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testland/jvm-unit-tests

Use whenever the task is to write, add, fix or review JVM unit tests in Java, Kotlin, Scala or Groovy: JUnit 6 (Jupiter) by default and JUnit 5 upgrades, plus Kotest, Spock, ScalaTest and TestNG. Covers parameterized tests, lifecycle hooks, Mockito, AssertJ, JaCoCo coverage and Maven or Gradle CI.

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ScalaTest - Scala-native testing (reference)

Companion reference for jvm-unit-tests. Consult for Scala projects (Scala 2.13 or 3) or existing ScalaTest codebases. For Java/Kotlin projects, JUnit 5 (SKILL.md) or Kotest (kotest.md) are more idiomatic.

Per scalatest.org:

ScalaTest is the de facto Scala testing framework, with multiple specification styles and a Matchers DSL; ScalaCheck is its canonical property-based pairing.

Install

build.sbt:

libraryDependencies += "org.scalatest" %% "scalatest" % "3.2.19" % Test
libraryDependencies += "org.scalatestplus" %% "scalacheck-1-17" % "3.2.18.0" % Test

Test files live under src/test/scala/.

Specification styles

Per scalatest.org/user_guide/selecting_a_style - pick one style per project:

FlatSpec (BDD-style, the recommended default):

import org.scalatest.flatspec.AnyFlatSpec
import org.scalatest.matchers.should.Matchers

class CalculatorSpec extends AnyFlatSpec with Matchers {
  "Calculator" should "add two numbers" in {
    Calculator.add(1, 2) should equal(3)
  }

  it should "throw on overflow" in {
    an [ArithmeticException] should be thrownBy {
      Calculator.add(Int.MaxValue, 1)
    }
  }
}

FunSuite (xUnit-style): test("add two numbers") { assert(Calculator.add(1, 2) == 3) }. WordSpec (deeply-nested BDD): "A UserService" when { "creating a user" should { "set default role" in { … } } }.

Matchers DSL

Per scalatest.org/user_guide/using_matchers:

result should equal(42)
result shouldBe 42                        // strict equality (uses ==)
list should have size 5
list should contain("alice")
list should contain only("alice", "bob")
list should contain inOrder("alice", "bob")
map should contain key("alice")
string should startWith("hello")
string should fullyMatch regex("\\d+")
opt shouldBe defined
either shouldBe Right(42)
result should be > 10
result should be (within(1.0) of 42.0)    // float tolerance

Async tests

AsyncFlatSpec test bodies return Future[Assertion] - ScalaTest handles the async lifecycle:

class AsyncSpec extends AsyncFlatSpec with Matchers {
  "fetchUser" should "return user data" in {
    fetchUser(1) map { user => user.id shouldBe 1 }
  }
}

Sync test bodies for async code are a silent-pass trap - the Future never resolves inside the assertion.

ScalaCheck integration

import org.scalatestplus.scalacheck.ScalaCheckPropertyChecks
import org.scalacheck.Gen

class PropertyCheckSpec extends AnyFlatSpec with Matchers
                          with ScalaCheckPropertyChecks {
  "addition" should "be commutative" in {
    forAll { (a: Int, b: Int) => a + b shouldBe b + a }
  }
}

forAll shrinks to a minimal counterexample on failure (scalacheck.org). For the property-based discipline see the qa-property-based plugin.

Lifecycle hooks and fixtures

BeforeAndAfterAll (beforeAll / afterAll) + BeforeAndAfter (before { … } / after { … }), or the loan-fixture pattern:

def withDatabase(test: Database => Unit): Unit = {
  val db = createTestDb()
  try test(db)
  finally db.close()
}

"createUser" should "persist to db" in withDatabase { db => ... }

Tagged tests

object Slow extends Tag("Slow")

"slow operation" should "work" taggedAs Slow in { ... }

Selective run: sbt 'testOnly * -- -n Slow' (include) / -l Slow (exclude).

CI

- run: sbt clean coverage test coverageReport

Coverage via the sbt-scoverage plugin (Scala-native, not JaCoCo); cross-language projects need JaCoCo separately for the Java/Kotlin side.

Anti-patterns

Anti-pattern	Why it fails	Fix
Mix specification styles in one project	Reader confusion	Pick one
`assert(x == y)` instead of Matchers DSL	Loses diff on failure	`x should equal(y)`
Sync test bodies for async code	Future never resolves; false pass	`AsyncFlatSpec`