Mockito for Scala language
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Mockito Scala


The most popular mocking framework for Java, now in Scala!!!

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Why separate project?

The library has independent developers, release cycle and versioning from core mockito library ( This is intentional because core Mockito developers don't use Scala and cannot confidently review PRs, and set the vision for the Scala library.


  • Artifact identifier: "org.mockito:mockito-scala_2.11:VERSION"
  • Artifact identifier: "org.mockito:mockito-scala_2.12:VERSION"
  • Latest version - see release notes
  • Repositories: Maven Central or JFrog's Bintray

Please ensure mockito-scala is your only mockito dependency

Note: For more examples and use cases than the ones shown below, please refer to the library's tests

Migration Notes for version 1.0.0

  • DefaultAnswer was moved from org.mockito.DefaultAnswer to org.mockito.stubbing.DefaultAnswer
  • The recommended way to use the pre-defined DefaultAnswers is via the object org.mockito.DefaultAnswers
  • * matcher is now defined in org.mockito.ArgumentMatchersSugar, mixin (or use the companion object) this trait whenever you wanna use it
  • argumentCaptor[String] was removed, replace by ArgCaptor[T] (Captor[T] was renamed to ArgCaptor[T] to add clarity), ValCaptor[T] was deprecated, (see Improved ArgumentCaptor)
  • The usage of org.mockito.Answer[T] was removed from the API in favour of Function Answers
  • If you were using something like doAnswer(_ => <something>).when ... to lazily compute a return value when the method is actually called you should now write it like doAnswer(<something>).when ..., no need of passing a function as that argument is by-name
  • If you have chained return values like when( thenReturn "a" thenReturn "b" etc... the syntax has changed a bit to when( thenReturn "a" andThen "b" etc...
  • Idiomatic syntax has some changes to remove postFix operations and also allow support for mixing values and argument matchers Mix-and-Match shouldCallRealMethod                              => shouldCall realMethod

aMock wasCalled on bar                                      => was called
aMock wasCalled onlyOn bar                                  => wasCalled onlyHere
aMock was never called on bar                               => wasNever called
aMock wasCalled twiceOn bar                                 => wasCalled twice
aMock wasCalled sixTimesOn bar                              => wasCalled sixTimes
aMock was never called                                      => wasNever called
aMock was never called again                                => wasNever calledAgain

"mocked!" willBe returned by aMock bar                      => "mocked!" willBe returned by
"mocked!" willBe answered by aMock bar                      => "mocked!" willBe answered by
((i: Int) => i * 10) willBe answered by aMock bar *         => ((i: Int) => i * 10) willBe answered by*)
theRealMethod willBe called by aMock bar                    => theRealMethod willBe called by
new IllegalArgumentException willBe thrown by aMock bar     => new IllegalArgumentException willBe thrown by
  • eqToVal matcher syntax was improved to look more natural Value Class Matchers NOTE: eqToVal has been deprecated in v 1.0.2 as eqTo is now aware of value classes
verify(myObj).myMethod(eqToVal[MyValueClass](456))    => verify(myObj).myMethod(eqToVal(MyValueClass(456)))
myObj.myMethod(eqToVal[MyValueClass](456)) was called => myObj.myMethod(eqToVal(MyValueClass(456))) was called

Getting started


For a more detailed explanation read this

This trait wraps the API available on org.mockito.Mockito from the Java version, but it provides a more Scala-like syntax, mainly

  • Fixes the compiler errors that sometimes occurred when using overloaded methods that use varargs like doReturn
  • Eliminates the need to use classOf[T]
  • Eliminates parenthesis when possible to make the test code more readable
  • Adds spyLambda[T] to allow spying lambdas (they don't work with the standard spy as they are created as final classes by the compiler)
  • Supports mocking inline mixins like mock[MyClass with MyTrait]
  • Supports by-name arguments in some scenarios
    • Full support when all arguments in a method are by-name
    • Full support when only some arguments in a method are by-name, but we use the any[T] matcher for every argument
    • Full support when only some arguments in a method are by-name, but we use NO matchers at all
    • Partial support when only some arguments in a method are by-name and we use specific matchers, in this scenario the stubbing will only work if the by-name arguments are the last ones in the method signature
  • Adds support for working with default arguments

The companion object also extends the trait to allow the usage of the API without mixing-in the trait in case that's desired


For a more detailed explanation read this

This trait exposes all the existent org.mockito.ArgumentMatchers but again it gives them a more Scala-like syntax, mainly

  • eq was renamed to eqTo to avoid clashing with the Scala eq operator for identity equality, eq also supports value classes out of the box and relies on org.scalactic.Equality[T] (see Scalactic integration)
  • any[T] works even when the type can't be inferred, removing the need of using the likes of anyString, anyInt, etc (see Notes)
  • any[T] also supports value classes (in this case you MUST provide the type parameter)
  • isNull and isNotNull are deprecated as using nulls in Scala is clear code smell
  • Adds support for value classes via anyVal[T] and eqToVal[T]() NOTE: both had been deprecated (use any[T] or eqTo[T] instead)
  • Adds function0 to easily match for a function that returns a given value
  • Adds argMatching that takes a partial function to match, i.e. argMatching({ case Baz(_, "pepe") => })

Again, the companion object also extends the trait to allow the usage of the API without mixing-in the trait in case that's desired

Value Class Matchers

eqTo and any[T] support value classes since v1.0.2, so no special syntax is needed for them (but you MUST provide the type param for any[T] otherwise you'll get a NPE)

Improved ArgumentCaptor

For a more detailed explanation read this

A new set of classes were added to make it easier, cleaner and more elegant to work with ArgumentCaptors, they also add support to capture value classes without any annoying syntax

There is a new object org.mockito.captor.ArgCaptor[T] that exposes a nicer API


val aMock  = mock[Foo]
val captor = argumentCaptor[String]

aMock.stringArgument("it worked!")


captor.getValue shouldBe "it worked!"


val aMock  = mock[Foo]
val captor = ArgCaptor[String]

aMock.stringArgument("it worked!")


captor hasCaptured "it worked!"

As you can see there is no need to call capture() nor getValue anymore (although they're still there if you need them)

There is another constructor ValCaptor[T] that should be used to capture value classes NOTE: Since version 1.0.2 ValCaptor[T] has been deprecated as ArgCaptor[T] now support both, standard and value classes

Both ArgCaptor[T] and ValCaptor[T] return an instance of Captor[T] so the API is the same for both


This is a wrapper around org.mockito.MockitoSession, it's main purpose (on top of having a Scala API) is to improve the search of mis-used mocks and unexpected invocations to reduce debugging effort when something doesn't work

To use it just wrap your code with it, e.g.

MockitoScalaSession().run {
    val foo = mock[Foo]
    when("pepe")) thenReturn "mocked""pepe") shouldBe "mocked"

That's it! that block of code will execute within a session which will take care of checking the use of the framework and, if the test fails, it will try to find out if the failure could be related to a mock being used incorrectly


For a more detailed explanation read this

If you mix-in this trait on your test class after your favourite Spec trait, you will get an automatic MockitoScalaSession around each one of your tests, so all of them will run in Strict Stub mode.

This trait also includes org.mockito.MockitoSugar and org.mockito.ArgumentMatchersSugar so you have pretty much all the mockito-scala API available in one go, i.e.

class MyTest extends WordSpec with MockitoFixture

In case you want to use the Idiomatic Syntax just do

class MyTest extends WordSpec with IdiomaticMockitoFixture

IMPORTANT: A session is defined on a per-test basis, and only the mocks created within the scope of the session are handled by it, so if you have class level fields with mocks, i.e. mocks that are not created within the test, they will be ignored by the session. If you use the same mocks in all or most of the tests and want to avoid the boilerplate while still usfing the advantages of strict stubbing then declare those mocks in a setup trait.

class MySpec extends WordSpec with MockitoFixture {
   trait Setup {
      val myMock = mock[Sth] 
      myMock.someMethod shouldReturn "something" /*stub common to **all** tests -notice that if it's not used by all of them then the session will find it as an unused stubbing on those-*/

   "some feature" should {
       "test whatever i want" in new Setup {
            myMock.someOtherMethod(*) shouldReturn None /*stub specific only to this test*/

      "test something else" in new Setup {
             myMock.someOtherMethod("expected value") shouldReturn Some("result")  /*stub specific only to this test*/

This will give you a fresh new instance of myMock for each test but at the same time you only declare the creation/common stubbing once.


Inspired by this StackOverflow question, mockito-scala provides this trait that helps to automatically reset any existent mock after each test is run The trait has to be mixed after org.mockito.MockitoSugar in order to work, otherwise your test will not compile The code shown in the StackOverflow question would look like this if using this mechanism

NOTE: MockitoFixture and ResetMocksAfterEachTest are mutually exclusive, so don't expect them to work together

class MyTest extends PlaySpec with MockitoSugar with ResetMocksAfterEachTest

private val foo = mock[Foo]

override def fakeApplication(): Application = new GuiceApplicationBuilder().overrides(bind[Foo].toInstance(foo)).build

The main advantage being we don't have to remember to reset each one of the mocks...

If for some reason we want to have a mock that is not reset automatically while using this trait, then it should be created via the companion object of org.mockito.MockitoSugar so is not tracked by this mechanism

Idiomatic Mockito

By adding the trait org.mockito.IdiomaticMockito you get access to some improved methods in the API

This API is heavily inspired on Scalatest's Matchers, so if have used them, you'll find it very familiar

Here we can see the old syntax on the left and the new one on the right

trait Foo {
  def bar: String
  def bar(v: Int): Int
val aMock = mock[Foo]  
when( thenReturn "mocked!"                            <=> shouldReturn "mocked!"
when( thenReturn "mocked!" thenReturn "mocked again!" <=> shouldReturn "mocked!" andThen "mocked again!"
when( thenCallRealMethod()                            <=> shouldCall realMethod
when([IllegalArgumentException]             <=>[IllegalArgumentException]
when( thenThrow new IllegalArgumentException          <=> shouldThrow new IllegalArgumentException
when( thenAnswer(_ => "mocked!")                      <=> shouldAnswer "mocked!"
when( thenAnswer(_.getArgument[Int](0) * 10)     <=>*) shouldAnswer ((i: Int) => i * 10)

doReturn("mocked!").when(aMock).bar                             <=> "mocked!" willBe returned by
doAnswer(_ => "mocked!").when(aMock).bar                        <=> "mocked!" willBe answered by
doAnswer(_.getArgument[Int](0) * 10).when(aMock).bar(any)       <=> ((i: Int) => i * 10) willBe answered by*)
doCallRealMethod.when(aMock).bar                                <=> theRealMethod willBe called by
doThrow(new IllegalArgumentException).when(aMock).bar           <=> new IllegalArgumentException willBe thrown by
verifyZeroInteractions(aMock)                                   <=> aMock wasNever called
verify(aMock).bar                                               <=> was called
verify(aMock).bar(any)                                          <=>*) was called

verify(aMock, only).bar                                         <=> wasCalled onlyHere
verify(aMock, never).bar                                        <=> wasNever called

verify(aMock, times(2)).bar                                     <=> wasCalled twice
verify(aMock, times(2)).bar                                     <=> wasCalled 2.times

verify(aMock, times(6)).bar                                     <=> wasCalled sixTimes
verify(aMock, times(6)).bar                                     <=> wasCalled 6.times

verify(aMock, atLeast(6)).bar                                   <=> wasCalled atLeastSixTimes
verify(aMock, atLeast(6)).bar                                   <=> wasCalled atLeast(sixTimes)
verify(aMock, atLeast(6)).bar                                   <=> wasCalled atLeast(6.times)

verify(aMock, atMost(6)).bar                                    <=> wasCalled atMostSixTimes
verify(aMock, atMost(6)).bar                                    <=> wasCalled atMost(sixTimes)
verify(aMock, atMost(6)).bar                                    <=> wasCalled atMost(6.times)

verifyNoMoreInteractions(aMock)                                 <=> aMock wasNever calledAgain

val order = inOrder(mock1, mock2)                               <=> InOrder(mock1, mock2) { implicit order =>
order.verify(mock2).someMethod()                                <=>   mock2.someMethod() was called
order.verify(mock1).anotherMethod()                             <=>   mock1.anotherMethod() was called
                                                                <=> }

As you can see the new syntax reads a bit more natural, also notice you can use * instead of any[T]

Check the tests for more examples

Default Answers

We defined a new type org.mockito.stubbing.DefaultAnswer which is used to configure the default behaviour of a mock when a non-stubbed invocation is made on it.

The object org.mockito.DefaultAnswers contains each one of the provided ones

All the mocks created will use ReturnsSmartNulls by default, this is different to the Java version, which returns null for any non-primitive or non-final class.

A "Smart Null", is nothing else than a mock of the type returned by the called method. The main advantage of doing that is that if the code tries to call any method on this mock, instead of failing with a NPE the mock will throw a different exception with a hint of the non-stubbed method that was called (including its params), this should make much easier the task of finding and fixing non-stubbed calls

Most of the Answers defined in org.mockito.Answers have it's counterpart in org.mockito.DefaultAnswers, and on top of that we also provide ReturnsEmptyValues which will try its best to return an empty object for well known types, i.e. Nil for List, None for Option etc. This DefaultAnswer is not part of the default behaviour as we think a SmartNull is better, to explain why, let's imagine we have the following code.

class UserRepository {
  def find(id: Int): Option[User]
class UserController(userRepository: UserRepository) {
  def get(userId: Int): Option[Json] = userRepository.find(userId).map(_.toJson)

class UserControllerTest extends WordSpec with IdiomaticMockito {

  "get" should {
     "return the expected json" in {
        val repo = mock[UserRepository]
        val testObj = new UserController(repo)

        testObj.get(123) shouldBe Some(Json(....)) //overly simplified for clarity

Now, in that example that test could fail in 3 different ways

  1. With the standard implementation of Mockito, the mock would return null and we would get a NullPointerException, which we all agree it's far from ideal, as it's hard to know where did it happen in non trivial code
  2. With the default/empty values, we would get a None, so the final result would be None and we will get an assertion error as None is not Some(Json(....)), but I'm not sure how much improvement over the NPE this would be, because in a non-trivial method we may have many dependencies returning Option and it could be hard to track down which one is returning None and why
  3. With a smart-null, we would return a mock[Option] and as soon as our code calls to .map() that mock would fail with an exception telling you what non-stubbed method was called and on which mock (in the example would say something you called the find method on some mock of type UserRepository)

And that's why we use option 3 as default

Of course you can override the default behaviour, for this you have 2 options

  1. If you wanna do it just for a particular mock, you can, at creation time do mock[MyType](MyDefaultAnswer)
  2. If you wanna do it for all the mocks in a test, you can define an implicit, i.e. implicit val defaultAnswer: DefaultAnswer = MyDefaultAnswer

DefaultAnswers are also composable, so for example if you wanted empty values first and then smart nulls you could do implicit val defaultAnswer: DefaultAnswer = ReturnsEmptyValues orElse ReturnsSmartNulls

Function Answers

org.mockito.Answer[T] can be a bit boilerplate-ish, mostly if you're still in Scala 2.11 (in 2.12 with SAM is much nicer), to simplify the usage for both versions is that we replaced it by standard scala functions, so instead of

when("bar", 42)) thenAnswer new Answer[String] {
  override def answer(invocation: InvocationOnMock): String = i.getArgument[String](0) + i.getArgument[Int](1)

We can now write: (this may be nothing new for users of 2.12, but at least now the API is consistent for both 2.11 and 2.12)

when("bar", 42)) thenAnswer ((i: InvocationOnMock) => i.getArgument[String](0) + i.getArgument[Int](1))

I guess we all agree that's much better, but, it gets even better, we can now pass standard functions that work over the arguments, we only need to take care to pass the right types, so the previous example would become

when("bar", 42)) thenAnswer ((v1: String, v2: Int) => v1 + v2)

Mix and match

Mixing normal values with argument matchers

Since mockito 1.0.0, when you use the idiomatic syntax, you are not forced anymore to use argument matchers for all your parameters as soon as you use one, so stuff like this is now valid (not a comprehensive list, just a bunch of examples)

trait Foo {
  def bar(v: Int, v2: Int, v3: Int = 42): Int
val aMock = mock[Foo],2) shouldReturn "mocked!",*) shouldReturn "mocked!"*,*) shouldReturn "mocked!"*,*, 3) shouldReturn "mocked!"

"mocked!" willBe returned by,2)
"mocked!" willBe returned by,*)
"mocked!" willBe returned by*,*)
"mocked!" willBe returned by*,*, 3),2) was called,*) was called*,*) was called*,*, 3) was called

So far there is one caveat, if you have a curried function that has default arguments on the second (or any following) argument list, the macro that achieves this will fail, this is related to how the default method is created by the compiler. I'll write a more detailed explanation at some point, but there are more than one reason why this is probably never going to work The workaround is quite easy though, just provide a value (or a matcher) for that argument and you are good to go.

Numeric matchers

A new set of matchers to deal with number comparison were introduced (see Scalactic tolerance for aproximation), the syntax is slightly different to make them more readable, so now we can write stuff like (notice the 'n')


aMock.method(n > 4.99) was called
aMock.method(n >= 5) was called
aMock.method(n < 5.1) was called
aMock.method(n <= 5) was called


Most matchers that makes sense to, work with varargs out of the box, the only thing to notice is that if you are passing more than one value and want to use eqTo then you should pass all of them to the same instance of eqTo e.g.

trait FooWithVarArgAndSecondParameterList {
  def bar(bells: String*)(cheese: String): String
}"cow", "blue"))(*) was called //RIGHT"cow"), eqTo("blue"))(*) was called //WRONG - it will complain it was expecting 2 matchers but got 3

Scalactic integration


Since version 1.0.0 the eqTo matcher uses the org.scalactic.Equality[T] typeclass, this doesn't change anything on the API and existent code will not be affected, but it allows you to override the standard equality of any class by just providing an implicit Equality in scope, e.g.

implicit val fooEquality: Equality[Foo] = new Equality[Foo] { 
  override def areEqual(a: Foo, b: Any): Boolean = /*Do the comparison as you like*/ 

aMock.method(eqTo(/*some foo instance/*))


You can use Scalactic's Spread[T] to deal with the precision errors in floating points, so you can now write stuff like


aMock.method(n =~ 5.0 +- 0.001) was called


Dead code warning

if you have enabled the compiler flag -Ywarn-dead-code, you will see the warning dead code following this construct when using the any or * matchers , this is because in some cases the compiler can not infer the return type of those matchers and it will default to Nothing, and this compiler warning is shown every time Nothing is found on our code. This will NOT affect the behaviour of Mockito nor your test in any way, so it can be ignored, but in case you want to get rid of it then you have 2 options:

  1. If you are not too fuss about dead code warnings in test code, you can add scalacOptions in Test -= "-Ywarn-dead-code" to your build.sbt and that warning will be ignored for your tests only
  2. If you wanna keep the warning enabled for potentially real dead code statements, but get rid of the warnings related to the matchers usage then you have to explicitly provide the type for the matcher, thus any would become any[MyType] and * would become *[MyType] (you can also use anyShort, anyInt, etc for the primitive types)

Scala 2.11

Please note that in Scala 2.11 the following features are not supported

  • Default arguments on methods defined in traits (they will behave as before, getting null or a default value if they are of a primitive type)
  • Any kind of ArgumentMatcher[T] for methods with by-name parameters (they'll throw an exception if used with ArgumentMatcher[T])


  • Bruno Bonanno - Initial work - bbonanno


This project is licensed under the MIT License - see the LICENSE file for details