# Migrating to Gradle Kotlin DSL - Groovy Closure

While migrating the real-life project’s Gradle build from Groovy to Kotlin, I collected some useful recommendations, code snippets, and explanations. Throughout the post series, we will together learn how to convert to Gradle Kotlin DSL faster and easier.

New to Gradle Kotlin DSL? Take a look at the first post for practical recommendations on migrating from Groovy to Kotlin build scripts. In the second post, we cover Kotlin tasks setup on Gradle Kotlin DSL scripts. Few more findings of mine towards buildSrc, plugins and extensions are presented in the third post.

In that post, I’ll share my findings for the migrating a Groovy Closure to Gradle Kotlin DSL. In that post you’ll learn how to step-by-step migrate a Gradle script with Groovy closure usages into Kotlin DSL.

## Dependencies

I’ve seen many times in different Gradle projects an attempt to list all dependencies in some file in the root project and fix versions of all used libraries in one place so that only a library name is used in a dependency and its version written only once for the whole project. It helps one to avoid an unexpected library version clashes in a multi-project Gradle project. For example, how many different versions of say OkHttp or Jackson do you have in your project? Of course, another version of the same dependency may come from transitive dependencies, it is another story, we will not cover it in that blog post.

I’ve implemented a similar pattern to list all dependencies in the root Gradle project. Let’s see how I migrated my Groovy solution to Kotlin.

## Groovy Script Dependencies

Let’s assume we need to add a dependency to OkHttp library in a sub-project. We’d like to use common library definition for all sub-projects. I found it elegant to create a Groovy closure in the root project for such a definition:

ext {
dependency_okhttp = { Project project ->
project.dependencies {
compile 'com.squareup.okhttp3:okhttp:3.12.1'
}
}
}


Do not ask me, why do I have the Project parameter in that function/closure. Technically, it is not needed, but I was lazy to remove it from the code. Now it is time to finally do that (and convert to Kotlin DSL)!

Meanwhile, the usage in Groovy was easy and strait forward:

ext.dependency_okhttp(project)


To follow the one-by-one migration strategy we need the way to call the same code from Kotlin DSL. I found the following code for it:

(project.extensions.extraProperties["dependency_okhttp"] as Closure<*>).call(project)


The code is indeed problematic. When I migrate the dependency_okhttp property definition to Kotlin, I will have to fix every usage of it and replace the cast to Closure<*> with a cast to, say, Function1<Project,*> or something different

## Moving ext to buildSrc and Kotlin

The better place for all extensions in Kotlin (also in Groovy) is buildSrc. We’ve covered it in the previous post. Let’s move the definition of the dependency_okhttp function into Kotlin code and place the code under the buildSrc folder. For that, I created a Kotlin file and added the following function there:

package x.y.z
fun DependencyHandlerScope.dependency_okhttp() {
"implementation"("com.squareup.okhttp3:okhttp:3.12.1")
}


Now we can use the function in Gradle/Kotlin directly in the dependencies{..} block:

import x.y.z.*
dependencies {
dependency_okhttp()
}


The usage is now clean and short, which is great. The only price for that is the import statement. We need to import the package, where our declaration is to all our build.gradle.kts project files. I wish it were possible to tell Gradle to implicitly import several more packages into Kotlin DSL script execution context.

The DependencyHandlerScope type is the receiver type of the lambda behind the dependencies{} function. There is yet another issue — we cannot use implementation inside dependencies{} block in the buildSrc code. Instead, we may to use "implementation" string. I’m looking for the answer to that. It seems the implementation function is generated on the fly by Gradle’s Kotlin scripts runtime, and it is not included into the buildSrc evaluation environment.

## Calling Kotlin buildSrc from Groovy

To start with, do not forget to use the new operator to create objects in Groovy. It is so easy to forget after dealing with Kotlin. I paid about a dozen minutes debugging that.

By that moment, we have all the extension function DependencyHandlerScope.dependency_okhttp declared in Kotlin DSL under the buildSrc. Let’s see how to call the function from build.gradle files and Groovy. In other words, it is the challenge to re-implement the older Groovy ext closures in all original gradle scripts.

Our goal is to call the DependencyHandlerScope.dependency_okhttp function from Groovy. We only need an instance of the DependencyHandlerScope, which is only used on Kotlin DSL, to make the call to our Kotlin extension function. The DependencyHandlerScope class has the of factory function in its companion object, but it is not a static function, it misses the @JvmStatic annotation!

## Kotlin to JVM Interop

Global functions (such as dependency_okhttp) are compiled by the Kotlin/JVM compiler to a FilenameKt class static member functions. We need to import static that class when calling Kotlin functions from Groovy or Java. The short solution, but it took several dozen minutes for me to figure out.

To simplify, we may just add a global import static <package>.<KotlinFileName>Kt.*; to our Groovy scripts and then call all our Kotlin global functions without a qualifier. That also applies to the dependency_okhttp() function that we have under buildSrc.

One more thing is that the dependency_okhttp() function is an extension function. How can we call it from Groovy?

Kotlin extension functions have the receiver parameter as the very first parameter at the JVM bytecode level. We may call extension functions from Groovy or Java as an ordinary method passing the receiver object instance as the first parameter.

Accessing functions from object or companion object is in general a bit more tricky. Use the following syntax to access Kotlin declarations from Groovy:

• @JvmStatic annotation is present — Call as a static function
• Kotlin object — use TypeName.INSTANCE.functionName
• Kotlin companion object — use [email protected]

You may find more details in the JVM bytecode for Kotlin Object and Companion Object declarations post. Frankly, I was happy to find the possibility to write @Companion in Groovy.

## Calling Kotlin from Groovy

The dependency_okhttp function is declared in the dependencies.kt file under the buildSrc project in my codebase. It is visible to Groovy as a member of the DependenciesKt class. We use the [email protected] trick to access the .of function of the DependencyHandlerScope class from Gradle. We have the following Groovy code to call our extension function declared in Kotlin from Groovy:

import static x.y.z.DependenciesKt.*
ext {
dependency_okhttp = { Project project ->
dependency_okhttp(DependencyHandlerScope.@Companion.of(project.dependencies))
}
}


That code is stable, and it is unlikely to break if we change something code under buildSrc. The Kotlin version is easy to use from Kotlin DSL, and the Groovy version is only needed for the time of the migration from Gradle to Kotlin. We’ve covered the migration plan in the very first post of this series.

## Conclusion

In the post we’ve seen how to deal with Groovy closures on Kotlin and how to move from ext properties to Kotlin extension functions declared in buildSrc.

Kotlin as a statically typed programming language seems to play well with writing Gradle build scripts. Thanks to the static type inference, the Kotlin compiler detects errors earlier and shows helpful compilation error messages and warnings. Both the IDE and the compiler use information about types to infer the available functions and properties in a given scope, even inside a 5th level nested lambda with receivers.

I will cover more aspects in the coming posts, stay tuned! Check out the

• first post — First steps of the migration
• third post — a buildSrc project with Kotlin, ad-hoc plugins and extensions