docs/html/resources/articles/zipalign.jd

page.title=Zipalign: an Easy Optimization
@jd:body

<p>The Android SDK includes a tool called <a
href="{@docRoot}guide/developing/tools/zipalign.html"><code>zipalign</code></a>
that optimizes the way an application is packaged. Running zipalign against your
application enables Android to interact it more efficiently at run time and thus
has the potential to make it and the overall system run faster. We strongly
encourage you to use <code>zipalign</code> on both new and already published
applications and to make the optimized version available &mdash; even if your
application targets a previous version of Android. This article describes how
<code>zipalign</code> helps performance and how to use it to optimize your
app.</p>

<p>In Android, data files stored in each application's apk are accessed by
multiple processes: the installer reads the manifest to handle the
permissions associated with that application; the Home application
reads resources to get the application's name and icon; the system
server reads resources for a variety of reasons (e.g. to display that
application's notifications); and last but not least, the resource
files are obviously used by the application itself.</p>

<p>The resource-handling code in Android can efficiently access resources when
they're aligned on 4-byte boundaries by memory-mapping them. But for resources
that are not aligned (that is, when <code>zipalign</code> hasn't been run on an
apk), it has to fall back to explicitly reading them &mdash; which is slower and
consumes additional memory.</p>

<p>For an application developer, this fallback mechanism is very
convenient. It provides a lot of flexibility by allowing for several
different development methods, including those that don't include
aligning resources as part of their normal flow.</p>

<p>Unfortunately, for users the situation is reversed &mdash; reading resources
from unaligned apks is slow and takes a lot of memory. In the best case, the
only visible result is that both the Home application and the unaligned
application launch slower than they otherwise should. In the worst case,
installing several applications with unaligned resources increases memory
pressure, thus causing the system to thrash around by having to constantly start
and kill processes. The user ends up with a slow device with a poor battery
life.</p>

<p>Luckily, it's very easy for you to align the resources in your application:</p>

<ul>
<li>Using ADT:</li>
<li style="list-style-type: none; list-style-image: none; list-style-position: outside;">
<ul>
<li>The ADT plugin for Eclipse (starting from version 0.9.3) will automatically
align release application packages if the export wizard is used to create them.
To use the wizard, right click the project and choose "Android Tools" &gt;
"Export Signed Application Package..." It can also be accessed from the first
page of the <code>AndroidManifest.xml</code> editor.</li>
</ul>
</li>
<li>Using Ant:</li><li style="list-style-type: none; list-style-image: none; list-style-position: outside;">

<ul>
<li>The <em>Ant</em> build script (starting from Android 1.6) can align
application packages. Targets for older versions of the Android platform are not
aligned by the <em>Ant</em> build script and need to be manually aligned.</li>
<li>Starting from the Android 1.6 SDK, Ant aligns and signs packages automatically, 
when building in debug mode.</li>
<li>In release mode, Ant aligns packages only if it has enough
information to sign the packages, since aligning has to happen after signing. In
order to be able to sign packages, and therefore to align them, <em>Ant</em>
needs to know the location of the keystore and the name of the key in
<code>build.properties</code>. The name of the properties are
<code>key.store</code> and <code>key.alias</code> respectively. If those
properties are present, the signing tool will prompt to enter the store/key
passwords during the build, and the script will sign and then align the apk
file. If the properties are missing, the release package will not be signed, and
therefore will not get aligned either.</li>
</ul>
</li>
<li>Manually:</li>
<li style="list-style-type: none; list-style-image: none; list-style-position: outside;">
<ul>
<li>In order to manually align a package, <code>zipalign</code>
is in the <code>tools/</code> folder of Android 1.6 and later SDKs. You can use
it to align application packages targeting any version of Android. You should run
it only after signing the apk file, using the following command:
<br><code>zipalign -v 4 source.apk destination.apk</code></li>
</ul>
</li>
<li>Verifying alignment:</li>
<li style="list-style-type: none; list-style-image: none; list-style-position: outside;">
<ul>
<li>The following command verifies that a package is aligned:<br><code>zipalign -c -v 4 application.apk</code>
</li>
</ul>
</li>
</ul>

<p>We encourage you manually run <code>zipalign</code>
on your currently published applications and to make the newly aligned
versions available to users. Also, don't forget to align any new
applications going forward!</p>