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Configure Apps with Over 64K Methods

As the Android platform has continued to grow, so has the size of Android apps. When your app and the libraries it references reach a certain size, you encounter build errors that indicate your app has reached a limit of the Android app build architecture. Earlier versions of the build system report this error as follows:

Conversion to Dalvik format failed:
Unable to execute dex: method ID not in [0, 0xffff]: 65536

More recent versions of the Android build system display a different error, which is an indication of the same problem:

trouble writing output:
Too many field references: 131000; max is 65536.
You may try using --multi-dex option.

Both these error conditions display a common number: 65,536. This number is significant in that it represents the total number of references that can be invoked by the code within a single Dalvik Executable (DEX) bytecode file. This page explains how to move past this limitation by enabling an app configuration known as multidex, which allows your app to build and read multiple DEX files.

About the 64K reference limit

Android app (APK) files contain executable bytecode files in the form of Dalvik Executable (DEX) files, which contain the compiled code used to run your app. The Dalvik Executable specification limits the total number of methods that can be referenced within a single DEX file to 65,536—including Android framework methods, library methods, and methods in your own code. In the context of computer science, the term Kilo, K, denotes 1024 (or 2^10). Because 65,536 is equal to 64 X 1024, this limit is referred to as the '64K reference limit'.

Multidex support prior to Android 5.0

Versions of the platform prior to Android 5.0 (API level 21) use the Dalvik runtime for executing app code. By default, Dalvik limits apps to a single classes.dex bytecode file per APK. In order to get around this limitation, you can use the multidex support library, which becomes part of the primary DEX file of your app and then manages access to the additional DEX files and the code they contain.

Note: If your project is configured for multidex with minSdkVersion 20 or lower, and you deploy to target devices running Android 4.4 (API level 20) or lower, Android Studio disables Instant Run.

Multidex support for Android 5.0 and higher

Android 5.0 (API level 21) and higher uses a runtime called ART which natively supports loading multiple DEX files from APK files. ART performs pre-compilation at app install time which scans for classesN.dex files and compiles them into a single .oat file for execution by the Android device. Therefore, if your minSdkVersion is 21 or higher, you do not need the multidex support library.

For more information on the Android 5.0 runtime, read ART and Dalvik.

Note: While using Instant Run, Android Studio automatically configures your app for multidex when your app's minSdkVersion is set to 21 or higher. Because Instant Run only works with the debug version of your app, you still need to configure your release build for multidex to avoid the 64K limit.

Avoid the 64K limit

Before configuring your app to enable use of 64K or more method references, you should take steps to reduce the total number of references called by your app code, including methods defined by your app code or included libraries. The following strategies can help you avoid hitting the DEX reference limit:

Using these techniques might help you avoid the need to enable multidex in your app while also decreasing the overall size of your APK.

Configure your app for multidex

Setting up your app project to use a multidex configuration requires that you make the following modifications to your app project, depending on the minimum Android version your app supports.

If your minSdkVersion is set to 21 or higher, all you need to do is set multiDexEnabled to true in your module-level build.gradle file, as shown here:

android {
    defaultConfig {
        ...
        minSdkVersion 21 
        targetSdkVersion 26
        multiDexEnabled true
    }
    ...
}

However, if your minSdkVersion is set to 20 or lower, then you must use the multidex support library as follows:

Now when you build your app, the Android build tools construct a primary DEX file (classes.dex) and supporting DEX files (classes2.dex, classes3.dex, and so on) as needed. The build system then packages all DEX files into your APK.

At runtime, the multidex APIs use a special class loader to search all of the available DEX files for your methods (instead of searching only in the main classes.dex file).

Limitations of the multidex support library

The multidex support library has some known limitations that you should be aware of and test for when you incorporate it into your app build configuration:

Declare classes required in the primary DEX file

When building each DEX file for a multidex app, the build tools perform complex decision-making to determine which classes are needed in the primary DEX file so that your app can start successfully. If any class that's required during startup is not provided in the primary DEX file, then your app crashes with the error java.lang.NoClassDefFoundError.

This shouldn't happen for code that's accessed directly from your app code because the build tools recognize those code paths, but it can happen when the code paths are less visible such as when a library you use has complex dependencies. For example, if the code uses introspection or invocation of Java methods from native code, then those classes might not be recognized as required in the primary DEX file.

So if you receive java.lang.NoClassDefFoundError, then you must manually specify these additional classes as required in the primary DEX file by declaring them with the multiDexKeepFile or the multiDexKeepProguard property in your build type. If a class is matched in either the multiDexKeepFile or the multiDexKeepProguard file, then that class is added to the primary DEX file.

multiDexKeepFile property

The file you specify in multiDexKeepFile should contain one class per line, in the format com/example/MyClass.class. For example, you can create a file called multidex-config.txt that looks like this:

com/example/MyClass.class
com/example/MyOtherClass.class

Then you can declare that file for a build type as follows:

android {
    buildTypes {
        release {
            multiDexKeepFile file 'multidex-config.txt'
            ...
        }
    }
}

Remember that Gradle reads paths relative to the build.gradle file, so the above example works if multidex-config.txt is in the same directory as the build.gradle file.

multiDexKeepProguard property

The multiDexKeepProguard file uses the same format as Proguard and supports the entire Proguard grammar. For more information about Proguard format and grammar, see the Keep Options section in the Proguard manual.

The file you specify in multiDexKeepProguard should contain -keep options in any valid ProGuard syntax. For example, -keep com.example.MyClass.class. You can create a file called multidex-config.pro that looks like this:

-keep class com.example.MyClass
-keep class com.example.MyClassToo

If you want to specify all classes in a package, the file looks like this:

-keep class com.example.** { *; } // All classes in the com.example package

Then you can declare that file for a build type as follows:

android {
    buildTypes {
        release {
            multiDexKeepProguard 'multidex-config.pro'
            ...
        }
    }
}

Optimize multidex in development builds

A multidex configuration requires significantly increased build processing time because the build system must make complex decisions about which classes must be included in the primary DEX file and which classes can be included in secondary DEX files. This means that incremental builds using multidex typically take longer and can potentially slow your development process.

To mitigate the longer build times for multidex output, create two build variants using productFlavors: a development flavor and a release flavor with different values for minSdkVersion.

For the development flavor, set minSdkVersion to 21. This setting enables a build feature called pre-dexing, which generates multidex output much faster using an ART format available only on Android 5.0 (API level 21) and higher. For the release flavor, set the minSdkVersion as appropriate for your actual minimum support level. This setting generates a multidex APK that is compatible with more devices, but takes longer to build.

The following build configuration sample demonstrates the how to set up these flavors in a Gradle build file:

android {
    defaultConfig {
        ...
        multiDexEnabled true
    }
    productFlavors {
        dev {
            // Enable pre-dexing to produce an APK that can be tested on
            // Android 5.0+ without the time-consuming DEX build processes.
            minSdkVersion 21
        }
        prod {
            // The actual minSdkVersion for the production version.
            minSdkVersion 14
        }
    }
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'),
                                                 'proguard-rules.pro'
        }
    }
}
dependencies {
    compile 'com.android.support:multidex:1.0.1'
}

After you complete this configuration change, you can use the devDebug variant of your app for incremental builds, which combines the attributes of the dev product flavor and the debug build type. That creates a debuggable app with multidex enabled and proguard disabled (because minifyEnabled is false by default). These settings cause the Android plugin for Gradle to do the following:

  1. Perform pre-dexing: Build each app module and each dependency as a separate DEX file.
  2. Include each DEX file in the APK without modification (no code shrinking).
  3. Most importantly, the module DEX files are not combined, and so the long-running calculation to determine the contents of the primary DEX file is avoided.

These settings result in fast, incremental builds, because only the DEX files of modified modules are recomputed and repackaged during subsequent builds. However, the APK from these builds can be used to test on Android 5.0 devices only. Yet, by implementing the configuration as a flavor, you preserve the ability to perform normal builds with the release-appropriate minimum API level and ProGuard code shrinking.

You can also build the other variants, including a prodDebug variant build, which takes longer to build, but can be used for testing outside of development. Within the configuration shown, the prodRelease variant would be the final testing and release version. For more information about using build variants, see Configure Build Variants.

Tip: Now that you have different build variants for different multidex needs, you can also provide a different manifest file for each variant (so only the one for API level 20 and lower changes the <application> tag name), or create a different Application subclass for each variant (so only the one for API level 20 and lower extends the MultiDexApplication class or calls MultiDex.install(this)).

Test multidex apps

When you write instrumentation tests for multidex apps, no additional configuration is required if you use a MonitoringInstrumentation (or an AndroidJUnitRunner) instrumentation. If you use another Instrumentation, then you must override its onCreate() method with the following code:

public void onCreate(Bundle arguments) {
  MultiDex.install(getTargetContext());
  super.onCreate(arguments);
  ...
}
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