1. Introduction

In the world of Kotlin unit testing, JUnit 5 and Kotest are two popular testing frameworks that aid developers in writing reliable and efficient unit tests. This tutorial aims to compare and contrast JUnit 5 and Kotest, helping developers understand the strengths and weaknesses of each framework and choose the one that best fits their testing needs.

2. JUnit 5

JUnit 5 is the latest version of the widely-used JUnit testing framework for Java. It introduces several significant improvements over its predecessor, JUnit 4. JUnit 5 provides a modular architecture, allowing developers to use only the components they need.

2.1. Core Features

When reviewing a JUnit 5 test, certain features will readily stand out:

  • Annotations: JUnit 5 comes with a set of annotations like @Test, @BeforeEach, @AfterEach, @BeforeAll, and @AfterAll, which help in defining and managing test methods and lifecycle hooks.
  • Assertions: JUnit 5 provides a rich set of built-in assertions such as assertEquals()assertTrue()assertFalse(), and assertNotNull(), allowing us to verify expected outcomes with ease.
  • Extensions: The extension model in JUnit 5 enables us to add custom functionality to tests, such as parameter resolution, custom reporting, and test instance post-processing.

2.2. Typical Test Case

Let’s see what a typical JUnit 5 test looks like with these features put to use. Let’s start by preparing a class with lifecycle hooks, where we can set up and tear down what is needed for the actual tests:

class JUnit5SampleTest {

    // This method will run once before all test methods
    fun setUpClass() {
        // Add setup logic here
        println("Setting up test class")

    // This method will run before each test method
    fun setUp() {
        // Add setup logic here
        println("Setting up test")

    // This method will run after each test method
    fun tearDown() {
        // Add cleanup logic here
        println("Tearing down test")

    // This method will run once after all test methods
    fun tearDownClass() {
        // Add cleanup logic here
        println("Tearing down test class")

In this example, we have a test class named JUnit5SampleTest. It contains four test-related annotations:

  • @BeforeAll: A method that should be executed once before all test methods in the class.
  • @BeforeEach: A method that should be executed before each test method in the class
  • @AfterEach: A method that should be executed after each test method in the class
  • @AfterAll: A method that should be executed once after all test methods in the class.

A test won’t always require all these hooks to be set up; they’re not obligatory. It’s important to note that @BeforeAll and @AfterAll require either that the methods be in a companion object as static methods or that the class is annotated with @TestInstance(Lifecycle.PER_CLASS).

Let’s finalize our class with a test method that performs some assertions:

class JUnit5SampleTest {

    fun testExample() {
        val result = 2 + 2

        assertEquals(4, result, "2 + 2 should be equal to 4")
        assertTrue(result > 0, "Result should be positive")
        assertFalse(result < 0, "Result should not be negative")
        assertNotNull(result, "Result should not be null")

        println("Executing testExample()")

The test method named testExample() is marked with the JUnit 5 annotation @Test. We have used various assertion methods provided by JUnit 5, such as assertEquals(), assertTrue(), assertFalse(), and assertNotNull(). These assertions help verify expected outcomes during testing.

3. Kotest

Kotest is a powerful testing framework specifically designed for Kotlin. It leverages Kotlin’s language features and provides a concise and expressive syntax for writing tests.

3.1. Core Features

When examining a Kotest test, specific attributes will be promptly noticeable:

  • Kotlin Syntax: Kotest takes full advantage of Kotlin’s concise syntax, making test code more readable and expressive.
  • Fluent DSL: Kotest offers a fluent Domain-Specific Language (DSL) that simplifies test specification.
  • Asynchronous Testing: Kotest provides seamless support for asynchronous testing using Kotlin’s coroutines.

3.2. Typical Specification

To see what a usual Kotest test looks like, we’ll rewrite our JUnit 5 test from before with Kotest. Let’s start with a FunSpec with lifecycle hooks:

class KotestSampleTest : FunSpec({
    // This block runs once before all test methods
    beforeSpec {
        println("Setting up test class")

    // This block runs once after all test methods
    afterSpec {
        println("Tearing down test class")

    // This block runs before each test method
    beforeEach {
        println("Setting up test")

    // This block runs after each test method
    afterEach {
        println("Tearing down test")

In this Kotest FunSpec style, we use lambda expressions within the FunSpec class to define the test structure.

The beforeSpec(), afterSpec(), beforeEach(), and afterEach() blocks are equivalent to the @BeforeAll, @AfterAll, @BeforeEach, and @AfterEach annotations in JUnit 5, respectively.

Incorporating a test into this FunSpec is straightforward — all it requires is calling the test() function within our lambda expression:

class KotestSampleTest : FunSpec({

    // This is a test
    test("Test Example") {
        val result = 2 + 2 

        result shouldBe 4 
        result shouldBeGreaterThan 0 
        result shouldNotBeLessThan 0 
        result shouldNotBe null 

        println("Executing Test Example") 

The test method is defined using the test() function and the test assertions are performed using Kotest’s matchers: shouldBe(), shouldBeGreaterThan(), shouldNotBeLessThan(), and shouldNotBe(). These matchers provide more expressive and readable test assertions compared to the JUnit 5 assertions.

4. JUnit 5 vs. Kotest Comparison

It’s not feasible to make a blanket statement declaring one framework superior to the other. To make informed decisions, we must consistently evaluate the pros and cons of the tools at our disposal. Keeping this in perspective, let’s delve into how both frameworks address issues, enabling us to establish a robust comparison.

4.1. Syntax and Test Structure

JUnit 5 follows the traditional Java syntax for annotations and test structure, while Kotest takes advantage of Kotlin’s more concise and expressive syntax, taking a more function-oriented approach. This can lead to more readable and compact test code in Kotest.

4.2. Assertion Styles

While both frameworks offer comparable assertion functionalities, a distinction emerges in the syntax employed for these assertions.

Certain developers may be drawn to the traditional JUnit 5 approach, which employs expressions such as assertEquals(expected, actual, “Expected should be equal to Actual”). This pattern is generalized as assertXXX(expected, actual) and assertNotXXX(expected, actual), with an optional description message.

In contrast, the Kotlin-centric style of Kotest employs extension functions directly on objects for assertions, favoring infix functions where appropriate to provide sentence-like statements. This can be exemplified by constructs like actual shouldBe expected, which is generalized as actual shouldBeXXX expected, along with actual shouldNotBeXXX expected.

4.3. Asynchronous Testing

While JUnit 5 supports asynchronous testing through CompletableFuture and @Timeout, Kotest offers native support for asynchronous testing using Kotlin coroutines, making it more convenient for Kotlin projects.

4.4. Integration and Ecosystem

JUnit 5 has a long history and a mature ecosystem, with widespread integration in build tools and IDEs. On the other hand, Kotest is gaining popularity in the Kotlin community and will have a more tailored ecosystem for Kotlin projects.

5. Conclusion

To sum it up, JUnit 5 and Kotest are both solid testing frameworks, each with its own benefits. JUnit 5 is great for Java folks who like a stable and familiar setup, while Kotest shines with its Kotlin-powered DSL, making tests shorter and clearer.

When picking between JUnit 5 and Kotest, it’s important to consider our testing style, project ecosystem, and the pros and cons of the testing frameworks, such as Kotest’s native coroutine support versus JUnit’s vast plugin support.

Kotest’s fancy DSL and its native coroutine support for async testing are valuable assets in any project that is exclusively Kotlin. On the other hand, if we’re switching from Java or have a solid Java background, JUnit 5 is a familiar and proven choice.

At the end of the day, our choice should be based on our project’s specific needs and our team’s comfort zone. Both frameworks do a good job at unit testing. By understanding what they’re good at and where the gaps are, we can pick what’s right for our project and team. As always, the code used in this article is available over on GitHub.

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