The best way to extend a functionality of a Gradle task is by adding an input or an output. Sadly, sometimes it is not a task you can modify easily, and thus you are stuck dealing with what you’ve got.

Let’s say I’m writing a JUnit test in a Gradle JVM project and I want my test to store some additional data (e.g. a diff image between a reference image and a rendered screenshot). Extending org.gradle.api.tasks.testing.Test can be non-trivial, especially when you are dealing with the Kotlin Gradle Plugin or the Android Gradle Plugin. So what can we do?

Naive approach would be to pass a directory path to the task via environment variable.

tasks.named<Test>("test").configure {
    val testOutput = layout.buildDirectory.dir("testOutput")
    environment("TEST_OUTPUT" to testOutput.get().asFile.absolutePath)
}

then in your test class you do the following

val testOutput = File(System.getenv("TEST_OUTPUT"))
File(testOutput, "myFile.txt").writeText("Hello world!")

If you run this test, you will get build/testOutput/myFile.txt created as expected. Sadly, we have a few issues at hand. First, if build/testOutput gets deleted, and you run the test task again, the directory will not be recreated. That is because we haven’t told Gradle this directory is an output for this task.

tasks.named<Test>("test").configure {
    val testOutput = layout.buildDirectory.dir("testOutput")
    outputs.dir(testOutput)
    environment("TEST_OUTPUT" to testOutput.get().asFile.absolutePath)
}

After this change, if build/testOutput directory is deleted, then Test task will rerun and the directory will be recreated! Yay!

You might think we are done, but sadly, there is another issue if you have a remote build cache enabled (and you should!) you will likely not cache hits. This will happen because you set TEST_OUTPUT environment variable to an absolute path that will likely be different on different machines. Luckily there is a cheeky fix!

tasks.named<Test>("test").configure {
    val testOutput = layout.buildDirectory.dir("testOutput")
    outputs.dir(testOutput)
    doFirst {
        environment("TEST_OUTPUT" to testOutput.get().asFile.absolutePath)
    }
}

What we did here is to move environment variable setting to after the task inputs are captured as doFirst actions runs during task execution phase. This means that Gradle will not see TEST_OUTPUT and the absolute path as an input at all, but your test will see it. This is a somewhat hacky and I’ve been told by Gradle developers that they will likely close this door down eventually by locking task inputs after configuration is complete, but it still works, so…

Hopefully, this post gives you ideas of how you can add outputs to Test and other tasks. You can also do similar things with Task.inputs, but if you do, remember to also normalize your input like

inputs.dir(testInput).withPathSensitivity(PathSensitivity.RELATIVE)

Happy tinkering!