Embracing AGP 9 and JDK 21: A Gradual Path to Android Build Optimization

As Android developers, major AGP (Android Gradle Plugin) updates are always significant. AGP 9 in particular promises a stricter, faster build environment, moving away from ambiguous configurations.

Instead of waiting for its full release and then scrambling to fix issues, why not start preparing your project now, gradually aligning it with "AGP 9 standards" through your gradle.properties file?

🤔 Why JDK 21 and AGP 9 Now? (The Ultimate Synergy)

When transitioning to AGP 9, updating to JDK 21 isn't just a "requirement"; it's a powerful "booster" that dramatically enhances your development experience.

Performance Synchronization: JDK 21's improved resource management, including features like Virtual Threads, allows Gradle to fully leverage its parallel build capabilities, leading to more stable and efficient builds.
Language Specification Alignment: By targeting Java 21, you bridge potential gaps in type inference and bytecode generation in mixed Java/Kotlin projects, especially as Kotlin 2.x gains traction.
Precision R8 Optimization: AGP 9 is optimized to parse and transform class files generated by JDK 21. This means that even with stricter settings, R8 can more accurately understand modern code structures, reducing the need for excessive keep rules while safely shrinking code.

This combination offers the kind of seamless experience you get from pairing the latest OS with the latest CPU.

🤔 Prepare with `gradle.properties`: 10 Flags to Enable Today

The strategy is simple: enable one flag at a time, fix any errors that arise, and then move to the next. This iterative approach is the most reliable way to prepare for AGP 9.

1. Structure Enforcement (Clean Up Your Project)

android.uniquePackageNames=true: Prevents duplicate package names across modules, eliminating resource conflicts.
android.usesSdkInManifest.disallowed=true: Enforces placing minSdk, targetSdk, etc., in build.gradle instead of AndroidManifest.xml.
android.defaults.buildfeatures.resvalues=true: Explicitly controls the generation of resValue entries.

2. Build Speed Enhancements

android.enableAppCompileTimeRClass=true: Uses lightweight R classes during app compilation, significantly improving build times for large projects.
android.sdk.defaultTargetSdkToCompileSdkIfUnset=true: Automatically sets targetSdk to compileSdk if unspecified, preventing inconsistent behavior.
android.dependency.useConstraints=true: Utilizes Gradle's "Constraints" feature for dependency resolution, making library version management more robust.

3. Aggressive R8 / Optimization Settings (The Biggest Hurdle)

android.r8.strictFullModeForKeepRules=true: Enables R8's Full Mode. This maximizes optimization but requires precise keep rules for code that relies on reflection, potentially leading to crashes if not handled correctly.
android.r8.optimizedResourceShrinking=true: Employs a more advanced algorithm for removing unused resources, leading to smaller app sizes.

4. Next-Gen Defaults

android.builtInKotlin=true: Prioritizes AGP's built-in Kotlin support.
android.newDsl=false: Use this to maintain the current DSL while preparing for future changes.

🤔 Conclusion: One Flag at a Time for a Smoother Future

The AGP 9 update is akin to a major cleanup. Attempting it all at once can be overwhelming, but tackling it gradually makes it incredibly rewarding.

Why not start with android.uniquePackageNames=true? With each flag you enable, your project will move closer to a more modern, robust, and efficient build environment.

👉 AGP 9.0 移行ガイド：新旧コード比較で見るモダンビルド設定