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Building Custom Gradle Plugins for Android Projects

#kotlin #android #gradle #buildtools

Building Custom Gradle Plugins for Android Projects

Large Android repositories often contain dozens of modules with repeated Gradle configuration.

Custom Gradle convention plugins allow teams to centralize build rules and apply them consistently.

Instead of repeating configuration in every module, a feature module can use a simple plugin:

plugins {
    id("company.android.feature")
}
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Why Custom Gradle Plugins?

Without convention plugins, modules may repeatedly configure:

  • Android Gradle Plugin
  • Kotlin
  • Java compatibility
  • Compose
  • Testing
  • Lint
  • Publishing

Duplicated configuration becomes difficult to maintain.

Project Structure

A scalable repository can contain:

project/
├── app/
├── feature/
├── core/
└── build-logic/
    └── convention/
        ├── build.gradle.kts
        └── src/main/kotlin/
            ├── AndroidApplicationConventionPlugin.kt
            ├── AndroidLibraryConventionPlugin.kt
            └── AndroidFeatureConventionPlugin.kt
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Convention Plugin

A plugin is implemented in Kotlin:

class AndroidLibraryConventionPlugin :
    Plugin<Project> {

    override fun apply(target: Project) {
        // Shared Android library configuration
    }
}
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The plugin receives the target Gradle project and configures it.

Android Configuration

A convention plugin can configure common Android settings:

target.extensions.configure<LibraryExtension> {
    compileSdk = 36

    defaultConfig {
        minSdk = 24
    }
}
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Use APIs compatible with the Android Gradle Plugin version used by your project.

Kotlin Configuration

Shared Kotlin compiler configuration can also be centralized:

tasks.withType<KotlinCompile>().configureEach {
    compilerOptions {
        // Shared compiler configuration
    }
}
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This keeps modules consistent.

Plugin IDs

Useful convention plugin IDs include:

company.android.application
company.android.library
company.android.feature
company.android.compose
company.android.test
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Then a module remains simple:

plugins {
    id("company.android.feature")
}
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Feature Convention Plugin

A feature plugin can bundle common configuration:

class AndroidFeatureConventionPlugin :
    Plugin<Project> {

    override fun apply(target: Project) {
        // Android configuration
        // Kotlin configuration
        // Testing configuration
    }
}
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This is particularly useful in multimodule projects.

Version Catalogs

Version catalogs can centralize dependency versions:

[versions]
kotlin = "..."
androidGradlePlugin = "..."
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A useful architecture is:

Version Catalog
       |
Convention Plugins
       |
Android Modules
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Build Logic

Modern large repositories often keep custom build logic in an included build-logic build.

This keeps build tooling separate from application modules and makes conventions easier to organize.

Dependency Management

Convention plugins can add dependencies, but avoid hiding too much application configuration.

Developers should still be able to understand what a module depends on.

Testing Gradle Plugins

Build logic deserves tests.

Verify that:

  • Plugins apply successfully
  • Expected Android configuration exists
  • Tasks are configured
  • Dependencies are added correctly
  • Invalid configurations fail clearly

For complex plugins, Gradle TestKit can execute real Gradle builds against test projects.

Avoiding Giant Plugins

Do not create one plugin that configures everything.

Prefer focused conventions:

android.application
android.library
android.feature
android.compose
android.testing
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This keeps build behavior predictable.

Common Mistakes

Avoid:

  • Hardcoded module names
  • Hidden dependencies
  • One giant convention plugin
  • Duplicated version definitions
  • Project-specific assumptions
  • Untested build logic

Conclusion

Custom Gradle plugins are extremely useful for large Kotlin Android repositories.

The goal is not to hide the build system. The goal is to remove duplication and make engineering conventions reusable.

When combined with multimodule architecture and version catalogs, convention plugins provide a strong foundation for maintaining large Android codebases.

Useful Links

SDK Flutter: https://github.com/v-modal/vmodal_sdk_flutter

SDK Android: https://github.com/v-modal/vmodal_sdk_android

Discord: https://discord.gg/K72z28KUx

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