topic/performance/reduce-apk-size.jd

page.title=Reduce APK Size
trainingnavtop=true

@jd:body

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

<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#apk-structure">Understand the APK Structure</a></li>
  <li><a href="#reduce-resources">Reduce Resource Count and Size</a></li>
  <li><a href="#reduce-code">Reduce Native and Java Code</a></li>
  <li><a href="#multiple-apks">Maintain Multiple Lean APKs</a></li>
</ol>

<h2>
  You should also read
</h2>

<ul>
  <li>
    <a href="{@docRoot}studio/build/shrink-code.html">Shrink Your Code and
    Resources</a>
  </li>
</ul>


</div>
</div>

<p>
  Users often avoid downloading apps that seem too large, particularly in
  emerging markets where devices connect to often-spotty 2G and
  3G networks or work on pay-by-the-byte plans. This article describes how to
  reduce your app's APK size, which enables more users to download your app.
</p>

<h2 id="apk-structure">
  Understand the APK Structure
</h2>

<p>
  Before discussing how to reduce the size of your app, it's helpful to
  understand the structure of an app's APK. An APK file consists of a ZIP
  archive that contains all the files that comprise your app. These files
  include Java class files, resource files, and a file containing compiled
  resources.
</p>

<p>
An APK contains the following directories:
</p>

<ul>
  <li>{@code META-INF/}: Contains the <code>CERT.SF</code> and
  <code>CERT.RSA</code> signature files, as well as the {@code MANIFEST.MF}
  manifest file.
  </li>

  <li>{@code assets/}: Contains the app's assets, which the app can retrieve
  using an {@link android.content.res.AssetManager} object.
  </li>

  <li>
  {@code res/}: Contains resources that aren't compiled into
  <code>resources.arsc</code>.
  </li>

  <li>{@code lib/}: Contains the compiled code that is specific to the software
  layer of a processor. This directory contains a subdirectory for each
  platform type, like <code>armeabi</code>, <code>armeabi-v7a</code>,
  <code>arm64-v8a</code>, <code>x86</code>, <code>x86_64</code>, and
  <code>mips</code>.
  </li>
</ul>

<p>
An APK also contains the following files. Among them,
only <code>AndroidManifest.xml</code> is mandatory.
</p>

<ul>
  <li>{@code resources.arsc}: Contains compiled resources. This file contains
  the XML content from all configurations of the <code>res/values/</code>
  folder. The packaging tool extracts this XML content, compiles it to binary
  form, and archives the content. This content includes language strings and
  styles, as well as paths to content that is not included directly in the
  <code>resources.arsc</code> file, such as layout files and images.
  </li>

  <li>{@code classes.dex}: Contains the classes compiled in the DEX file format
  understood by the Dalvik/ART virtual machine.
  </li>

  <li>{@code AndroidManifest.xml}: Contains the core Android manifest file.
  This file lists the name, version, access rights, and referenced library
  files of the app. The file uses Android's binary XML format.
  </li>
</ul>

<h2 id="reduce-resources">
  Reduce Resource Count and Size
</h2>

<p>
  The size of your APK has an impact on how fast your app loads, how much
  memory it uses, and how much power it consumes. One of the simple ways to
  make your APK smaller is to reduce the number and size of the
  resources it contains. In particular, you can remove resources
  that your app no longer uses, and you can use scalable {@link
  android.graphics.drawable.Drawable} objects in place of image files. This
  section discusses these methods as well as several other ways that you can
  reduce the resources in your app to decrease the overall size of your APK.
</p>

<h3 id="remove-unused">
  Remove Unused Resources
</h3>

<p>
  The <a href="{@docRoot}studio/write/lint.html">{@code lint}</a> tool, a
  static code analyzer included in Android Studio, detects resources in your
  <code>res/</code> folder that your code doesn't reference. When the
  <code>lint</code> tool discovers a potentially unused resource in your
  project, it prints a message like the following example.
</p>

<pre class="no-pretty-print">
res/layout/preferences.xml: Warning: The resource R.layout.preferences appears
    to be unused [UnusedResources]
</pre>
<p class="note">
  <strong>Note:</strong> The <code>lint</code> tool doesn't scan the {@code
  assets/} folder, assets that are referenced via reflection, or library files
  that you've linked to your app. Also, it doesn't remove resources; it only
  alerts you to their presence.
</p>

<p>
  Libraries that you add to your code may include unused resources. Gradle can
  automatically remove resources on your behalf if you enable <a href=
  "{@docRoot}studio/build/shrink-code.html">{@code shrinkResources}</a> in
  your app's <code>build.gradle</code> file.
</p>

<pre class="prettyprint">
android {
    // Other settings

    buildTypes {
        release {
            minifyEnabled true
            shrinkResources true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
</pre>
<p>
  To use {@code shrinkResources}, you must enable code shrinking. During the
  build process, first <a href=
  "{@docRoot}studio/build/shrink-code.html">ProGuard</a> removes unused code
  but leaves unused resources. Then Gradle removes the unused resources.
</p>

<p>
  For more information about ProGuard and other ways
  Android Studio helps you reduce APK size, see <a href=
  "{@docRoot}studio/build/shrink-code.html">Shrink Your Code and Resources</a>.
</p>

<p>
  In Android Gradle Plugin 0.7 and higher, you can declare the configurations
  that your app supports. Gradle passes this information to the build system
  using the {@code resConfig} and {@code resConfigs} flavors and the
  <code>defaultConfig</code> option. The build system then prevents resources
  from other, unsupported configurations from appearing in the APK, reducing
  the APK's size. For more information about this feature, see <a href=
  "{@docRoot}studio/build/shrink-code.html#unused-alt-resources">Remove unused
  alternative resources</a>.
</p>

<h3 id="minimize">
  Minimize Resource Use from Libraries
</h3>

<p>
  When developing an Android app, you usually use external libraries to improve
  your app's usability and versatility. For example, you might reference the
  <a href="{@docRoot}topic/libraries/support-library/index.html">Android
  Support Library</a> to improve the user experience on older devices, or you
  could use <a class="external-link" href=
  "https://developers.google.com/android/guides/overview">Google Play
  Services</a> to retrieve automatic translations for text within your app.
</p>

<p>
  If a library was designed for a server or desktop, it can include many
  objects and methods that your app doesnt need. To include only the parts of
  the library that your app needs, you can edit the library's files if the
  license allows you to modify the library. You can also use an alternative,
  mobile-friendly library to add specific functionality to your app.
</p>

<p class="note">
  <strong>Note:</strong> <a href=
  "{@docRoot}studio/build/shrink-code.html">ProGuard</a> can clean up some
  unnecessary code imported with a library, but it can't remove a library's
  large internal dependencies.
</p>

<h3 id="support-densities">
  Support Only Specific Densities
</h3>

<p>
  Android supports a very large set of devices, encompassing a variety of
  screen densities. In Android 4.4 (API level 19) and higher, the framework
  supports various densities: <code>ldpi</code>, <code>mdpi</code>,
  <code>tvdpi</code>, <code>hdpi,</code> <code>xhdpi</code>,
  <code>xxhdpi</code> and <code>xxxhdpi</code>. Although Android supports all
  these densities, you don't need to export your rasterized assets to each
  density.
</p>

<p>
  If you know that only a small percentage of your users have devices with
  specific densities, consider whether you need to bundle those densities into
  your app. If you don't include resources for a specific screen density,
  Android automatically scales existing resources originally designed for other
  screen densities.
</p>

<p>
  If your app needs only scaled images, you can save even more space by having
  a single variant of an image in <code>drawable-nodpi/</code>. We recommend
  that every app include at least an <code>xxhdpi</code> image variant.
</p>

<p>
  For more information screen densities, see <a class="external-link" href=
  "{@docRoot}about/dashboards/index.html#Screens">Screen Sizes and
  Densities</a>.
</p>

<h3 id="reduce-frames">
  Reduce Animation Frames
</h3>

<p>
  Frame-by-frame animations can drastically increase the size of your APK.
  Figure 1 shows an example of a frame-by-frame animation separated into
  multiple PNG files within a directory. Each image is one frame in the
  animation.
</p>

<p>
  For each frame that you add to the animation, you increase the number of
  images stored in the APK. In Figure 1, the image animates at 30 FPS within
  the app. If the image animated at only 15 FPS instead, the animation would
  require only half the number of needed frames.
</p>

<figure id="fig-frame-animations">
  <img src="{@docRoot}images/training/performance/animation-frames.png" srcset=
  "{@docRoot}images/training/performance/animation-frames.png 1x, {@docRoot}images/training/performance/animation-frames_2x.png 2x"
  width="803" alt="">
  <figcaption>
    <strong>Figure 1.</strong> Frame by frame animations stored as resources.
  </figcaption>
</figure>

<h3 id="use-drawables">
  Use Drawable Objects
</h3>

<p>
  Some images don't require a static image resource; the framework can
  dynamically draw the image at runtime instead. {@link
  android.graphics.drawable.Drawable} objects (<code>&lt;shape&gt;</code> in
  XML) can take up a tiny amount of space in your APK. In addition, XML {@link
  android.graphics.drawable.Drawable} objects produce monochromatic images
  compliant with material design guidelines.
</p>

<h3 id="reuse-resources">
  Reuse Resources
</h3>

<p>
  You can include a separate resource for variations of an image, such as
  tinted, shaded, or rotated versions of the same image. We recommend, however,
  that you reuse the same set of resources, customizing them as needed at
  runtime.
</p>

<p>
  Android provides several utilities to change the color of an asset, either
  using the {@code android:tint} and {@code tintMode} attributes on Android 5.0
  (API level 21) and higher. For lower versions of the platform, use the {@link
  android.graphics.ColorFilter} class.
</p>
<p>
  You can also omit resources that are only a rotated equivalent of another
  resource. The following code snippet provides an example of turning an
  "expand" arrow into a "collapse" arrow icon by simply rotating the original
  image 180 degrees:
</p>

<pre class="prettyprint">
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;rotate xmlns:android="http://schemas.android.com/apk/res/android"
    android:drawable="@drawable/ic_arrow_expand"
    android:fromDegrees="180"
    android:pivotX="50%"
    android:pivotY="50%"
    android:toDegrees="180" /&gt;
</pre>
<h3 id="render-code">
  Render From Code
</h3>

<p>
  You can also reduce your APK size by procedurally rendering your images.
  Procedural rendering frees up space because you no longer store an image file
  in your APK.
</p>

<h3 id="crunch">
  Crunch PNG Files
</h3>

<p>
  The <code>aapt</code> tool can optimize the image resources placed in
  <code>res/drawable/</code> with lossless compression during the build
  process. For example, the <code>aapt</code> tool can convert a true-color PNG
  that does not require more than 256 colors to an 8-bit PNG with a color
  palette. Doing so results in an image of equal quality but a smaller memory
  footprint.
</p>

<p>
  Keep in mind that the <code>aapt</code> has the following limitations:
</p>

<ul>
  <li>The <code>aapt</code> tool does not shrink PNG files contained in the
  <code>asset/</code> folder.
  </li>

  <li>Image files need to use 256 or fewer colors for the <code>aapt</code>
  tool to optimize them.
  </li>

  <li>The <code>aapt</code> tool may inflate PNG files that have already been
  compressed. To prevent this, you can use the <code>cruncherEnabled</code>
  flag in Gradle to disable this process for PNG files:
  </li>
</ul>

<pre class="prettyprint">
aaptOptions {
    cruncherEnabled = false
}
</pre>
<h3 id="compress">
  Compress PNG and JPEG Files
</h3>

<p>
  You can reduce PNG file sizes without losing image quality using tools like
  <a class="external-link" href=
  "http://pmt.sourceforge.net/pngcrush/">pngcrush</a>, <a class="external-link"
  href="https://pngquant.org/">pngquant</a>, or <a class="external-link" href=
  "https://github.com/google/zopfli">zopflipng</a>. All of these tools can
  reduce PNG file size while preserving image quality.
</p>

<p>
  The {@code pngcrush} tool is particularly effective: This tool iterates over
  PNG filters and zlib (Deflate) parameters, using each combination of filters
  and parameters to compress the image. It then chooses the configuration that
  yields the smallest compressed output.
</p>

<p>
  For JPEG files, you can use tools like <a class="external-link" href=
  "http://www.elektronik.htw-aalen.de/packjpg/">packJPG</a> that compress JPEG
  files into a more compact form.
</p>

<h3 id="use-webp">
  Use WebP File Format
</h3>

<p>
  Instead of using PNG or JPEG files, you can also use the <a class=
  "external-link" href="https://developers.google.com/speed/webp/">WebP</a>
  file format for your images. The WebP format provides lossy compression (like
  JPEG) as well as transparency (like PNG) but can provide better compression
  than either JPEG or PNG.
</p>

<p>
  Using the WebP file format has a few notable drawbacks, however. First,
  support for WebP is not available in versions of the platform lower than
  Android 3.2 (API level 13). Second, it takes a longer amount of time for the
  system to decode WebP than PNG files.
</p>

<p class="note">
  <strong>Note:</strong> Google Play accepts APKs only if the included icons
  use the PNG format. You can't use other file formats like JPEG or WebP for
  app icons if you intend to publish your app through Google Play.
</p>

<h3 id="vector">
  Use Vector Graphics
</h3>

<p>
  You can use vector graphics to create resolution-independent icons and other
  scalable media. Using these graphics can greatly reduce your APK footprint.
  Vector images are represented in Android as {@link
  android.graphics.drawable.VectorDrawable} objects. With a {@link
  android.graphics.drawable.VectorDrawable } object, a 100-byte file can
  generate a sharp image the size of the screen.
</p>

<p>
  However, it takes a significant amount of time for the system to render each
  {@link android.graphics.drawable.VectorDrawable} object, and larger images
  take even longer to appear on the screen. Therefore, consider using these
  vector graphics only when displaying small images.
</p>

<p>
  For more information on working with {@link
  android.graphics.drawable.VectorDrawable } objects, see <a class=
  "external-link" href="{@docRoot}training/material/drawables.html">Working
  with Drawables</a>.
</p>

<h2 id="reduce-code">
  Reduce Native and Java Code
</h2>

<p>
  There are several methods you can use to reduce the size of the Java and
  native codebase in your app.
</p>

<h3 id="remove-generated">
  Remove Unnecessary Generated Code
</h3>

<p>
  Make sure to understand the footprint of any code which is automatically
  generated. For example, many protocol buffer tools generate an excessive
  number of methods and classes, which can double or triple the size of your
  app.
</p>

<h3 id="remove-enums">
  Remove Enumerations
</h3>

<p>
  A single enum can add about 1.0 to 1.4 KB of size to your app's
  <code>classes.dex</code> file. These additions can quickly accumulate for
  complex systems or shared libraries. If possible, consider using the
  <code>@IntDef</code> annotation and <a href=
  "{@docRoot}studio/build/shrink-code.html">ProGuard</a> to strip enumerations
  out and convert them to integers. This type conversion preserves all of the
  type safety benefits of enums.
</p>

<h3 id="reduce-binaries">
  Reduce the Size of Native Binaries
</h3>

<p>
  If your app uses native code and the Android NDK, you can also reduce the
  size of your app by optimizing your code. Two useful techniques are
  removing debug symbols and not extracting native libraries.
</p>

<h4 id="remove-debug">
  Remove Debug Symbols
</h4>

<p>
  Using debug symbols makes sense if your application is in development and
  still requires debugging. Use the <code>arm-eabi-strip</code> tool, provided
  in the Android NDK, to remove unnecessary debug symbols from native
  libraries. After that, you can compile your release build.
</p>

<h4 id="extract-false">
  Avoid Extracting Native Libraries
</h4>

<p>
  Store {@code .so} files uncompressed in the APK, and set the {@code
  android:extractNativeLibs} flag to false in the <a href=
  "{@docRoot}guide/topics/manifest/application-element.html">{@code
  <application>}</a> element of your app manifest. This will prevent
  {@link android.content.pm.PackageManager} from copying out {@code .so} files
  from the APK to the filesystem during installation and will have the added
  benefit of making delta updates of your app smaller.
</p>

<h2 id="multiple-apks">
  Maintain Multiple Lean APKs
</h2>

<p>
  Your APK can contain content that users download but never use, like regional
  or language information. To create a minimal download for your users, you can
  segment your app into several APKs, differentiated by factors such as screen
  size or GPU texture support.
</p>

<p>
  When a user downloads your app, their device receives the correct APK based
  on the device's features and settings. This way, devices don't receive assets
  for features that the devices don't have. For example, if a user has a
  <code>hdpi</code> device, they dont need <code>xxxhdpi</code> resources that
  you might include for devices with higher density displays.
</p>

<p>
  For more information, see <a href=
  "{@docRoot}studio/build/configure-apk-splits.html">Configure APK Splits</a>
  and <a class="external-link" href=
  "{@docRoot}training/multiple-apks/index.html">Maintaining Multiple APKs</a>.
</p>