This lesson covers testing with Specs, a Behavior-Driven Design (BDD) Framework for Scala.

extends Specification

Let’s just jump in.

import org.specs._

object ArithmeticSpec extends Specification {
  "Arithmetic" should {
    "add two numbers" in {
      1 + 1 mustEqual 2
    "add three numbers" in {
      1 + 1 + 1 mustEqual 3

Arithmetic is the System Under Specification

add is a context.

add two numbers and add three numbers are examples.

mustEqual indicates an expectation

1 mustEqual 1 is a common placeholder expectation before you start writing real tests. All examples should have at least one expectation.


Notice how two tests both have add in their name? We can get rid of that by nesting expectations.

import org.specs._

object ArithmeticSpec extends Specification {
  "Arithmetic" should {
    "add" in {
      "two numbers" in {
        1 + 1 mustEqual 2
      "three numbers" in {
        1 + 1 + 1 mustEqual 3

Execution Model

object ExecSpec extends Specification {
  "Mutations are isolated" should {
    var x = 0
    "x equals 1 if we set it." in {
      x = 1
      x mustEqual 1
    "x is the default value if we don't change it" in {
      x mustEqual 0

Setup, Teardown

doBefore & doAfter

"my system" should {
  doBefore { resetTheSystem() /** user-defined reset function */ }
  "mess up the system" in {...}
  "and again" in {...}
  doAfter { cleanThingsUp() }

NOTE doBefore/doAfter are only run on leaf examples.

doFirst & doLast

doFirst/doLast is for single-time setup. (need example, I don’t use this)

"Foo" should {
  doFirst { openTheCurtains() }
  "test stateless methods" in {...}
  "test other stateless methods" in {...}
  doLast { closeTheCurtains() }


You have data, you want to make sure it’s right. Let’s tour the most commonly used matchers. (See Also Matchers Guide)


We’ve seen several examples of mustEqual already.

1 mustEqual 1

"a" mustEqual "a"

Reference equality, value equality.

elements in a Sequence

val numbers = List(1, 2, 3)

numbers must contain(1)
numbers must not contain(4)

numbers must containAll(List(1, 2, 3))
numbers must containInOrder(List(1, 2, 3))

List(1, List(2, 3, List(4)), 5) must haveTheSameElementsAs(List(5, List(List(4), 2, 3), 1))

Items in a Map

map must haveKey(k)
map must notHaveKey(k)

map must haveValue(v)
map must notHaveValue(v)


a must beGreaterThan(b)
a must beGreaterThanOrEqualTo(b)

a must beLessThan(b)
a must beLessThanOrEqualTo(b)

a must beCloseTo(b, delta)


a must beNone

a must beSome[Type]

a must beSomething

a must beSome(value)


a must throwA[WhateverException]

This is shorter than a try catch with a fail in the body.

You can also expect a specific message

a must throwA(WhateverException("message"))

You can also match on the exception:

a must throwA(new Exception) like {
  case Exception(m) => m.startsWith("bad")

Write your own Matchers

import org.specs.matcher.Matcher

As a val

"A matcher" should {
  "be created as a val" in {
    val beEven = new Matcher[Int] {
      def apply(n: => Int) = {
        (n % 2 == 0, "%d is even".format(n), "%d is odd".format(n))
    2 must beEven

The contract is to return a tuple containing whether the expectation is true, and a message for when it is and isn’t true.

As a case class

case class beEven(b: Int) extends Matcher[Int]() {
  def apply(n: => Int) =  (n % 2 == 0, "%d is even".format(n), "%d is odd".format(n))

Using a case class makes it more shareable.


import org.specs.Specification
import org.specs.mock.Mockito

class Foo[T] {
  def get(i: Int): T

object MockExampleSpec extends Specification with Mockito {
  val m = mock[Foo[String]]

  m.get(0) returns "one"


  there was one(m).get(0)

  there was no(m).get(1)

See Also Using Mockito


Spies can also be used in order to do some “partial mocking” of real objects:

val list = new LinkedList[String]
val spiedList = spy(list)

// methods can be stubbed on a spy
spiedList.size returns 100

// other methods can also be used

// and verification can happen on a spy
there was one(spiedList).add("one")

However, working with spies can be tricky:

// if the list is empty, this will throws an IndexOutOfBoundsException
spiedList.get(0) returns "one"

doReturn must be used in that case:


Run individual specs in sbt

> test-only com.twitter.yourservice.UserSpec

Will run just that spec.

> ~ test-only com.twitter.yourservice.UserSpec

Will run that test in a loop, with each file modification triggering a test run.