A small library to help you (or not) with Mockito + Scala3 and ZIO.


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A small library to help you (or not) with Mockito + Scala 3 + ZIO (I think it is possible with Cats Effect as well).

I have decided to publish it because it helped me with a small project that I have been working on.

The following examples use ZIO, Try, Either and Option as Effect types.


Add the following dependencies in your sbt build, remember for now it's available only for Scala 3.

    "one.estrondo" %% "sweet-mockito" % <version> % Test,
    "one.estrondo" %% "sweet-mockito-zio" % <version> % Test

Returning succeed and failed ZIO effects.

First, use these imports:

import one.estrondo.sweetmockito.SweetMockito
import one.estrondo.sweetmockito.zio.given // Remember, it's necessary for Scala 3.

Trait and class used by the examples:

  trait Foo:
    def tellMe(name: String): Task[Message]
    def canIPass(name: String): IO[String, Message]

  case class Message(message: String)

Mocking success results

val mock = SweetMockito[Foo]
  .whenF2(mock.tellMe("Einstein")) // there is a method called whenF, but for ZIO use whenF2.
  .thenReturn(Message("You were smart!"))

for result <- mock.tellMe("Einstein")
yield assertTrue(result == Message("You were smart!"))

Mocking failures (not dying)

val mock          = SweetMockito[Foo]
val expectedError = IOException("You? Maybe... look at that bird!")

for exit <- mock.tellMe("Me").exit
yield assert(exit)(Assertion.fails(Assertion.equalTo(expectedError)))

If you need more control, you can do this

In this case, you want to run some code to answer who is calling the mocked method, for this, you have to inform SweetMockit what kind the answer will be returned using Answer.succeed or Answer.failed. Please import this object:

import one.estrondo.sweetmockito.Answer

Here is an example:

val mock          = SweetMockito[Foo]
val expectedError = IOException("You're a machine, not a man!")

  .whenF2(mock.tellMe(ArgumentMatchers.any())) // As you can see, here I'm using ArgumentMatchers from Mockito.
  .thenAnswer { invocation => // Here invocation is an InvocationOnMock from Mockito too.
    invocation.getArgument[String](0) match
      case "Morpheus"    => Answer.succeed(Message("A good leader"))
      case "Agent Smith" => Answer.failed(expectedError)
      case value         => Answer.succeed(Message(s"Who are you $value?"))

  morpheus   <- mock.tellMe("Morpheus").exit
  agentSmith <- mock.tellMe("Agent Smith").exit
  neo        <- mock.tellMe("Neo").exit
yield TestResult.all(
  assert(morpheus)(Assertion.succeeds(Assertion.equalTo(Message("A good leader")))),
  assert(neo)(Assertion.succeeds(Assertion.equalTo(Message("Who are you Neo?"))))

Mocking ZIO Services

It is quite similar, but do need to import this:

import one.estrondo.sweetmockito.zio.SweetMockitoLayer

Mocking success services

  _      <- SweetMockitoLayer[Foo]
              .whenF2(_.canIPass("Michael Jackson")) // Remember, use whenF2 for ZIO.
              .thenReturn(Message("Yes! Please take this money."))
  mock   <- ZIO.service[Foo]
  result <- mock.canIPass("Michael Jackson")

yield assertTrue(result == Message("Yes! Please take this money."))

Mocking failed services

  _    <- SweetMockitoLayer[Foo]
            .whenF2(_.canIPass("This is my friend"))
            .thenFail("I don't think so, I've never seen him!")
  mock <- ZIO.service[Foo]
  exit <- mock.canIPass("This is my friend").exit

yield assert(exit)(Assertion.fails(Assertion.equalTo("I don't think so, I've never seen him!")))

If you need more control over mocking services, you will do this:


import one.estrondo.sweetmockito.Answer


  _     <- SweetMockitoLayer[Foo]
              .thenAnswer { invocation =>
                invocation.getArgument[String](0) match
                  case "Dr. Spock"   => Answer.succeed(Message("Long live and prosper, please go ahead."))
                  case "Darth Vader" => Answer.failed("Nooooooo!")
                  case other         => Answer.failed(s"$other, you're not Spock!")
  mock  <- ZIO.service[Foo]
  spock <- mock.canIPass("Dr. Spock").exit
  vader <- mock.canIPass("Darth Vader").exit
  data  <- mock.canIPass("Data").exit
yield TestResult.all(
  assert(spock)(Assertion.succeeds(Assertion.equalTo(Message("Long live and prosper, please go ahead.")))),
  assert(data)(Assertion.fails(Assertion.equalTo("Data, you're not Spock!")))

Bonus! Mocking with scala.util.Try, scala.util.Either, scala.Option and Iterable classes.

Imports that you need:

import one.estrondo.sweetmockito.SweetMockito
import one.estrondo.sweetmockito.Answer // Just if you need .thenAnswer method, there are examples bellow.

These examples came from my testing codes and I'm using ScalaTest.


For scala.util.Try (maybe in coming support for Cats Effect) you have to call whenF instead whenF2 used by Either and ZIO mocking.

Success example:

val mock = SweetMockito[Foo]

assert(mock.returnAsTry(10) == Success("Ok!"))

Failure example:

val mock      = SweetMockito[Foo]
val exception = IllegalArgumentException("I'm marcian!")


assert(mock.failAsTry("earth") == Failure(exception))


Right example:

val mock = SweetMockito[Foo]
  .thenReturn(Message("Earth is OK!"))

assert(mock.returnAsEither("Earth") == Right(Message("Earth is OK!")))

Left example:

val mock = SweetMockito[Foo]

assert(mock.failAsEither("42") == Left("Ouch!"))


Some example:

val mock = SweetMockito[Foo]
  .thenReturn(Message("Earth is OK!"))

assert(mock.returnAsOption("Earth") == Some(Message("Earth is OK!")))

None example, for this you can use the method thenEmpty which is available for some types.

val mock = SweetMockito[Foo]

assert(mock.returnAsOption("Pluto") == None)

Iterable classes

Here I tested only scala.collection.immutable.Vector. SweetMockito looks for a given (implicit) scala.collection.Factory for the type that you want to return in your mock. So, any type that has this Factory available SweetMockito could be used for it.

One element:

val mock = SweetMockito[Foo]

  .thenReturn(Message("Earth is OK!"))

assert(mock.returnAsVector("Earth") == Vector(Message("Earth is OK!")))

Multiple elements. I'm sorry! I forgot! That will be available soon.

Empty vector, like Option you can return an empty Iterable.

val mock = SweetMockito[Foo]

assert(mock.returnAsVector("Pluto") == Nil)

You are welcomed to contribute.

If you have any suggestions or more ideas for features, please contact me here in github.