java-build-optimization

name	java-build-optimization
description	Expert guidance on optimizing a Java Android Gradle build — configuration cache, R8 optimization, resource shrinking, parallel execution, and remote build cache. Use this when build times regress or APK/AAB size bloats.

Java Android Build Optimization

Instructions

Slow builds compound: every developer pays the cost every day. Make optimization continuous, not a one-time sprint.

1. Gradle JVM Settings

gradle.properties:

org.gradle.jvmargs=-Xmx4g -XX:+UseG1GC -XX:MaxMetaspaceSize=1g -Dfile.encoding=UTF-8
org.gradle.parallel=true
org.gradle.caching=true
org.gradle.configureondemand=true
org.gradle.configuration-cache=true
org.gradle.configuration-cache.problems=warn

android.useAndroidX=true
android.nonTransitiveRClass=true
android.nonFinalResIds=true

• parallel executes modules concurrently.
• caching enables the local build cache.
• configuration-cache stores the configured task graph — huge wins on incremental builds.
• nonTransitiveRClass shrinks each module's R.java and speeds up compilation.

2. R8 (Code Shrinking)

Enable for release only — debug builds must stay fast.

android {
    buildTypes {
        release {
            minifyEnabled true
            shrinkResources true
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'),
                          'proguard-rules.pro'
        }
    }
}

• proguard-android-optimize.txt enables full optimization passes (not just obfuscation).
• Add -keep rules for reflection users (Gson, Moshi, Room schemas referenced by migrations).
• Verify release builds in CI by running instrumentation tests against assembleReleaseAndroidTest so R8-stripped reflection bugs surface before Play.

Example proguard-rules.pro:

# Keep data classes read by Gson
-keep class com.example.app.data.dto.** { *; }

# Keep Retrofit service interfaces and parameter annotations
-keep,allowobfuscation interface retrofit2.** { *; }
-keepattributes Signature, *Annotation*, RuntimeVisibleAnnotations

# Room
-keep class * extends androidx.room.RoomDatabase { *; }
-keep @androidx.room.Entity class *

3. Resource Shrinking

shrinkResources true removes resources that R8-stripped code no longer references. Write a res/raw/keep.xml if you load resources by name:

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools"
    tools:keep="@drawable/dynamic_ic_*"
    tools:discard="@layout/debug_only_overlay" />

Enable android.enableResourceOptimizations=true (default on AGP 8) to dedupe strings across configurations.

4. Split APKs / App Bundles

Ship .aab, not .apk:

android {
    bundle {
        language { enableSplit = true }
        density  { enableSplit = true }
        abi      { enableSplit = true }
    }
}

Typical savings: 30–60% reduction in on-device size.

5. Dependency Hygiene

• Run ./gradlew :app:dependencies on every merge; reject new dependencies over 200kb without justification.
• Prefer the -android variant of Guava (com.google.guava:guava:X.Y.Z-android).
• Remove implementation dependencies that are no longer used — ruler (Spotify) reports the top contributors to APK size.
• Turn on android.defaults.buildfeatures.buildconfig=false globally and re-enable per module only where needed.

6. Incremental Annotation Processing

Most common processors (Hilt, Room, Glide) are incremental by default. Verify:

./gradlew assembleDebug --info | grep "non-incremental"

If you see non-incremental warnings, upgrade the processor or replace it. Prefer KSP-compatible alternatives for processors that offer it (some Room tooling).

7. Remote Build Cache

For teams > 3 developers, stand up a gradle-build-cache-node (Gradle Enterprise) or a simple HTTP cache:

// settings.gradle
buildCache {
    local { enabled = true }
    remote(HttpBuildCache) {
        url = uri("https://cache.example.com/cache/")
        push = System.getenv('CI') == 'true'
    }
}

Only CI pushes — developer machines read.

8. Measure

• ./gradlew assembleDebug --scan to generate a build scan URL (gradle.com).
• --profile produces a local HTML report under build/reports/profile/.
• Track clean + assemble and incremental assemble times per release in CI.

Checklist

• gradle.properties has parallel, caching, and configuration-cache enabled.
• Release builds enable minifyEnabled and shrinkResources.
• proguard-rules.pro keeps only what reflection needs; nothing is over-kept.
• App is shipped as .aab with language/density/abi splits.
• android.nonTransitiveRClass=true is set and each module's R is shrunk.
• No annotation processor reports "non-incremental".
• A build scan is generated in CI and used to diagnose regressions.