MultiDex

MultiDex is a technique used in Android development to allow an application to have more than the 65,536 method limit imposed by a single DEX file. Each DEX file encodes the app’s bytecode for the Dalvik/ART runtime, and exceeding the limit requires splitting code across multiple DEX files.

The method count limit arises from a 16-bit index used within a DEX file. As apps grow with libraries and frameworks, the total number of methods can surpass this threshold. MultiDex enables the APK to contain several DEX files (for example, classes.dex, classes2.dex, classes3.dex, etc.), allowing access to a much larger set of methods without refactoring.

Android platform support and configuration: On devices running Android 5.0 Lollipop (API 21) and later, the platform provides native support for multiple DEX files, so secondary dex files can be loaded automatically. For older devices, developers typically include the multidex support library and configure the app to load multiple DEX files. In Gradle, developers enable multiDexEnabled true in defaultConfig and add the appropriate multidex dependency. If the minimum SDK version is below 21, the application must initialize multidex support (for example, by using MultiDexApplication or by calling MultiDex.install(this) in the application class).

How it works in practice: During build, the code is partitioned into multiple DEX files. At runtime, the class loader can load classes from all included DEX files, with the primary dex loaded at startup and additional dex files loaded as needed.

Considerations: MultiDex can impact startup and loading times, so shrinking methods with tools like R8/ProGuard and reducing dependencies are common practices. Modern workflows and App Bundles can further optimize delivery by reducing unused methods and modules.