Merge "docs: performance: Reduce APK Sizes" into nyc-dev

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 doesn’t 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">

<?xml version="1.0" encoding="utf-8"?>

<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" />

</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 don’t 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>