Testing with Scala

After working with Scala for a while now, I thought it would be good to write down a couple of notes on my current testing setup, in particular with regards to which libraries I've settled on and which style of testing I've ended up using.

Tests end up in the same package as the code that's tested. A group of tests are always in a class with the Tests suffix, e.g. FooTests. If it's about a particular class Foo the same applies.

scalatest is used as the testing framework, using AnyWordSpec, that means we're using the should / in pattern.

For mocking the only addition is MockitoSugar to make things more Scala-ish.

How does it look like?

package com.example.foo

import org.mockito.MockitoSugar
import org.scalatest.wordspec.AnyWordSpec

class FooTests extends AnyWordSpec with MockitoSugar {
  "Foo" should {
    "do something" in {
      val bar = mock[Bar]
      val foo = new Foo(bar)
      foo.baz(42L)
      verify(bar).qux(42L)
    }
  }
}

Easy enough. There's also some more syntactic sugar for other Mockito features, meaning ArgumentMatchersSugar should also be imported when needed. Same as scalatest has a number of additional helpers for particular types like Option or Either, e.g. OptionValues and EitherValues.

class BarTests extends AnyWordSpec with Matchers with EitherValues with OptionValues {
  "Bar" should {
    "do something else" in {
      val bar = new Bar
      bar.qux(42L).left.value should be(empty)
      bar.quux().value shouldBe "a value"
    }
  }
}

This can be done to the extreme, but usually it looks easier to me to simply assign highly nested values to a variable and continue with matchers on that variable instead.

Since sbt is often used, the two test dependencies would look like this:

libraryDependencies ++= Seq(
  "org.scalatest" %% "scalatest"               % "3.1.1"  % Test,
  "org.mockito"   %% "mockito-scala-scalatest" % "1.13.0" % Test,
)