Gradle Build Optimization: Where Android Build Time Actually Goes

# A Build Scan: a web report of where the time went
./gradlew assembleDebug --scan

# A local profile report
./gradlew assembleDebug --profile

# The comparison that tells you whether incremental
# builds are working at all
./gradlew clean assembleDebug   # everything
./gradlew assembleDebug         # nothing changed

# gradle.properties

# Off by default: run tasks from different projects in parallel
org.gradle.parallel=true

# Off by default: reuse task outputs for the same inputs
org.gradle.caching=true

# Opt-in until Gradle 10
org.gradle.configuration-cache=true

# Heap for the Gradle daemon. Raise it only if the Build
# Analyzer shows garbage collection above 15% of build time.
org.gradle.jvmargs=-Xmx4g

# Not needed, do not add:
#   org.gradle.daemon=true              (already the default)
#   android.nonTransitiveRClass=true    (default since AGP 8.0)
#   org.gradle.configuration-cache.problems=warn
#       (a migration aid, not a setting to keep)

// settings.gradle.kts
dependencyResolutionManagement {
    repositories {
        google()        // serves most Android dependencies: first
        mavenCentral()
    }
}

// build.gradle.kts of a module that uses Room
dependencies {
    implementation(libs.room.runtime)
    ksp(libs.room.compiler)   // not kapt(...)
}

// core/model/build.gradle.kts
plugins {
    id("com.android.library")
}

dependencies {
    // api: Instant appears in this module's public signatures,
    // so consumers need it on their compile classpath
    api(libs.kotlinx.datetime)

    // implementation: invisible to consumers. A change here
    // does not recompile the modules that depend on this one.
    implementation(libs.kotlinx.serialization.json)
}

// feature/tasks/build.gradle.kts
dependencies {
    implementation(project(":core:model"))
    implementation(project(":data:tasks"))
}

# .github/workflows/android-ci.yml
jobs:
  build:
    runs-on: ubuntu-latest
    timeout-minutes: 15
    steps:
      - uses: actions/checkout@v7

      - uses: actions/setup-java@v6
        with:
          distribution: zulu
          java-version: 17

      # Restores the Gradle caches. Writes them back only
      # from the default branch.
      - uses: gradle/actions/setup-gradle@v6

      - name: Build and test
        run: ./gradlew assembleDebug testDebugUnitTest --no-daemon

      - name: Upload build reports
        if: failure()
        uses: actions/upload-artifact@v7
        with:
          name: build-reports
          path: "**/build/reports/"