Damien Le Berrigaud's Blog

Testing Kotlin with a custom DSL for Aspen

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Where that feature came from

When I started working on Aspen, I was focusing on only a few things: I want an easy way to regroup my tests for a given function, and I want to be able to name my tests with a simple sentence but I don’t want to have to name my tests. That lead to version 1.0.

Then came Spring integration, version 1.1. Then we started talking to more people to see what they wanted, and it became quickly obvious that a very simple DSL was not going to be enough for some. So I started working on another DSL that would allow nesting of the tree structure, like rSpec does. This lead to version 1.2.

During the creation of that second DSL, I tried to figure out what was the right abstraction for it. It became obvious that we are building a tree structure and running that. So now I’m going to show you how to create your own DSL that builds a tree structure for Aspen.

Building a TestTree

The goal of the DSL is to build a TestTree that can then be run by the TestTreeRunner from Aspen. For example, with the built-in DSL, the following code…

class MyTests: Test({

    describe("foo") {
        test("a") {


        test("b") {


    describe("bar") {
        test("c") {


…would build the following structure.

Test Tree Structure

The TestTree Interface

An Aspen DSL implements the TestTree Interface.

interface TestTree {
    fun readTestBody()
    fun getRoot(): TestBranch

The readTestBody function is the one that is supposed to evaluate the body of the tests and build the tree structure, thus making the root available. It is invoked at a particular time in the SpringTestTreeRunner for instance, the default TestTreeRunner invokes it as soon as it needs to browse the tree.

The getRoot function is more obvious, it returns the first branch of your Tree.

A TestBranch has children and optional before and after blocks. Its children can be of type TestBranch or TestLeaf. A TestLeaf represents an actual test, it has a block of code to execute and can have a focused flag.

All the code for the TestTree is in a single file: https://github.com/dam5s/aspen/blob/master/libraries/aspen/src/main/kotlin/io/damo/aspen/TestTree.kt

The Test class

Let’s start with the easy part, implementing the TestTree interface:

open class Test : TestTree {
    private val root = TestBranch.createRoot()
    private val body: Test.() -> Unit

    constructor(body: Test.() -> Unit) {
        this.body = body

    override fun getRoot() = root

    override fun readTestBody() {

This is an open class that is run with the TestTreeRunner and implements TestTree. The root is created with TestBranch.createRoot, creating a branch without parent, without name… The body is an extension function for the class itself, this is the block of code that will contain your tests and build the TestTree structure, it is evaluated in the readTestBody function.

Now you can start adding some functions that will help you build the tree.

This will allow specifying the before and after block:

fun before(block: () -> Unit) {
    root.before = block

fun after(block: () -> Unit) {
    root.after = block

This will add a test at the root level:

fun test(name: Any = "unnamed test", block: () -> Unit) {
    root.addChildLeaf(name.toString(), block)

Now in order to add branches that do not allow nesting, we have to create a function that will take an extension function for a different type. If it was for the same type, then we would be able to call that same function over and over.

fun describe(name: Any, block: TestDescription.() -> Unit) {
    val newBranch = root.addChildBranch(name.toString())

The TestDescription only allows adding tests:

class TestDescription(private val branch: TestBranch) {
    fun test(name: String = "unnamed test", block: () -> Unit) {
        branch.addChildLeaf(name, block)

And that’s it! From that point you can start making your DSL even smarter.

For instance, I have been working on making a DSL for nicer table based tests. You can see its first implementation on Github. It would allow for writing a test like this:

class MathTest : Test({

    class MaxData(context: String, val a: Int, val b: Int, val c: Int)
        : TestData(context)

    val testData = listOf(
        MaxData("when equal", 2, 2, 2),
        MaxData("when a is greater", 3, 1, 3),
        MaxData("when b is greater", 3, 4, 4)

    tableTest(testData) {
        test {
            assertThat(Math.max(a, b), equalTo(c))

But it is still a work in progress, as I still need to figure out exactly how I want my test to look like (that test block does not look useful for instance).

I hope this convinced you that it’s very simple to write your own type safe DSL for testing with Aspen. Let me know what you think about it!