Writing Tests With Kotlin Test

Writing Tests With kotlin.test — One Assertion API Across Every Target

kotlin.test is Kotlin's own thin test library: framework-agnostic assert* functions plus @Test/@BeforeTest/@AfterTest annotations that typealias onto whatever runner is on the classpath (JUnit4, JUnit5, TestNG, the JS/Native runners). Write import kotlin.test.* once and the same test source compiles in commonTest, on the JVM, and on every Kotlin Multiplatform target. This skill covers the API surface, the @OnlyInputTypes compile-time guard, the Asserter extension point, and the Gradle wiring. It is the assertion layer; the runner layer for Android instrumentation is ../../../jvm-tests/runner/configuring-junit4-on-android/SKILL.md.

When to use this skill

• The module is Kotlin Multiplatform and tests live in commonTest — JUnit's org.junit.* is not available there; kotlin.test is.
• The user wants one assertion vocabulary that reads the same on JVM, Android, JS, and Native.
• The user reaches for assertFailsWith<IllegalStateException> { … }, assertContentEquals(...), assertIs<Foo>(value), assertContains(list, x), expect(3) { compute() }, or @BeforeTest/@AfterTest.
• A JVM module has testImplementation(kotlin("test")) and the test "runs on JUnit but I never added JUnit" — explaining the capability-based resolution.
• kotlin.test.assertEquals failures render as a plain AssertionError with no diff, and the user wants the JUnit-style comparison failure back.

When NOT to use this skill

• The user wants rich fluent assertions (assertThat(x).isEqualTo(...), soft assertions, collection matchers) — that is AssertJ / Google Truth / Kotest assertions, deliberately out of kotlin.test's minimal scope.
• The user is writing an Android instrumentation test and asking about the runner (@RunWith(AndroidJUnit4::class), AndroidJUnitRunner, androidx.test:*) — use ../../../jvm-tests/runner/configuring-junit4-on-android/SKILL.md and ../../../instrumentation/runner/running-instrumented-tests-with-androidjunit4/SKILL.md. (You can still use kotlin.test assertions inside those tests.)
• The user is testing coroutines / Flow — runTest, TestScope, Dispatchers.setMain is ../../../jvm-tests/coroutines/testing-coroutines-with-runtest/SKILL.md; Turbine is ../../../jvm-tests/coroutines/testing-flows-with-turbine/SKILL.md. (kotlin.test supplies the assert* calls inside those.)
• The user wants Compose UI assertions (assertIsDisplayed, assertTextEquals) — that is the compose/assertions/ skills, unrelated to kotlin.test.

Prerequisites

• Gradle: for a JVM/Android module, testImplementation(kotlin("test")). For a Kotlin Multiplatform module, kotlin.test is added to the commonTest source set: sourceSets { commonTest.dependencies { implementation(kotlin("test")) } }.
• The kotlin("test") shorthand is a capability-based dependency (Kotlin 1.4+). On the JVM it resolves to the JUnit4 variant (kotlin-test-junit) by default; if the Test task is configured with useJUnitPlatform(), it resolves to the JUnit5 variant (kotlin-test-junit5) instead. You can also depend on a specific adapter explicitly: org.jetbrains.kotlin:kotlin-test-junit, :kotlin-test-junit5, :kotlin-test-testng, :kotlin-test-js.
• A test runner on the classpath for the chosen target — kotlin.test's @Test is only an annotation; something has to execute it (JUnit, TestNG, the Kotlin/JS or Kotlin/Native test infra). The adapter artifact pulls it in.

Workflow

• 1. Add kotlin("test") to the test source set, not a hardcoded JUnit dependency. In a multiplatform module put it in commonTest; in a JVM/Android module use testImplementation(kotlin("test")). Do not also add junit:junit or org.junit.jupiter:* directly unless you have framework-specific needs — the adapter brings the right one.

• 2. Import the framework-agnostic symbols. import kotlin.test.* (or the specific ones). The annotations come from kotlin.test, not org.junit:

import kotlin.test.Test
import kotlin.test.BeforeTest
import kotlin.test.AfterTest
import kotlin.test.Ignore
import kotlin.test.assertEquals

class CartTest {
    private lateinit var cart: Cart

    @BeforeTest fun setUp() { cart = Cart() }      // == @Before (JUnit4) / @BeforeEach (JUnit5)
    @AfterTest  fun tearDown() { cart.clear() }    // == @After  (JUnit4) / @AfterEach  (JUnit5)

    @Test fun emptyCartTotalsZero() {
        assertEquals(Money.ZERO, cart.total())
    }

    @Ignore @Test fun flaky() { /* skipped — == @Disabled on JUnit5 */ }
}

On JVM-JUnit4, kotlin.test.Test is typealias'd to org.junit.Test, BeforeTest to org.junit.Before, AfterTest to org.junit.After, Ignore to org.junit.Ignore. On JUnit5 they map to org.junit.jupiter.api.Test/BeforeEach/AfterEach/Disabled. So the same source runs on either engine, and in commonTest.

• 3. Pick the precise assertion. kotlin.test is small on purpose — there is exactly one obvious call per intent:

• 4. Assert on a thrown exception with assertFailsWith, not a try/catch. It returns the exception so you can assert its message/cause:

@Test fun withdrawOverdraws() {
    val e = assertFailsWith<InsufficientFundsException> { account.withdraw(BigDecimal(100)) }
    assertEquals("balance is 20", e.message)
}

• 5. Lean on @OnlyInputTypes — type mismatches are compile errors, not runtime failures. assertEquals, assertContentEquals, assertContains, assertSame, expect are declared with <@OnlyInputTypes T>, so assertEquals(1, "1") does not compile. Don't widen to Any to "make it pass" — fix the test.

• 6. Restore JUnit-style diffs on the JVM by keeping the JUnit adapter on the classpath. kotlin.test resolves an Asserter at runtime via ServiceLoader<AsserterContributor>. The kotlin-test-junit artifact registers a contributor that, when org.junit.Assert is present, returns a JUnitAsserter delegating assertEquals to org.junit.Assert.assertEquals — which throws ComparisonFailure, giving the clickable expected/actual diff in IDEs. With no contributor, DefaultAsserter throws a plain AssertionError. So kotlin.test failures look like JUnit failures automatically as long as the adapter is there (it is, via kotlin("test")). Only implement a custom Asserter + register an AsserterContributor (JVM: META-INF/services/kotlin.test.AsserterContributor) if you need a non-JUnit reporting backend.

Patterns

Pattern: importing org.junit.* annotations in a multiplatform-shared test

// WRONG — in src/commonTest, or in src/test that you later want to share
import org.junit.Test            // unresolved in commonTest; couples JVM source to JUnit4
import org.junit.Assert.assertEquals
class FooTest {
    @Test fun bar() { assertEquals(2, foo()) }
}
// WRONG because: org.junit is a JVM-JUnit4 dependency. It does not exist in commonMain/commonTest,
// it pins you to JUnit4 (not 5), and it cannot run on Kotlin/JS or Kotlin/Native.

// RIGHT — kotlin.test only; runs everywhere, on whatever engine the build selects
import kotlin.test.Test
import kotlin.test.assertEquals
class FooTest {
    @Test fun bar() { assertEquals(2, foo()) }
}

Pattern: hand-rolled exception checking

// WRONG
@Test fun parseRejectsGarbage() {
    try {
        parse("garbage")
        fail("expected ParseException")
    } catch (e: ParseException) {
        // ok — but verbose, and a different exception type slips through as a test error not a failure
    }
}
// WRONG because: assertFailsWith already does exactly this, returns the exception for assertions,
// and reports a clear "expected ParseException but was X" message.

// RIGHT
@Test fun parseRejectsGarbage() {
    val e = assertFailsWith<ParseException> { parse("garbage") }
    assertEquals(0, e.offset)
}

Pattern: assertTrue(a == b) instead of assertEquals

// WRONG
assertTrue(result == expected)
// WRONG because: on failure the message is just "Expected value to be true." — no expected/actual.
// assertEquals reports both values (and a diff via the JUnit asserter).

// RIGHT
assertEquals(expected, result)
// reserve assertTrue for genuine boolean predicates: assertTrue(result.isValid, "result was rejected: ${result.errors}")

Pattern: comparing list content

// WRONG — assertEquals on lists works, but assertEquals on the wrong shape is silent type widening
assertEquals(expected as List<Any>, actual)         // never widen to Any to dodge a type error

// RIGHT — assertContentEquals is the explicit "same elements, same order" check; @OnlyInputTypes keeps it honest
assertContentEquals(listOf(1, 2, 3), service.ids())
// for a Set (order-insensitive) use assertEquals: assertEquals(setOf("a", "b"), service.tags())

Mandatory rules

• MUST depend on kotlin("test") (in commonTest for multiplatform, testImplementation(kotlin("test")) for JVM/Android) rather than importing org.junit.* directly, so the test source is engine-agnostic and multiplatform-portable.
• MUST import annotations from kotlin.test (@Test, @BeforeTest, @AfterTest, @Ignore), not org.junit / org.junit.jupiter.api.
• MUST use assertEquals/assertNotEquals/assertContentEquals/assertIs/assertContains for their specific intents instead of collapsing everything into assertTrue(...) — the specific calls produce expected/actual diagnostics.
• MUST check thrown exceptions with assertFailsWith<E> { … } (returning the exception for message/cause assertions), not try/catch + fail.
• MUST NOT widen argument types (as Any, as List<Any>) to silence an @OnlyInputTypes compile error — that error is catching a real mismatch.
• MUST NOT call assertContentEquals on a Set (deprecated, ambiguous); use assertEquals for unordered set equality.
• MUST NOT drop the kotlin-test-junit/kotlin-test-junit5 adapter on the JVM — without an AsserterContributor on the classpath, failures degrade to bare AssertionError with no diff.
• PREFERRED: assertNotNull(x) / assertIs<T>(x) over x!! / x as T in tests — they fail with a clear message instead of an NPE/ClassCastException and return the narrowed value.
• PREFERRED: the assertTrue(message) { expensiveCondition() } lazy-block overload when computing the condition or message is costly.

Verification

• No import org.junit.* or import org.junit.jupiter.api.* in source sets meant to be multiplatform/shared; grep -rn 'import org.junit' src/commonTest src/test is empty (or justified per-platform only).
• ./gradlew :module:dependencies --configuration testRuntimeClasspath (JVM) shows kotlin-test-junit or kotlin-test-junit5 resolved via the kotlin("test") capability.
• Test classes use @Test/@BeforeTest/@AfterTest from kotlin.test.
• Exception cases use assertFailsWith<…> { … } rather than try/catch.
• No as Any / as List<Any> widening was added to make an assertion compile.
• A deliberately failing assertEquals in a JVM module renders as a JUnit ComparisonFailure (clickable diff) — proof the AsserterContributor is on the classpath.

References

• kotlinlang.org/api/latest/kotlin.test/ — the kotlin.test API reference: every assert* function, expect, fail, the annotations, Asserter/AsserterContributor/DefaultAsserter.
• kotlinlang.org/docs/jvm-test-using-junit.html and kotlinlang.org/docs/multiplatform-run-tests.html — using kotlin("test") on the JVM (JUnit4/JUnit5 selection) and across multiplatform targets.
• libraries/kotlin.test/annotations-common/src/main/kotlin/kotlin.test/Annotations.kt (Kotlin repo) — expect annotation class Test/Ignore/BeforeTest/AfterTest; the JVM actual typealiases in libraries/kotlin.test/junit/src/main/kotlin/Annotations.kt (→ org.junit.*) and libraries/kotlin.test/junit5/src/main/kotlin/Annotations.kt (→ org.junit.jupiter.api.*).
• libraries/kotlin.test/common/src/main/kotlin/kotlin/test/Assertions.kt (Kotlin repo) — assertTrue/assertFalse/assertEquals/assertNotEquals/assertSame/assertNotSame/assertNull/assertNotNull/assertIs/assertIsNot/assertContains/assertContentEquals/fail/expect/assertFails/assertFailsWith signatures, the @OnlyInputTypes annotations, and the Asserter / AsserterContributor interfaces.
• libraries/kotlin.test/jvm/src/main/kotlin/AsserterLookup.kt and libraries/kotlin.test/junit/src/main/kotlin/JUnitSupport.kt (Kotlin repo) — ServiceLoader<AsserterContributor> lookup; JUnitContributor/JUnitAsserter delegating to org.junit.Assert when present, else DefaultAsserter.
• Cross-set: ../../../jvm-tests/runner/configuring-junit4-on-android/SKILL.md — the JUnit4 runner / AndroidJUnit4 stack; kotlin.test assertions slot into those tests unchanged.
• Cross-set: ../../../jvm-tests/coroutines/testing-coroutines-with-runtest/SKILL.md — runTest is the surrounding scope; kotlin.test supplies the assert* calls inside it.
• Cross-set: ../../../jvm-tests/coroutines/testing-flows-with-turbine/SKILL.md — Turbine's awaitItem() paired with kotlin.test's assertEquals.
• Cross-set: ../../../fundamentals/strategies/organizing-test-source-sets/SKILL.md — where these tests live (src/test/, src/commonTest/) and the testImplementation vs androidTestImplementation split.
• Cross-set: ../../../fundamentals/doubles/picking-test-doubles/SKILL.md — fakes/stubs the assertions are checking.