frameworks_base/docs/html/tools/help/proguard.jd

page.title=Shrink Your Code and Resources
parent.title=Tools
parent.link=index.html
page.metaDescription=Make your APK file smaller and more secure by shrinking your code and resources.
@jd:body

 <div id="qv-wrapper">
    <div id="qv">
      <h2>In this document</h2>
<ol>
  <li><a href="#shrink-code">Shrink Your Code</a>
    <ol>
      <li><a href="#keep-code">Customize which code to keep</a></li>
      <li><a href="#decode-stack-trace">Decode an obfuscated stack trace</a></li>
    </ol>
  </li>
  <li><a href="#shrink-resources">Shrink Your Resources</a>
    <ol>
      <li><a href="#keep-resources">Customize which resources to keep</a></li>
      <li><a href="#strict-reference-checks">Enable strict reference checks</a></li>
      <li><a href="#unused-alt-resources">Remove unused alternative resources</a></li>
      <li><a href="#merge-resources">Merge duplicate resources</a></li>
      <li><a href="#troubleshoot-resource-shrink">Troubleshoot resource shrinking</a></li>
    </ol>
  </li>
</ol>

      <h2>See also</h2>
      <ul>
        <li>
          <a class="external-link"
          href="http://stuff.mit.edu/afs/sipb/project/android/sdk/android-sdk-linux/tools/proguard/docs/index.html#manual/introduction.html">ProGuard
          Manual</a>
        </li>
      </ul>
    </div>
  </div>



<p>To make your APK file as small as possible, you should enable shrinking for
your code and resources in your release build. This guide describes how to do
both and how to specify what to keep or discard during a build.</p>

<p>Code shrinking is available with ProGuard, which detects and removes unused
classes, fields, methods, and attributes from your packaged app, including
those from included code libraries (making it a valuable tool for working
around the <a href="{@docRoot}tools/building/multidex.html">64k reference
limit</a>). ProGuard also optimizes the bytecode, removes unused code
instructions, and obfuscates the remaining classes, fields, and methods with
short names. The obfuscated code makes your APK difficult to reverse engineer,
which is especially valuable when your app uses security-sensitive features,
such as <a href="{@docRoot}google/play/licensing/overview.html">licensing
verification</a>.</p>

<p>Resource shrinking is available with the Android Plugin for Gradle, which
removes unused resources from your packaged app, including unused resources in
code libraries. It works in conjunction with code shrinking such that once
unused code has been removed, any resources no longer referenced can be safely
removed as well.</p>


<p>Features in this document depend on:</p>
<ul>
<li><a href="{@docRoot}tools/sdk/tools-notes.html">SDK Tools</a> 25.0.10
or higher</li>
<li><a href="{@docRoot}tools/revisions/gradle-plugin.html">Android Plugin
for Gradle</a> 2.0.0 or higher</li>
</ul>

<h2 id="shrink-code">Shrink Your Code</h2>

<p>To enable code shrinking with ProGuard, add <code>minifyEnabled true</code>
to the appropriate build type in your <code>build.gradle</code> file.</p>

<p>Be aware that code shrinking slows down the build time, so you should avoid
using it on your debug build if possible. However, it's important that you
do enable code shrinking on your final APK used for testing, because it might
introduce bugs if you do not sufficiently <a href="#keep-code">customize which
code to keep</a>.</p>

<p>For example, the following snippet from a <code>build.gradle</code> file
enables code shrinking for the release build:</p>

<pre class="no-pretty-print">
android {
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile(‘proguard-android.txt'),
                    'proguard-rules.pro</a>'
        }
    }
    ...
}
</pre>

<p class="note"><strong>Note</strong>: Android Studio disables ProGuard when
using <a href=
"{@docRoot}tools/building/building-studio.html#instant-run">Instant
Run</a>.</p>

<p>In addition to the <code>minifyEnabled</code> property, the
<code>proguardFiles</code> property defines the ProGuard rules:</p>

<ul>
<li>The <code>getDefaultProguardFile(‘proguard-android.txt')</code> method gets
the default ProGuard settings from the Android SDK <code>tools/proguard/</code>
folder.
<p><strong>Tip</strong>: For even more code shrinking, try the
<code>proguard-android-optimize.txt</code> file that's in the same location. It
includes the same ProGuard rules, but with other optimizations that perform
analysis at the bytecode level—inside and across methods—to reduce your APK
size further and help it run faster.</p>
</li>
<li>The <code>proguard-rules.pro</code> file is where you can add custom
ProGuard rules. By default, this file is located at the root of the module
(next to the <code>build.gradle</code> file).</li>
</ul>

<p>To add more ProGuard rules that are specific to each build variant, add
another <code>proguardFiles</code> property in the corresponding
<code>productFlavor</code> block. For example, the following Gradle file adds
<code>flavor2-rules.pro</code> to the <code>flavor2</code> product flavor. Now
<code>flavor2</code> uses all three ProGuard rules because those from the
<code>release</code> block are also applied.</p>

<pre class="no-pretty-print">
android {
    ...
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
                   'proguard-rules.pro'</span>
        }
    }
    productFlavors {
        flavor1 {
        }
        flavor2 {
            proguardFile 'flavor2-rules.pro'
        }
    }
}
</pre>

<p>With each build, ProGuard outputs the following files:</p>
<dl>
<dt><code>dump.txt</code></dt>
<dd>Describes the internal structure of all the class files in the APK.</dd>

<dt><code>mapping.txt</code></dt>
<dd>Provides a translation between the original and obfuscated class, method, and
field names.</dd>

<dt><code>seeds.txt</code></dt>
<dd>Lists the classes and members that were not obfuscated.</dd>

<dt><code>usage.txt</code></dt>
<dd>Lists the code that was removed from the APK.</dd>
</dl>

<p>These files are saved at
<code>&lt;module-name&gt;/build/outputs/mapping/release/</code>.</p>


<h3 id="keep-code">Customize which code to keep</h3>

<p>For some situations, the default ProGuard configuration file
(<code>proguard-android.txt</code>) is sufficient and ProGuard removes all—and
only—the unused code. However, many situations are difficult for ProGuard to
analyze correctly and it might remove code your app actually needs. Some
examples of when it might incorrectly remove code include:</p>

<ul>
<li>When your app references a class only from the
<code>AndroidManifest.xml</code> file</li>
<li>When your app calls a method from the Java Native Interface (JNI)</li>
<li>When your app manipulates code at runtime (such as with reflection or
introspection)</li>
</ul>

<p>Testing your app should reveal any errors caused by inappropriately removed
code, but you can also inspect what code was removed by reviewing the
<code>usage.txt</code> output file saved in
<code>&lt;module-name&gt;/build/outputs/mapping/release/</code>.</p>

<p>To fix errors and force ProGuard to keep certain code, add a <code><a
href="https://stuff.mit.edu/afs/sipb/project/android/sdk/android-sdk-linux/tools/proguard/docs/index.html#manual/usage.html"
>-keep</a></code> line in the ProGuard configuration file. For example:</p>

<pre>
-keep public class MyClass
</pre>

<p>Alternatively, you can add the <code><a href=
"{@docRoot}reference/android/support/annotation/Keep.html">@Keep</a></code>
annotation to the code you want to keep. Adding <code>@Keep</code> on a class
keeps the entire class as-is. Adding it on a method or field will keep the
method/field (and it's name) as well as the class name intact. Note that this
annotation is available only when using the <a href=
"{@docRoot}tools/support-library/features.html#annotations">Annotations Support
Library</a>.</p>


<p>There are many considerations you should make when using the
<code>-keep</code> option; for more information about customizing your
configuration file, read the <a href=
"http://stuff.mit.edu/afs/sipb/project/android/sdk/android-sdk-linux/tools/proguard/docs/index.html#manual/introduction.html">
ProGuard Manual</a>. The <a href=
"http://stuff.mit.edu/afs/sipb/project/android/sdk/android-sdk-linux/tools/proguard/docs/index.html#manual/troubleshooting.html">
Troubleshooting</a> section outlines other common problems you might encounter
when your code gets stripped away.</p>


<h3 id="decode-stack-trace">Decode an obfuscated stack trace</h3>

<p>After ProGuard shrinks your code, reading a stack trace is difficult (if not
impossible) because the method names are obfuscated. Fortunately, ProGuard
creates a <code>mapping.txt</code> file each time it runs, which shows the
original class, method, and field names mapped to the obfuscated names.
ProGuard saves the file in the app
<code>&lt;module-name&gt;/build/outputs/mapping/release/</code> directory.</p>


<p>Be aware that the <code>mapping.txt</code> file is overwritten every time
you create a release build with ProGuard, so you must carefully save a copy
each time you publish a new release. By retaining a copy of the
<code>mapping.txt</code> file for each release build, you'll be able to debug a
problem if a user submits an obfuscated stack trace from an older version of
your app.</p>

<p>When publishing your app on Google Play, you can upload the
<code>mapping.txt</code> file for each version of your APK. Then Google Play
will deobfuscate incoming stack traces from user-reported issues so you can
review them in the Google Play Developer Console. For more information, see the
Help Center article about how to <a href=
"https://support.google.com/googleplay/android-developer/answer/6295281">deobfuscate
crash stack traces</a>.</p>

<p>To convert an obfuscated stack trace to a readable one yourself, use the
<code>retrace</code> script (<code>retrace.bat</code> on Windows;
<code>retrace.sh</code> on Mac/Linux). It is located in the
<code>&lt;sdk-root&gt;/tools/proguard/</code> directory. The script takes the
<code>mapping.txt</code> file and your stack trace, producing a new, readable
stack trace. The syntax for using the retrace tool is:</p>

<pre class="no-pretty-print">
retrace.bat|retrace.sh [-verbose] mapping.txt [&lt;stacktrace_file&gt;]
</pre>

<p>For example:</p>

<pre class="no-pretty-print">
retrace.bat -verbose mapping.txt obfuscated_trace.txt
</pre>

<p>If you do not specify the stack trace file, the retrace tool reads from
standard input.</p>

<h2 id="shrink-resources">Shrink Your Resources</h2>

<p>Resource shrinking works only in conjunction with code shrinking. After the
code shrinker removes all unused code, the resource shrinker can identify which
resources the app still uses. This is especially true when you add code
libraries that include resources—you must remove unused library code so the
library resources become unreferenced and, thus, removable by the resource
shrinker.</p>

<p>To enable resource shrinking, set the <code>shrinkResources</code> property
to <code>true</code> in your <code>build.gradle</code> file (alongside
<code>minifyEnabled</code> for code shrinking). For example:</p>

<pre class="no-pretty-print">
android {
    ...
    buildTypes {
        release {
            shrinkResources true
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
                    'proguard-rules.pro'
        }
    }
}
</pre>

<p>If you haven't already built your app using <code>minifyEnabled</code> for
code shrinking, then try that before enabling <code>shrinkResources</code>,
because you might need to edit your <code>proguard-rules.pro</code> file to
keep classes or methods that are created or invoked dynamically before you
start removing resources.</p>

<p class="note"><strong>Note</strong>: The resource shrinker currently does not
remove resources defined in a <code>values/</code> folder (such as strings,
dimensions, styles, and colors). This is because the Android Asset Packaging
Tool (AAPT) does not allow the Gradle Plugin to specify predefined versions for
resources. For details, see <a href=
"https://code.google.com/p/android/issues/detail?id=70869">issue 70869</a>.</p>


<h3 id="keep-resources">Customize which resources to keep</h3>

<p>If there are specific resources you wish to keep or discard, create an XML
file in your project with a <code>&lt;resources&gt;</code> tag and specify each
resource to keep in the <code>tools:keep</code> attribute and each resource to
discard in the <code>tools:discard</code> attribute. Both attributes accept a
comma-separated list of resource names. You can use the asterisk character as a
wild card.</p>

<p>For example:</p>

<pre>
&lt;?xml version=1.0" encoding="utf-8"?&gt;
&lt;resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="&#64;layout/l_used*_c,&#64;layout/l_used_a,&#64;layout/l_used_b*"
    tools:discard="&#64;layout/unused2" /&gt;
</pre>

<p>Save this file in your project resources, for example, at
<code>res/raw/keep.xml</code>. The build does not package this file into your
APK.</p>

<p>Specifying which resources to discard might seem silly when you could
instead delete them, but this can be useful when using build variants. For
example, you might put all your resources into the common project directory,
then create a different <code>keep.xml</code> file for each build variant when
you know that a given resource appears to be used in code (and therefore not
removed by the shrinker) but you know it actually won't be used for the given
build variant.</p>

<h3 id="strict-reference-checks">Enable strict reference checks</h3>

<p>Normally, the resource shrinker can accurately determine whether a resource
is used. However, if your code makes a call to {@link
android.content.res.Resources#getIdentifier(String,String,String)
Resources.getIdentifier()} (or if any of your libraries do that—the <a href=
"{@docRoot}tools/support-library/features.html#v7-appcompat">AppCompat</a>
library does), that means your code is looking up resource names based on
dynamically-generated strings. When you do this, the resource shrinker behaves
defensively by default and marks all resources with a matching name format as
potentially used and unavailable for removal.</p>

<p>For example, the following code causes all resources with the
<code>img_</code> prefix to be marked as used.</p>

<pre>
String name = String.format("img_%1d", angle + 1);
res = getResources().getIdentifier(name, "drawable", getPackageName());
</pre>

<p>The resource shrinker also looks through all the string constants in your
code, as well as various <code>res/raw/</code> resources, looking for resource
URLs in a format similar to
<code>file:///android_res/drawable//ic_plus_anim_016.png</code>. If it finds
strings like this or others that look like they could be used to construct URLs
like this, it doesn't remove them.</p>

<p>These are examples of the safe shrinking mode that is enabled by default.
You can, however, turn off this "better safe than sorry" handling, and specify
that the resource shrinker keep only resources that it's certain are used. To
do this, set <code>shrinkMode</code> to <code>strict</code> in the
<code>keep.xml</code> file, as follows:</p>

<pre>
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;resources xmlns:tools="http://schemas.android.com/tools"
    tools:shrinkMode="strict" /&gt;
</pre>

<p>If you do enable strict shrinking mode and your code also references
resources with dynamically-generated strings, as shown above, then you must
manually keep those resources using the <code>tools:keep</code> attribute.</p>

<h3 id="unused-alt-resources">Remove unused alternative resources</h3>

<p>The Gradle resource shrinker removes only resources that are not referenced
by your app code, which means it will not remove <a href=
"{@docRoot}guide/topics/resources/providing-resources.html#AlternativeResources">
alternative resources</a> for different device configurations. If necessary,
you can use the Android Gradle plugin's <code>resConfigs</code> property to
remove alternative resource files that your app does not need.</p>

<p>For example, if you are using a library that includes language resources
(such as AppCompat or Google Play Services), then your APK includes all
translated language strings for the messages in those libraries whether the
rest of your app is translated to the same languages or not. If you'd like to
keep only the languages that your app officially supports, you can specify
those languages using the <code>resConfig</code> property. Any resources for
languages not specified are removed.</p>

<p>The following snippet shows how to limit your language resources to just
English and French:</p>

<pre class="no-pretty-print">
android {
    defaultConfig {
        ...
        resConfigs "en", "fr"
    }
}
</pre>

<p>To customize which screen density or ABI resources to include in your APK,
instead use <a href=
"http://tools.android.com/tech-docs/new-build-system/user-guide/apk-splits">APK
splits</a> to build different APKs for different devices.</p>

<h3 id="merge-resources">Merge duplicate resources</h3>

<p>By default, Gradle also merges identically named resources, such as
drawables with the same name that might be in different resource folders. This
behavior is not controlled by the <code>shrinkResources</code> property and
cannot be disabled, because it is necessary to avoid errors when multiple
resources match the name your code is looking up.</p>

<p>Resource merging occurs only when two or more files share an identical
resource name, type, and qualifier. Gradle selects which file it considers to
be the best choice among the duplicates (based on a priority order described
below) and passes only that one resource to the AAPT for distribution in the
APK file.</p>

<p>Gradle looks for duplicate resources in the following locations:</p>

<ul>
<li>The main resources, associated with the main source set, generally
located in <code>src/main/res/</code>.</li>
<li>The variant overlays, from the build type and build flavors.</li>
<li>The library project dependencies.</li>
</ul>

<p>Gradle merges duplicate resources in the following cascading priority order:</p>

<p>Dependencies → Main → Build flavor → Build type</p>

<p>For example, if a duplicate resource appears in both your main resources and
a build flavor, Gradle selects the one in the build flavor.</p>

<p>If identical resources appear in the same source set, Gradle cannot merge
them and emits a resource merge error. This can happen if you define multiple
source sets in the <code>sourceSet</code> property of your
<code>build.gradle</code> file—for example if both <code>src/main/res/</code>
and <code>src/main/res2/</code> contain identical resources.</p>

<h3 id="troubleshoot-resource-shrink">Troubleshoot resource shrinking</h3>

<p>When you shrink resources, the Gradle Console shows a summary of the
resources that it removed from the app package. For example:</p>

<pre class="no-pretty-print">
:android:shrinkDebugResources
Removed unused resources: Binary resource data reduced from 2570KB to 1711KB: Removed 33%
:android:validateDebugSigning
</pre>

<p>Gradle also creates a diagnostic file named <code>resources.txt</code> in
<code>&lt;module-name&gt;/build/outputs/mapping/release/</code> (the same
folder as ProGuard's output files). This file includes details such as which
resources reference other resources and which resources are used or
removed.</p>

<p>For example, to find out why <code>&#64;drawable/ic_plus_anim_016</code> is
still in your APK, open the <code>resources.txt</code> file and search for that
file name. You might find that it's referenced from another resource, as
follows:</p>

<pre class="no-pretty-print">
16:25:48.005 [QUIET] [system.out] &#64;drawable/add_schedule_fab_icon_anim : reachable=true
16:25:48.009 [QUIET] [system.out]     &#64;drawable/ic_plus_anim_016
</pre>

<p>You now need to know why <code>&#64;drawable/add_schedule_fab_icon_anim</code>
is reachable—and if you search upwards you'll find that resource is listed
under "The root reachable resources are:". This means there is a code reference
to <code>add_schedule_fab_icon_anim</code> (that is, its R.drawable ID was
found in the reachable code).</p>

<p>If you are not using strict checking, resource IDs can be marked as reachable
if there are string constants that look like they might be used to construct
resource names for dynamically loaded resources. In that case, if you search
the build output for the resource name, you might find a message like this:</p>

<pre class="no-pretty-print">
10:32:50.590 [QUIET] [system.out] Marking drawable:ic_plus_anim_016:2130837506
    used because it format-string matches string pool constant ic_plus_anim_%1$d.
</pre>

<p>If you see one of these strings and you are certain that the string is not
being used to load the given resource dynamically, you can use the
<code>tools:discard</code> attribute to inform the build system to remove it,
as described in the section about how to <a href="#keep-resources"
>customize which resources to keep</a>.</p>