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Testing Gradle precompiled script plugins

In the previous article I discussed how to write and publish Gradle. Today, I want to show you how to write integration tests for them.

Gradle and Kotlin
© JetBrains Blog

As before I will be using my gradle-kotlin-setup-plugin Gradle plugin, which you can use as a reference.

Let’s start be setting up the source directory for our tests.

gradle-kotlin-setup-plugin
    src/test/kotlin
        com/driver733
            GradleKotlinSetupPluginTest.kts

Next, we’ll need to set up a test class and a test method. I am using the KoTest testing framework, but any other framework will work fine too.

class GradleKotlinSetupPluginTest : FunSpec() {
    init {
        test("build") {
            //
        }
    }   
}

An integration test for a Gradle plugin consist of the following parts:

  1. Create a temporary Gradle project folder with the setting.gradle.kts and build.gradle.kts files in it, which describe a project which uses your plugin.
  2. Run a Gradle command using the GradleRunner, specifying the project directory from the first step.
  3. Verify the Gradle command result, output and anything else you expect to happen.

Let’s go through the first step:

We start by creating a temporary directory by using the createTempDir() standard library method. Then we put the settings.gradle.kts file inside this directory, which declares out project. Lastly, we add the build.gradle.kts file and add our plugin to the Plugins DSL block. If your plugin relies on third-party dependencies, you should also add a repository, which provides these dependencies.

Here’s the code for the first step of the test:

val projectDir = createTempDir().apply {
        resolve("setting.gradle.kts").apply {
            appendText("rootProject.name = \"gradle-kotlin-setup-plugin-test\"")
        }
        resolve("build.gradle.kts").apply {
            appendText(
                """
                    plugins {
                        id("com.driver733.gradle-kotlin-setup-plugin")
                    }
                    repositories {
                        mavenCentral()
                    }
            """
            )
        }
    }

The GradleRunner allows to run a Gradle build using just a few lines of code:

val buildResult = GradleRunner.create()
    .withProjectDir(projectDir)
    .withArguments("build")
    .withPluginClasspath()
    .build()

The build result allows us to check the outcome of each task that was executed:

buildResult.task("build")?.outcome shouldBe TaskOutcome.SUCCESS

As well as the output of the build:

output.contains("BUILD SUCCESSFUL")

Below is the resulting code for our test:

class GradleKotlinSetupPluginTest : FunSpec() {
    val projectDir = createTempDir().apply {
        resolve("setting.gradle.kts").apply {
            appendText("rootProject.name = \"gradle-kotlin-setup-plugin-test\"")
        }
        resolve("build.gradle.kts").apply {
            appendText(
                """
                    plugins {
                        id("com.driver733.gradle-kotlin-setup-plugin")
                    }
                    repositories {
                        mavenCentral()
                    }
            """
            )
        }
    }

    init {
        test("build") {
            val actual = GradleRunner.create()
                .withProjectDir(projectDir)
                .withArguments("build")
                .withPluginClasspath()
                .build()
            
            actual.output shouldContainIgnoringCase "kotlin"
            actual.task(":build")?.outcome shouldBe TaskOutcome.SUCCESS
            output.contains("BUILD SUCCESSFUL")
        }
    }   
}

In case you need more examples, feel free to use the tests for my gradle-kotlin-setup-plugin Gradle plugin.