Mocking a gradle project in MockK doesn't call the stubbed method

Question

I am writing a unit test in kotlin using JUnit and MockK to test my custom Gradle task.

open class CopySourceTask @Inject constructor(
    private val config: MyCustomExtension
) : DefaultTask() {

    @TaskAction
    override fun run() {
        if (!config.enableCopyConfig) {
            return
        }

        val sourceSetContainer =
            project.extensions.getByType(SourceSetContainer::class.java)

        project.copy {
            // Copy project source
            it.from(sourceSetContainer.getByName(SourceSet.MAIN_SOURCE_SET_NAME))
            it.into("${project.buildDir}/private/src")
        }
    }
}

Here’s my unit test:

@ExtendWith(MockKExtension::class)
class CopySourceTaskTest {
    @MockK
    lateinit var customExtension: MyCustomExtension

    @MockK
    lateinit var dependencyEnabled: Property<Boolean>

    @MockK
    lateinit var sourceSetContainer: SourceSetContainer

    @BeforeEach
    fun setup() {
        every { customExtension.enableCopyConfig } returns dependencyEnabled
    }

    @Test
    fun testCopy() {
        val project = spyk(ProjectBuilder.builder().build())
        every { dependencyEnabled.get() } returns true
        every { project.extensions.getByType(SourceSetContainer::class.java) } returns sourceSetContainer
        val gradleTask = project.tasks.register(
            "CopySourceTask",
            CopySourceTask::class.java,
            customExtension
        )
       
        gradleTask.get().run()

       // Verify statements here
    }
}

The unit test is erroring due to this error: org.gradle.api.UnknownDomainObjectException: Extension of type ‘SourceSetContainer’ does not exist. Currently registered extension types: [ExtraPropertiesExtension].

Looks like the every { project.extensions.getByType(…) statement isn’t honored and it tries to load the SourceSetContainer from the project instead of the mocked statement. Please let me know what I’m missing.

Answer 1

Disclaimer: Unverified code. This code is meant to convey the idea, not be copy pasted blindly.

Since your Project is an actual instance, not a mock, the call to getByType cannot be mocked away. You need to spy on the extensions object:

val extensionsSpy = spyk(project.extensions)
project.extensions = extensionsSpy

If you can do that, it will work. If you cannot replace the extensions instance, you need to 'break in' with the crow bar method

val field = project::class.declaredFields.filter { it.name == "extensions" }.firstOrNull()!!
field.isAccessible = true
field.set(project, extensionsSpy)

Be aware that if you do not replace extensions in your project instance, internal calls from project to its own attribute will never be hit by the mocking framework. The same is true for calls to own methods. So you can only ever capture calls on the boundary between objects.