scalamock

ScalaMock: Native Scala Mocking

To use ScalaMock, you need the relevant MockFactoryBase trait implementation:

• for ScalaTest, use org.scalamock.scalatest.MockFactory,
• for Specs2 use org.scalamock.specs2.IsolatedMockFactory or org.scalamock.specs2.MockContext

At present, ScalaMock can only mock traits, Java interfaces, and non-final classes that define a default constructor. A future version will be able to mock any class, and singleton/companion objects.

ScalaMock supports two different mocking styles - expectations first and record then verify. These styles can be mixed within a single test.

Expectations-First Style

In the expectations-first style, expectations are set on mock objects before exercising the system under test. If these expectations are not met, the test fails.

A mock function that supports this style is created with mockFunction. For example, to create a mock function taking a single Int argument and returning a String:

val m = mockFunction[Int, String]

A mock object that supports this style is created with mock. For example, to create a mock that implements the Turtle trait:

val m = mock[Turtle]

Expectations can then be set using expects:

(m.setPosition _).expects(10.0, 10.0)
(m.forward _).expects(5.0)
(m.getPosition _).expects().returning(15.0, 10.0)

drawLine(m, (10.0, 10.0), (15.0, 10.0))

Record-then-Verify (Mockito) Style

In the record then verify style, expectations are verified after the system under test has executed.

A stub function that supports this style is created with stubFunction. For example:

val m = stubFunction[Int, String]

A stub object that supports this style is created with stub. For example:

val m = stub[Turtle]

Return values that are used by the system under test can be set up by using when. Calls are verified using verify:

(m.getPosition _).when().returns(15.0, 10.0)

drawLine(m, (10.0, 10.0), (15.0, 10.0))

(m.setPosition _).verify(10.0, 10.0)
(m.forward _).verify(5.0)

Argument matching

ScalaMock supports two types of generalised matching: wildcards and epsilon matching.

Wildcards

Wildcard values are specified with an * (asterisk). For example:

m expects ("this", *)

will match any of the following:

m("this", 42)
m("this", 1.0)
m("this", null)

Epsilon matching

Epsilon matching is useful when dealing with floating point values. An epsilon match is specified with the ~ (tilde) operator:

m expects (~42.0)

will match:

m(42.0)
m(42.0001)
m(41.9999)

but will not match:

m(43.0)
m(42.1)

Repeated parameters

Repeated parameters are represented as a Seq. For example, given:

def takesRepeatedParameter(x: Int, ys: String*)

you can set an expectation with:

(m.takesRepeatedParameter _).expects(42, Seq("red", "green", "blue"))

Predicate matching

More complicated argument matching can be implemented by using where to pass a predicate:

m = mockFunction[Double, Double, Unit]
m expects (where { _ < _ })

Return values

By default mocks and stubs return null. You can return a computed return value (or throw a computed exception) with onCall:

val mockIncrement = mockFunction[Int, Int]
m expects (*) onCall { _ + 1 }

Overloaded, curried and polymorphic methods

Overloaded, curried and polymorphic methods can be mocked by specifying either argument types or type parameters. For example:

trait Foo {
  def overloaded(x: Int): String
  def overloaded(x: String): String
  def overloaded[T](x: T): String
  def curried(x: Int)(y: Double): String
  def polymorphic[T](x: List[T]): String
}

val m = mock[Foo]
(m.overloaded(_: Int)).expects(10)
(m.overloaded(_: String)).expects("foo")
(m.overloaded[Double] _).expects(1.23)
(m.curried(_: Int)(_: Double)).expects(10, 1.23)
(m.polymorphic(_: List[Int])).expects(List(1, 2, 3))
(m.polymorphic[String] _).expects("foo")

Exceptions

Instead of a return value, mocks and stubs can be instructed to throw:

m expects ("this", "that") throws new RuntimeException("what's that?")

Call count

By default, mocks and stubs expect exactly one call. Alternative constraints can be set with repeat:

m1.expects(42).returns(42).repeat(3 to 7)
m2 expects (3) repeat 10

There are various aliases for common expectations and styles:

m1.expects("this", "that").once
m2.expects().returns("foo").noMoreThanTwice
m3.expects(42).repeated(3).times

For a full list, see org.scalamock.handlers.CallHandler.

Ordering

By default, expectations can be satisfied in any order. For example:

m expects (1)
m expects (2)
m(2)
m(1)

A specific sequence can be enforced with inSequence:

inSequence {
  m expects (1)
  m expects (2)
}
m(2) // throws ExpectationException
m(1)

Multiple sequences can be specified. As long as the calls within each sequence happen in the correct order, calls within different sequences can be interleaved. For example:

inSequence {
  m expects (1)
  m expects (2)
}
inSequence {
  m expects (3)
  m expects (4)
}

m(3)
m(1)
m(2)
m(4)

To specify that there is no constraint on ordering, use inAnyOrder (there is an implicit inAnyOrder at the top level). Calls to inSequence and inAnyOrder can be arbitrarily nested. For example:

(m.a _).expects()
inSequence {
  (m.b _).expects()
  inAnyOrder {
    (m.c _).expects()
    inSequence {
      (m.d _).expects()
      (m.e _).expects()
    }
    (m.f _).expects()
  }
  (m.g _).expects()
}

Threads

ScalaMock will work with tests that are run in parallel (Specs2 runs tests in parallel by default, and ScalaTest does so with ParallelTestExecution).

You can call mocks from other threads within tests, but any such calls must be complete before the test completes - it's an error to call a mock afterwards.