a></li>
<li><a
href="{@docRoot}guide/practices/screens_support.html">Supporting Multiple Screens</a></li>
<li style="margin-top:3px;"><code><a
href="{@docRoot}guide/topics/manifest/supports-screens-element.html">&lt;supports-screens&gt;</a></code></li>
<li><code><a
href="{@docRoot}guide/topics/manifest/uses-configuration-element.html">&lt;uses-configuration&gt;</a></code></li>
<li><code><a
href="{@docRoot}guide/topics/manifest/uses-feature-element.html">&lt;uses-feature&gt;</a></code></li>
<li><code><a
href="{@docRoot}guide/topics/manifest/uses-library-element.html">&lt;uses-library&gt;</a></code></li>
<li><code><a
href="{@docRoot}guide/topics/manifest/uses-permission-element.html">&lt;uses-permission&gt;</a></code></li>
<li><code><a
href="{@docRoot}guide/topics/manifest/uses-sdk-element.html">&lt;uses-sdk&gt;</code></a></li>
</ol>


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<p>Android is designed to run on many different types of devices. For
developers, the range and number of devices means a huge potential audience: the
more devices that run Android apps, the more users who can access your app. In
exchange, however, it also means that your apps will have to cope with that same
variety of hardware.</p>

<p>Fortunately, Android has built-in tools and support that make it easy for
your apps to do that, while at the same time letting you maintain control of
what types of devices your app is available to. With a bit of forethought and
some minor changes in your app's manifest file, you can ensure that users
whose devices can’t run your app will never see it in the Android Market, and
will not get in trouble by downloading it. This page explains how you can
control which devices have access to your apps, and how to prepare your apps to
make sure they reach the right audience.</p>


<h3 id="defined">What does “compatibility” mean?</h3>

<p>A device is “Android compatible” if it can correctly run apps written for the
<em>Android execution environment</em>. The exact details of the Android execution
environment</em> are defined by the Android Compatibility Definition Document,
but the single most important characteristic of a compatible device is the 
ability to install and correctly run an Android <code>.apk</code> file.</p>

<p>There is exactly one Android API for each <a
href="{@docRoot}guide/appendix/api-levels.html">API level</a>, and it’s the same
API no matter what kind of device it’s installed on. No parts of the API are
optional, and you never have to worry about parts of the API missing on some
devices. Every compatible Android device your app will land on will include
every class and every API for that API level.</p>

<p>Of course, some APIs won’t work correctly if a particular device lacks the
corresponding hardware or feature. But that’s not a problem: we also designed
Android to prevent apps from being visible to devices which don’t have features
the apps require. We’ve built support for this right into the SDK tools, and
it’s part of the Android platform itself, as well as Android Market.</p>

<p>As a developer, you have complete control of how and where your apps are
available. Android provides tools as a first-class part of the platform that let
you manage this. You control the availability of your apps, so that they reach
only the devices capable of running them.</p>

<h3 id="how">How does it work?</h3>

<p>You manage your app’s availability through a simple three-step process:</p>

<ol>
<li>You state the features your app requires by declaring <a
href="{@docRoot}guide/topics/manifest/uses-feature-element.html"><code>&lt;uses-feature&gt;</code></a>
elements its manifest file.</li>
<li>Devices are required to declare the features they include to Android
Market.</li>
<li>Android Market uses your app’s stated requirements to filter it from devices
that don’t meet those requirements.</li>
</ol>

<p>This way, users never even see apps that won’t work properly on their
devices. As long as you accurately describe your app’s requirements, you don’t
need to worry about users blaming you for compatibility problems.</p>

<p>If you’re familiar with web development, you may recognize this model as
“capability detection”. Web developers typically prefer this approach to
“browser detection”, because it’s very difficult to keep up as new browsers and
new versions of current browsers are released. By checking for support for
specific required capabilities instead of the current browser, web developers
get better fine-grained control. That’s the same approach Android uses: since
it’s impossible to keep up with all the Android devices being released, you
instead use the fine-grained controls Android provides.</p>

<h3>Filtering for technical reasons</h3>

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    <img src="{@docRoot}assets/images/icon_market.jpg" style="float:left;margin:0;padding:0;"> 
    <p style="color:#669999;">Filtering on Android Market</p>

    <p>Android Market filters the applications that are visible to users, so
that users can see and download only those applications that are compatible with
their devices.</p>

    <p style="margin-top:1em;">One of the ways Market filters applications is by
feature compatibility. To do this, Market checks the
<code>&lt;uses-feature&gt;</code> elements in each application's manifest, to
establish the app's feature needs. Market then shows or hides the application to
each user, based on a comparison with the features available on the user's
device. 

<p style="margin-top:1em;">For information about other filters that you can
use to control the availability of your apps, see the 
<a href="{@docRoot}guide/appendix/market-filters.html">Market
Filters</a> document.</p>
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</div>

<p>Android includes support for a lot of features, some hardware and some
software. Examples include compass and accelerometer sensors, cameras, and Live
Wallpapers. However, not every device will support every feature. For instance,
some devices don’t have the hardware horsepower to display Live Wallpapers
well.</p>

<p>To manage this, Android defines <em>feature IDs</em>. Every capability has a
corresponding feature ID defined by the Android platform. For instance, the
feature ID for compass is <code>“android.hardware.sensor.compass”</code>, 
while the feature
ID for Live Wallpapers is <code>“android.software.live_wallpapers”</code>. Each of these IDs
also has a corresponding Java-language constant on the
{@link android.content.pm.PackageManager} class that you can use to query whether
feature is supported at runtime. As Android adds support for new features in
future versions, new feature IDs will be added as well.</p>

<p>When you write your application, you specify which features your app requires
by listing their feature IDs in <code>&lt;uses-feature&gt;</code> elements in
the <code>AndroidManifest.xml</code> file.  This is the information that Android
Market uses to match your app to devices that can run it. For instance, if you
state that your app requires android.software.live_wallpapers, it won’t be shown
to devices that don’t support Live Wallpapers.</p>

<p>This puts you in total control of your app &mdash; because you don’t have to
declare these features. Consider an example involving cameras.</p>

<p>If you’re building a really impressive next-generation augmented-reality app,
your app won’t function at all without a camera. However, if you’re building a
shopping app that only uses the camera for barcode scanning, users without
cameras might still find it useful even if they can’t scan barcodes. While both
apps need to acquire the permission to access the camera, only the first app
needs to state that it requires a camera.  (The shopping app can simply check at
runtime and disable the camera-related features if there’s no camera
present.)</p>

<p>Since only you can say what the best approach is for your app, Android
provides the tools and lets you make your own tradeoff between maximizing
audience size and minimizing development costs.</p>


<h3 id="filtering">Filtering for business reasons</h3>

<p>It’s possible that you may need to restrict your app’s availability for
business or legal reasons. For instance, an app that displays train schedules
for the London Underground is unlikely to be useful to users outside the United
Kingdom. Other apps might not be permitted in certain countries for business or
legal reasons. For cases such as these, Android Market itself provides
developers with filtering options that allow them control their app’s
availability for non-technical reasons.</p>

<p>The help information for Android Market provides full details, but in a
nutshell, developers can use the Market publisher UI to:</p>

<ul>
<li>List the countries an app is available in.</li>
<li>Select which carrier’s users are able to access the app.</li>
</ul>

<p>Filtering for technical compatibility (such as required hardware components)
is always based on information contained within your <code>.apk</code> file. But
filtering for non-technical reasons (such as geographic restrictions) is always
handled in the Market user interface.</p>

<h3 id="futureproofing">Future-proofing</h3>

<p>There’s one additional quirk that we haven’t yet addressed: protecting apps
from changes made to future versions of Android.  If the Android platform
introduces a new feature or changes how existing features are handled, what
happens to existing apps that were written without any knowledge of the new
behavior?</p>

<p>Simply put, Android commits to not making existing apps available to devices
where they won’t work properly, even when the platform changes. The best way to
explain this is through examples, so here are two:</p>

<ul>
<li>Android 1.0 through 1.5 required a 2 megapixel camera with auto-focus.
However, with version 1.6, Android devices were permitted to omit the auto-focus
capability, though a (fixed-focus) camera was still required. Some apps such as
barcode scanners do not function as well with cameras that do not auto-focus. To
prevent users from having a bad experience with those apps, existing apps that
obtain permission to use the Camera were assumed by default to require
auto-focus. This allowed Android Market to filter those apps from devices that
lack auto-focus.</li>

<li>Android 2.2, meanwhile, allowed the microphone to be optional on some
devices, such as set-top boxes. Android 2.2 included a new feature ID for the
microphone which allows developers to filter their apps if necessary, but
&mdash; as with camera &mdash; apps that obtain permission to record audio are
assumed to require the microphone feature by default. If your app can use a
microphone but doesn’t strictly need it, you can explicitly state that you don’t
require it; but unless you do that, your app won’t be shown to devices without
microphones.</li>
</ul>

<p>In other words, whenever Android introduces new features or changes existing
ones, we will always take steps to protect existing applications so that they
don’t end up being available to devices where they won’t work.</p>

<p>This is implemented, in part, using the <code>aapt</code> tool in the SDK. 
To see which features your app explicitly requires or is implicitly assumed to
require, you can use the command <code>aapt dump badging</code>.</p>

<h3 id="conclusion">Conclusion</h3>

<p>The goal of Android is to create a huge installed base for developers to take
advantage of. One of the ways we will achieve this is through different kinds of
hardware running the same software environment. But we also recognize that only
developers know which kinds of devices their apps make sense on. We’ve built in
tools to the SDK and set up policies and requirements to ensure that developers
remain in control of their apps, today and in the future. With the information
you just read, and the resources listed in the sidebar of this document, you 
can publish your app with the confidence that only users who can run it will
see it.</p>

<p>For more information about Android device compatibility, please visit:</p>

<p style="margin-left:2em;"><a href="http://source.android.com/compatibility/index.html">http://source.android.com/compatibility/index.html</a></p>