Why Can’t a Kotlin Library Module Depend on an Android Library Module?

Are you a developer trying to create a Kotlin library module that depends on an Android library module? Have you encountered errors and warnings that leave you wondering why it can’t be done? Well, you’re not alone! In this article, we’ll dive into the reasons behind this limitation and provide a solution to help you overcome it.

The Problem

When you try to add a dependency on an Android library module in your Kotlin library module, Android Studio will throw an error. The error message might look something like this:

Error: Module 'your-kotlin-module' depends on 'your-android-module', which is not a jar

This error occurs because Kotlin library modules are designed to be pure Kotlin modules, whereas Android library modules contain Android-specific resources and configurations. As a result, Kotlin library modules can’t directly depend on Android library modules.

Why is This a Limitation?

The main reason behind this limitation is the fundamental difference between Kotlin library modules and Android library modules. Here are a few key differences:

• Kotlin library modules: These are pure Kotlin modules that contain only Kotlin code and dependencies. They can be used in any Kotlin-based project, including JVM, Android, and multiplatform projects.
• Android library modules: These are Android-specific modules that contain Android resources, such as layouts, drawables, and AndroidManifest.xml. They are designed to be used in Android projects only.

Because of these differences, Kotlin library modules can’t directly depend on Android library modules. If they could, it would introduce Android-specific dependencies into the Kotlin module, making it incompatible with non-Android projects.

Solution

So, how can you create a Kotlin library module that depends on an Android library module? Well, there’s a workaround! Here’s a step-by-step guide to help you achieve this:

Step 1: Create an Interface Module

Create a new Kotlin library module (let’s call it `interface-module`) that contains only the interface definitions. This module will act as an abstraction layer between your Kotlin library module and the Android library module.

// interface-module/build.gradle
apply plugin: 'kotlin'

dependencies {
  implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.5.31'
}

In this module, define the interfaces that you want to expose to your Kotlin library module. For example:

// interface-module/src/main/kotlin/MyInterface.kt
interface MyInterface {
  fun doSomething()
}

Step 2: Create an Android Implementation Module

Create a new Android library module (let’s call it `android-implementation-module`) that implements the interfaces defined in the interface module.

// android-implementation-module/build.gradle
apply plugin: 'com.android.library'
apply plugin: 'kotlin-android'

android {
  compileSdkVersion 30
  defaultConfig {
    minSdkVersion 21
    targetSdkVersion 30
  }
}

dependencies {
  implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.5.31'
  implementation project(':interface-module')
}

In this module, implement the interfaces using Android-specific resources and configurations. For example:

// android-implementation-module/src/main/kotlin/MyImplementation.kt
class MyImplementation : MyInterface {
  override fun doSomething() {
    // Android-specific implementation
  }
}

Step 3: Create a Kotlin Library Module that Depends on the Interface Module

Create a new Kotlin library module (let’s call it `kotlin-library-module`) that depends on the interface module.

// kotlin-library-module/build.gradle
apply plugin: 'kotlin'

dependencies {
  implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.5.31'
  implementation project(':interface-module')
}

In this module, use the interfaces defined in the interface module to interact with the Android implementation module. For example:

// kotlin-library-module/src/main/kotlin/MyKotlinClass.kt
class MyKotlinClass(private val myInterface: MyInterface) {
  fun doSomething() {
    myInterface.doSomething()
  }
}

Benefits of this Approach

This solution provides several benefits, including:

• Decoupling: The Kotlin library module is decoupled from the Android library module, making it reusable in non-Android projects.
• Abstraction: The interface module acts as an abstraction layer, allowing you to change the implementation without affecting the Kotlin library module.
• Modularity: This approach promotes modularity, making it easier to maintain and update individual modules without affecting other parts of the project.

Conclusion

In conclusion, while it’s not possible to directly depend on an Android library module from a Kotlin library module, there’s a clever workaround that involves creating an interface module and an Android implementation module. By following this approach, you can create a Kotlin library module that depends on the interface module, which in turn depends on the Android implementation module. This solution provides decoupling, abstraction, and modularity, making it a scalable and maintainable approach for your project.

Module	Description	Dependencies
Interface Module	Contains interface definitions	None
Android Implementation Module	Implements interfaces using Android-specific resources	Interface Module
Kotlin Library Module	Depends on the interface module	Interface Module

By following this guide, you’ll be able to create a Kotlin library module that depends on an Android library module, while maintaining a decoupled and modular architecture.

FAQs

Q: Why can’t I just use an Android library module instead of a Kotlin library module?

A: While you could use an Android library module, it would limit the reusability of your code in non-Android projects. By using a Kotlin library module, you can create a module that can be used in a variety of projects, including JVM, Android, and multiplatform projects.

Q: How do I handle Android-specific resources in my Kotlin library module?

A: You shouldn’t handle Android-specific resources in your Kotlin library module. Instead, create an Android implementation module that handles these resources and provides an implementation of the interfaces defined in the interface module.

Q: Can I use this approach for other types of modules?

A: Yes, this approach can be applied to other types of modules, such as Java library modules or multiplatform modules. The key is to decouple the modules using an interface module and an implementation module, allowing for greater flexibility and reusability.

Frequently Asked Question

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