docs/html/guide/appendix/api-levels.jd

page.title=Android API Levels
@jd:body

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

  <h2>In this document</h2>
<ol>
  <li><a href="intro">What is API Level?</a></li>
  <li><a href="#uses">Uses of API Level</a></li>
  <li><a href="#considerations">Development Considerations</a>
    <ol>
      <li><a href="#fc">Application forward compatibility</a></li>
      <li><a href="#bc">Application backward compatibility</a></li>
      <li><a href="#platform">Selecting a platform version for compiling</a></li>
      <li><a href="#apilevel">Declaring a minimum API Level</a></li>
      <li><a href="#testing">Testing against higher API Levels</a></li>
    </ol>
  </li>
  <li><a href="#filtering">Filtering the documentation</a></li>
</ol>

  <h2>See also</h2>
  <ol>
    <li><a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html">&lt;uses-sdk&gt;</a> manifest element</li>
  </ol>

</div>
</div>

<p>As you develop your application on Android, it's useful to understand the
platform's general approach to API change management. It's also important to
understand the API Level identifier and the role it plays in ensuring your 
application's compatibility with devices on which it may be installed. </p>

<p>The sections below provide information about API Level and how it affects
your applications. </p>

<p>For information about how to use the "Filter by API Level" control
available in the API reference documentation, see 
<a href="#filtering">Filtering the documentation</a> at the 
end of this document. </p>

<h2 id="intro">What is API Level?</h2>

<p>API Level is an integer value that uniquely identifies the framework API
revision offered by a version of the Android platform.</p>

<p>The Android platform provides a framework API that applications can use to
interact with the underlying Android system. The framework API consists of:</p>

<ul>
<li>A core set of packages and classes</li>
<li>A set of XML elements and attributes for declaring a manifest file</li>
<li>A set of XML elements and attributes for declaring and accessing resources</li>
<li>A set of Intents</li>
<li>A set of permissions that applications can request, as well as permission
enforcements included in the system</li>
</ul>

<p>Each successive version of the Android platform can include updates to the
Android application framework API that it delivers. </p>

<p>Updates to the framework API are designed so that the new API remains
compatible with earlier versions of the API. That is, most changes in the API
are additive and introduce new or replacement functionality. As parts of the API
are upgraded, the older replaced parts are deprecated but are not removed, so
that existing applications can still use them. In a very small number of cases,
parts of the API may be modified or removed, although typically such changes are
only needed to ensure API robustness and application or system security. All
other API parts from earlier revisions are carried forward without
modification.</p>

<p>The framework API that an Android platform delivers is specified using an
integer identifier called "API Level". Each Android platform version supports
exactly one API Level, although support is implicit for all earlier API Levels
(down to API Level 1). The initial release of the Android platform provided 
API Level 1 and subsequent releases have incremented the API Level.</p>

<p>The following table specifies the API Level supported by each version of the
Android platform.</p>

<table>
  <tr><th>Platform Version</th><th>API Level</th></tr>
  <tr><td>Android 1.6</td><td>4</td></tr>
  <tr><td>Android 1.5</td><td>3</td></tr>
  <tr><td>Android 1.1</td><td>2</td></tr>
  <tr><td>Android 1.0</td><td>1</td></tr>
</table>


<h2 id="uses">Uses of API Level in Android</h2>

<p>The API Level identifier serves a key role in ensuring the best possible
experience for users and application developers: 

<ul>
<li>It lets the Android platform describe the maximum framework API revision 
that it supports</li>
<li>It lets applications describe the framework API revision that they
require</li>
<li>It lets the system negotiate the installation of applications on the user's
device, such that version-incompatible applications are not installed.</li>
</ul>

<p>Each Android platform version stores its API Level identifier internally, in
the Android system itself. </p>

<p>Applications can use a manifest element provided by the framework API &mdash;
<code>&lt;uses-sdk&gt;</code> &mdash; to describe the minimum and maximum API
Levels under which they are able to run, as well as the preferred API Level that
they are designed to support. The element offers two key attributes:</p>

<ul>
<li><code>android:minSdkVersion</code> &mdash; Specifies the minimum API Level
on which the application is able to run. The default value is "1".</li>
<li><code>android:maxSdkVersion</code> &mdash; Specifies the maximum API Level
on which the application is able to run.</li>
</ul>

<p>For example, to specify the minimum system API Level that an application
requires in order to run, the application would include in its manifest a
<code>&lt;uses-sdk&gt;</code> element with a <code>android:minSdkVersion</code>
attribute. The value of <code>android:minSdkVersion</code> would be the integer
corresponding to the API Level of the earliest version of the Android platform
under which the application can run. </p>

<p>When the user attempts to install an application, the Android system first
checks the <code>&lt;uses-sdk&gt;</code> attributes in the application's
manifest and compares them against its own internal API Level. The system
allows the installation to begin only if these conditions are met:</p>

<ul>
<li>If a <code>android:minSdkVersion</code> attribute is declared, its value
must be less than or equal to the system's API Level integer. If not declared,
the system assumes that the application requires API Level 1. </li>
<li>If a <code>android:maxSdkVersion</code> attribute is declared, its value
must be equal to or greater than the system's API Level integer. 
If not declared, the system assumes that the application
has no maximum API Level. </li>
</ul>

<p>When declared in an application's manifest, a <code>&lt;uses-sdk&gt;</code>
element might look like this: </p>

<pre>&lt;manifest&gt;
  ...
  &lt;uses-sdk android:minSdkVersion="4" /&gt;
  ...
&lt;/manifest&gt;</pre>

<p>The principal reason that an application would declare an API Level in
<code>android:minSdkVersion</code> is to tell the Android system that it is
using APIs that were <em>introduced</em> in the API Level specified. If the
application were to be somehow installed on a platform with a lower API Level,
then it would crash at run-time when it tried to access APIs that don't exist.
The system prevents such an outcome by not allowing the application to be
installed if the lowest API Level it requires is higher than that of the
platform version on the target device.</p>

<p>For example, the {@link android.appwidget} package was introduced with API
Level 3. If an application uses that API, it must declare a
<code>android:minSdkVersion</code> attribute with a value of "3". The
application will then be installable on platforms such as Android 1.5 (API Level
3) and Android 1.6 (API Level 4), but not on the Android 1.1 (API Level 2) and
Android 1.0 platforms (API Level 1).</p>

<p>For more information about how to specify an application's API Level
requirements, see the <a
href="{@docRoot}guide/topics/manifest/uses-sdk-element.html"><code>&lt;uses-sdk&gt;</code></a>
 section of the manifest file documentation.</p>


<h2 id="considerations">Development Considerations</h2>

<p>The sections below provide information related to API level that you should
consider when developing your application.</p>

<h3 id="fc">Application forward compatibility</h3>

<p>Android applications are generally forward-compatible with new versions of
the Android platform.</p>

<p>Because almost all changes to the framework API are additive, an Android
application developed using any given version of the API (as specified by its
API Level) is forward-compatible with later versions of the Android platform and
higher API levels. The application should be able to run on all later versions
of the Android platform, except in isolated cases where the application uses a
part of the API that is later removed for some reason. </p>

<p>Forward compatibility is important because many Android-powered devices
receive over-the-air (OTA) system updates. The user may install your
application and use it successfully, then later receive an OTA update to a new
version of the Android platform. Once the update is installed, your application
will run in a new run-time version of the environment, but one that has the API
and system capabilities that your application depends on. </p>

<p>In some cases, changes <em>below</em> the API, such those in the underlying
system itself, may affect your application when it is run in the new
environment. For that reason it's important for you, as the application
developer, to understand how the application will look and behave in each system
environment. To help you test your application on various versions of the Android
platform, the Android SDK includes multiple platforms that you can download.
Each platform includes a compatible system image that you can run in an AVD, to
test your application. </p>

<h3 id="bc">Application backward compatibility</h3>

<p>Android applications are not necessarily backward compatible with versions of
the Android platform older than the version against which they were compiled.
</p>

<p>Each new version of the Android platform can include new framework APIs, such
as those that give applications access to new platform capabilities or replace
existing API parts. The new APIs are accessible to applications when running on
the new platform and, as mentioned above, also when running on later versions of
the platform, as specified by API Level. Conversely, because earlier versions of
the platform do not include the new APIs, applications that use the new APIs are
unable to run on those platforms.</p>

<p>Although it's unlikely that an Android-powered device would be downgraded to
a previous version of the platform, it's important to realize that there are
likely to be many devices in the field that run earlier versions of the
platform. Even among devices that receive OTA updates, some might lag and
might not receive an update for a significant amount of time. </p>

<h3 id="platform">Selecting a platform version and API Level</h3>

<p>When you are developing your application, you will need to choose
the platform version against which you will compile the application. In
general, you should compile your application against the lowest possible
version of the platform that your application can support. 

<p>You can determine the lowest possible platform version by compiling the
application against successively lower build targets. After you determine the
lowest version, you should create an AVD using the corresponding platform
version (and API Level) and fully test your application. Make sure to declare a
<code>android:minSdkVersion</code> attribute in the application's manifest and
set its value to the API Level of the platform version. </p>

<h3 id="apilevel">Declaring a minimum API Level</h3>

<p>If you build an application that uses APIs or system features introduced in
the latest platform version, you should set the
<code>android:minSdkVersion</code> attribute to the API Level of the latest
platform version. This ensures that users will only be able to install your
application if their devices are running a compatible version of the Android
platform. In turn, this ensures that your application can function properly on
their devices. </p>

<p>If your application uses APIs introduced in the latest platform version but
does <em>not</em> declare a <code>android:minSdkVersion</code> attribute, then
it will run properly on devices running the latest version of the platform, but
<em>not</em> on devices running earlier versions of the platform. In the latter
case, the application will crash at runtime when it tries to use APIs that don't
exist on the earlier versions.</p>

<h3 id="testing">Testing against higher API Levels</h3>

<p>After compiling your application, you should make sure to test it on the
platform specified in the application's <code>android:minSdkVersion</code>
attribute. To do so, create an AVD that uses the platform version required by
your application. Additionally, to ensure forward-compatibility, you should run
and test the application on all platforms that use a higher API Level than that
used by your application. </p>

<p>The Android SDK includes multiple platform versions that you can use,
including the latest version, and provides an updater tool that you can use to
download other platform versions as necessary. </p>

<p>To access the updater, use the <code>android</code> command-line tool,
located in the &lt;sdk&gt;/tools directory. You can launch the Updater by using
the <code>android</code> command without specifying any options. You can
also simply double-click the android.bat (Windows) or android (OS X/Linux) file.
In ADT, you can also access the updater by selecting 
<strong>Window</strong>&nbsp;>&nbsp;<strong>Android SDK and AVD
Manager</strong>.</p>

<p>To run your application against different platform versions in the emulator,
create an AVD for each platform version that you want to test. For more
information about AVDs, see <a
href="{@docRoot}guide/developing/tools/avd.html">Android Virtual Devices</a>. If
you are using a physical device for testing, ensure that you know the API Level
of the Android platform it runs. See the table at the top of this document for
a list of platform versions and their API Levels. </p>


<h2 id="filtering">Filtering the Reference Documentation by API Level</h2>

<p>Reference documentation pages on the Android Developers site offer a "Filter
by API Level" control in the top-right area of each page. You can use the 
control to show documentation only for parts of the API that are actually 
accessible to your application, based on the API Level that it specifies in 
the <code>android:minSdkVersion</code> attribute of its manifest file. </p>

<p>To use filtering, set the control to the same API Level as that specified
by your application. Notice that APIs introduced in a later API Level are 
then grayed out and their content is masked, since they would not be
accessible to your application. </p>

<p>Filtering by API Level in the documentation does not provide a view
of what is new or introduced in each API Level &mdash; it simply provides a way
to view the entire API associated with a given API Level, while excluding API
elements introduced in later API Levels.</p>

<p>By default, API Level filtering is enabled and set to show the latest API
Level. If you do not want to use filtering reference documentation, 
simply select the highest available API Level. </p>

<p>Also note that the reference documentation for individual API elements
specifies the API Level at which the elements were introduced. The API Level 
for packages and classes is specified as "Since &lt;api level&gt;" at the 
top-right corner of the content area on each documentation page. The API Level 
for class members is specified in their detailed description headers, 
at the right margin. </p>