assertk - assertions for kotlin inspired by assertj - Kotlin Resources


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assertions for kotlin inspired by assertj



repositories {

dependencies {
  testCompile 'com.willowtreeapps.assertk:assertk-jvm:0.21'


Replace dependency on assertk-jvm with assertk-js or assertk to use it in JavaScript and common projects, respectively.


Simple usage is to wrap the value or property you are testing in assertThat() and call assertion methods on the result.

import assertk.assertThat
import assertk.assertions.*

class PersonTest {
    val person = Person(name = "Bob", age = 18)

    fun testName() {
        // -> expected:<["Alice"]> but was:<["Bob"]>

    fun testAge() {
        assertThat(person.age, "age").isGreaterThan(20)
        // -> expected [age] to be greater than:<20> but was:<18>

    fun testNameProperty() {
        // -> expected [name]:<["Alice"]> but was:<["Bob"]>

You can see all built-in assertions in the docs.


Since null is a first-class concept in kotlin’s type system, you need to be explicit in your assertions.

val nullString: String? = null

will not compile, since hasLength() only makes sense on non-null values. You can chain isNotNull() to handle this.

val nullString: String? = null
// -> expected to not be null

This will first ensure the string is not null before running any other checks.

Multiple assertions

You can assert multiple things on a single value by providing a lambda as the second argument. All assertions will be run even if the first one fails.

val string = "Test"
assertThat(string).all {
// -> The following 2 assertions failed:
//    - expected to start with:<"L"> but was:<"Test">
//    - expected to have length:<3> but was:<"Test"> (4)

You can wrap multiple assertions in an assertAll to ensure all of them get run, not just the first one.

assertAll {
// -> The following 2 assertions failed:
//    - expected to be true
//    - expected to be false


If you expect an exception to be thrown, you can use the version of assertThat that takes a lambda.

assertThat {
    throw Exception("error")
// -> expected [message] to be:<["wrong"]> but was:<["error"]>

This method also allows you to assert on successfully returned values.

assertThat { 1 + 1 }.isSuccess().isNegative()
// -> expected to be negative but was:<2>

Table Assertions

If you have multiple sets of values you want to test with, you can create a table assertion.

tableOf("a", "b", "result")
    .row(0, 0, 1)
    .row(1, 2, 4)
    .forAll { a, b, result ->
        assertThat(a + b).isEqualTo(result)
// -> the following 2 assertions failed:
//    on row:(a=<0>,b=<0>,result=<1>)
//    - expected:<[1]> but was:<[0]>
//    on row:(a=<1>,b=<2>,result=<4>)
//    - expected:<[4]> but was:<[3]>

Up to 4 columns are supported.

Custom Assertions

One of the goals of this library is to make custom assertions easy to make. All assertions are just extension methods.

fun Assert<Person>.hasAge(expected: Int) {
    prop("age", Person::age).isEqualTo(expected)

// -> expected [age]:<1[0]> but was:<1[8]> (Person(age=18))

For completely custom assertions, you can access the actual value with given and fail with expected() and show().

fun Assert<Person>.hasAge(expected: Int) = given { actual ->
    if (actual.age == expected) return
    expected("age:${show(expected)} but was age:${show(actual.age)}")

// -> expected age:<10> but was age:<18>

Contributing to assertk

Contributions are more than welcome! Please see the Contributing Guidelines and be mindful of our Code of Conduct.

Known Issues

  1. You get java.lang.AssertionError: java.lang.NoClassDefFoundError: org/opentest4j/AssertionFailedError when running a failing test from intellij.

    I’ve filed a bug about this, it works correctly when running on the cmdline with gradle. To workaround, you can explicilty add opentest4j as a dependency.

    testComple 'org.opentest4j:opentest4j:1.1.1'
  2. Gradle fails to find the correct variant if the kapt plugin is applied:

        > Could not resolve com.willowtreeapps.assertk:assertk-jvm:0.19.
          Required by:
              project :core-test
           > Cannot choose between the following variants of com.willowtreeapps.assertk:assertk-jvm:0.19:
               - jvm-api
               - jvm-runtime
               - metadata-api

    This is a known issue with the kapt plugin, you can add the below to your gradle file to work around it

     configurations.all { configuration ->
         // Workaround for kapt bug with MPP dependencies
         if (name.contains('kapt')) {
             attributes.attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage.class, Usage.JAVA_RUNTIME))
compile "com.willowtreeapps.assertk:assertk:0.21"

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