Code Shrinking with R8

Android

DEX bytecode not optimized by R8. Enabling code shrinking, tree shaking, and obfuscation typically reduces app size by 10-65%.

R8 is Android's default code optimizer and shrinker, integrated into the Android Gradle Plugin. It operates directly on DEX bytecode in a single compilation step, performing four core optimizations: tree shaking (removing unreachable code), code optimization (inlining methods, merging classes, removing dead branches), obfuscation (shortening class/field/method names to reduce string table size), and desugaring. R8 full mode (default since AGP 8.0) applies more aggressive optimizations: default constructors are not implicitly kept, access modification is enabled by default, and annotations are only preserved for items matched by explicit keep rules.

Why this happens

Third-party library bloat: Libraries like Google Play Services, Jetpack, or Gson include thousands of classes, but apps typically use only a fraction of them. Without R8, all library code ships in the APK.
Unused code paths from feature flags, dead code from previous iterations, and debug-only utilities inflate the DEX file.
Verbose class/method names (e.g., com.example.myapp.feature.analytics.AnalyticsEventTracker) consume significant space in the DEX string table.
Auto-generated code from annotation processors, data binding, and protocol buffer generators creates substantial boilerplate that may be partially unused.
Multi-DEX overhead: Without shrinking, apps easily exceed the 64K method limit, requiring multiple DEX files with duplicated metadata.

Size impact

R8 is often the single highest-impact optimization for Android apps. Full mode delivers even greater savings due to aggressive class merging and inlining.

Reddit (R8 full mode)

14% reduction in app size

LinkedIn (R8 + AGP 4.1)

10.5 MB total decrease

Google IOSched sample

65% DEX size reduction

R8 vs ProGuard

~10% compaction vs 8.5%

How we detect it

Uses APK Analyzer to inspect the classes.dex file size and method count. Checks whether R8 outputs (mapping.txt, usage.txt) indicate code was removed. Compares referenced vs. defined methods to gauge shrinking effectiveness.

How to fix

Enable R8 in release builds

Set isMinifyEnabled and isShrinkResources to true in your release build type. Use the default proguard-android-optimize.txt rules file.

build.gradle.kts (app module)

android {
    buildTypes {
        release {
            isMinifyEnabled = true
            isShrinkResources = true
            proguardFiles(
                getDefaultProguardFile("proguard-android-optimize.txt"),
                "proguard-rules.pro"
            )
        }
    }
}

Ensure R8 full mode is active

R8 full mode is default since AGP 8.0. Verify it is not explicitly disabled in gradle.properties.

Add keep rules for reflection-dependent code

R8 cannot detect code accessed via reflection, Class.forName(), or JNI. Add explicit keep rules for data models used with Gson/Moshi, JNI-called methods, and @Keep-annotated classes.

proguard-rules.pro

# Keep classes used via reflection (e.g., Gson data models)
-keepclassmembers class com.example.myapp.model.** {
    <fields>;
}

# Keep JNI-called methods
-keepclasseswithmembers class * {
    native <methods>;
}

# Aggressive optimization
-repackageclasses
-allowaccessmodification

Enable optimized resource shrinking (AGP 8.12.0+)

Combines code and resource analysis in a single R8 pass for better removal of unused resources.

gradle.properties

android.r8.optimizedResourceShrinking=true

Important: Reflection crashes are the #1 pitfall. Missing keep rules cause ClassNotFoundException or NoSuchMethodException at runtime. Always test release builds on physical devices. R8 full mode does not implicitly keep default constructors, so code using getDeclaredConstructor().newInstance() will fail without explicit keep rules.

References

Enable app optimization with R8

Add keep rules

Use R8 in full mode

Troubleshoot the optimization

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