JUnit multiplied by Kotlin
JUnit is great for quickly writing and running tests as part of a TDD workflow, but try to do anything unusual and you have to reach for the documentation and specially written annotations.
Minutest extends JUnit with a simple model that allows you to solve your own problems with plain Kotlin.
For example
JUnit has a special annotation
@Test
@EnabledIfEnvironmentVariable(named = "ENV", matches = "staging-server")
fun onlyOnStagingServer() {
// ...
}Minutest is just Kotlin
if (getenv("ENV") == "staging-server" ) test("only on staging server") {
// ...
}JUnit has three annotations
@DisplayName("Fruit tests")
@ParameterizedTest(name = "{index} ==> fruit=''{0}'', rank={1}")
@CsvSource("apple, 1", "banana, 2", "'lemon, lime', 3")
fun testWithCustomDisplayNames(fruit: String, rank, String) {
// ...
}Minutest is just Kotlin
context("Fruit tests") {
listOf("apple" to 1, "banana" to 2, "lemon, lime" to 3).forEachIndexed { index, (fruit, rank) ->
test("$index ==> fruit='$fruit', rank=$rank") {
// ...
}
}
}JUnit needs more annotations
@DisplayName("A stack")
class TestingAStackDemo {
var stack: Stack<Any> = Stack()
@Nested
@DisplayName("when new")
inner class WhenNew {
@Test
fun `is empty`() {
assertTrue(stack.isEmpty())
}
}
@Nested
@DisplayName("after pushing an element")
inner class AfterPushing {
var anElement = "an element"
@BeforeEach
fun pushAnElement() {
stack.push(anElement)
}
@Test
fun `it is no longer empty`() {
assertFalse(stack.isEmpty())
}
}
}../core/src/test/kotlin/dev/minutest/examples/StackExampleTestsJUnit.kt
Minutest is just Kotlin
class StackExampleTests : JUnit5Minutests {
fun tests() = rootContext<Stack<Any>> {
given { Stack() }
context("when new") {
test("is empty") {
assertTrue(it.isEmpty())
}
}
context("after pushing an element") {
beforeEach {
it.push("an element")
}
test("it is no longer empty") {
assertFalse(it.isEmpty())
}
}
}
}../core/src/test/kotlin/dev/minutest/examples/StackExampleTests.kt
Minutest brings the power of Kotlin to JUnit, providing
- A clean DSL to define nested contexts and tests
- Generation and manipulation of tests at runtime
- Much easier reuse of test code
For more information on how why Minutest is like it is, see My New Test Model .
Here is a version of the JUnit 5 first test case, converted to Kotlin.
class MyFirstJUnitJupiterTests {
private val calculator = Calculator()
@Test
fun addition() {
calculator.add(2)
assertEquals(2, calculator.currentValue)
}
@Test
fun subtraction() {
calculator.subtract(2)
assertEquals(-2, calculator.currentValue)
}
}../core/src/test/kotlin/dev/minutest/examples/MyFirstJUnitJupiterTests.kt
In Minutest it looks like this
// Mix-in JUnit5Minutests to run Minutests with JUnit 5
//
// (JUnit 4 support is also available, see [JUnit4Minutests].)
class MyFirstMinutests : JUnit5Minutests {
// tests are grouped in a context
fun tests() = rootContext<Calculator> {
// We need to tell Minutest how to build the fixture
given { Calculator() }
// define a test with a test block
test("addition") {
// inside tests, the fixture is `it`
it.add(2)
assertEquals(2, it.currentValue)
}
// each new test gets its own new fixture
test("subtraction") { calculator ->
subtract(2)
assertEquals(-2, calculator.currentValue)
}
}
}../core/src/test/kotlin/dev/minutest/examples/MyFirstMinutests.kt
Most tests require access to some state. The collection of state required by the tests is called the test fixture. In JUnit we use the fields of the test class as the fixture - in this case just the calculator. JUnit uses a fresh instance of the test class for each test method run, which is why the state of calculator after addition does not affect the result of subtraction.
Minutest does not create a fresh instance of the test class for each test, instead it invokes a fixture block in a context and passes the result into tests as this.
Tests for cooperating components will typically have more state than just the thing we are testing. In this case make the fixture hold all the state.
class ControlPanel(
private val beep: () -> Unit,
private val launchRocket: () -> Unit
) {
private var keyTurned: Boolean = false
fun turnKey() {
keyTurned = true
}
fun pressButton() {
if (keyTurned)
launchRocket()
else
beep()
}
val warningLightOn get() = keyTurned
}
class CompoundFixtureExampleTests : JUnit5Minutests {
// The fixture consists of all the state affected by tests
class Fixture {
var beeped = false
var launched = false
val controlPanel = ControlPanel(
beep = { beeped = true },
launchRocket = { launched = true }
)
}
fun tests() = rootContext<Fixture> {
given { Fixture() }
context("key not turned") {
test("light is off") {
assertFalse(controlPanel.warningLightOn)
}
test("cannot launch when pressing button") {
controlPanel.pressButton()
assertTrue(beeped)
assertFalse(launched)
}
}
context("key turned") {
beforeEach {
controlPanel.turnKey()
}
test("light is on") {
assertTrue(controlPanel.warningLightOn)
}
test("launches when pressing button") {
controlPanel.pressButton()
assertFalse(beeped)
assertTrue(launched)
}
}
}
}../core/src/test/kotlin/dev/minutest/examples/fixtures/CompoundFixtureExampleTests.kt
Understanding fixtures is key to Minutest - read more
The key to Minutest is that by separating the fixture from the test code, both are made available to manipulate as data.
For example, parameterised tests require special handling in JUnit, but not in Minutest.
class ParameterisedExampleTests : JUnit5Minutests {
fun tests() = rootContext {
context("palindromes") {
// Creating a test for each of multiple parameters is as easy as
// calling `test()` for each one.
listOf("a", "oo", "racecar", "able was I ere I saw elba").forEach { candidate ->
test("$candidate is a palindrome") {
assertTrue(candidate.isPalindrome())
}
}
}
context("not palindromes") {
listOf("", "ab", "a man a plan a canal pananma").forEach { candidate ->
test("$candidate is not a palindrome") {
assertFalse(candidate.isPalindrome())
}
}
}
// Minutest will check that the following tests are run
willRun(
"▾ tests",
" ▾ palindromes",
" ✓ a is a palindrome",
" ✓ oo is a palindrome",
" ✓ racecar is a palindrome",
" ✓ able was I ere I saw elba is a palindrome",
" ▾ not palindromes",
" ✓ is not a palindrome",
" ✓ ab is not a palindrome",
" ✓ a man a plan a canal pananma is not a palindrome"
)
}
}
fun String.isPalindrome(): Boolean =
if (length == 0) false
else (0 until length / 2).find { index -> this[index] != this[length - index - 1] } == null../core/src/test/kotlin/dev/minutest/examples/ParameterisedExampleTests.kt
More complicated scenarios can be approached by writing your own function that returns a test or a context.
If you want to reuse the same tests for different concrete implementations, define a context with a function and call it for subclasses. Some people call this a contract.
// To run the same tests against different implementations, first define a ContextBuilder extension function
// that defines the tests you want run.
fun ContextBuilder<MutableCollection<String>>.behavesAsMutableCollection() {
context("behaves as MutableCollection") {
test("is empty when created") {
assertTrue(isEmpty())
}
test("can add") {
add("item")
assertEquals("item", first())
}
}
}
// Now tests can supply the fixture and invoke the function to create the tests to verify the contract.
class ArrayListTests : JUnit5Minutests {
fun tests() = rootContext<MutableCollection<String>> {
given {
ArrayList()
}
behavesAsMutableCollection()
}
}
// We can reuse the contract for different collections.
class LinkedListTests : JUnit5Minutests {
fun tests() = rootContext<MutableCollection<String>> {
given {
LinkedList()
}
behavesAsMutableCollection()
}
}../core/src/test/kotlin/dev/minutest/examples/ContractsExampleTests.kt
When your tests grow so that they need more structure, Minutest has extensions to support Given When Then blocks
class ControlPanel(
private val beep: () -> Unit,
private val launchRocket: () -> Unit
) {
private var keyTurned: Boolean = false
fun turnKey() {
keyTurned = true
}
fun pressButton(): Boolean =
when {
keyTurned -> {
launchRocket()
true
}
else -> {
beep()
false
}
}
val warningLightOn get() = keyTurned
}
class ScenariosExampleTests : JUnit5Minutests {
class Fixture {
var beeped = false
var launched = false
val controlPanel = ControlPanel(
beep = { beeped = true },
launchRocket = { launched = true }
)
}
fun tests() = rootContext<Fixture> {
// Scenario defines a nested context
Scenario("Cannot launch without key switch") {
// GivenFixture sets up the fixture for the scenario
GivenFixture("key not turned") {
Fixture()
}.Then("warning light is off") {
// Then can check the setup
assertFalse(controlPanel.warningLightOn)
}
// When performs the operation
When("pressing the button") {
controlPanel.pressButton()
}.Then("result was false") { result ->
// Then has access to the result
assertFalse(result)
}.And("it beeped") {
// And makes another Thens with checks
assertTrue(beeped)
}.And("rocket was not launched") {
assertFalse(launched)
}
}
Scenario("Will launch with key switch") {
GivenFixture("key turned") {
Fixture().apply {
controlPanel.turnKey()
}
}.Then("warning light is on") {
assertTrue(controlPanel.warningLightOn)
}
When("pressing the button") {
controlPanel.pressButton()
}.Then("result was true") { result ->
assertTrue(result)
}.And("it didn't beep") {
assertFalse(beeped)
}.And("rocket was launched") {
assertTrue(launched)
}
}
}
}../core/src/test/kotlin/dev/minutest/examples/scenarios/ScenariosExampleTests.kt
The Cookbook shows other ways to use Minutest.
We're pretty happy with the core Minutest language and expect not to make any breaking changes without a major version update. Features like JUnit 4 support and test annotations are public but experimental - if you use anything in an experimental package you should expect it to change between minor releases, and move completely once adopted into the stable core.
Note that we aim for source and not binary compatibility. Some implementation may move from methods to extension functions, or from constructors to top level or companion-object functions.
The best bet for feedback and help is the #minutest channel on the Kotlin Slack. See you there.
