How can I use the Mock annotation in Scala? - scala

Question

I'm using scalatest and Scala 2.x. The following compiles/works:
import org.mockito.Mockito._
import org.mockito._
import org.mockito.ArgumentMatchers._
class Test1 extends AnyFunSuite {
val action = Mockito.mock(classOf[Action])
when(action.process(anyInt())).thenReturn("x")
...
}
This doesn't:
import org.mockito.Mockito._
import org.mockito._
import org.mockito.ArgumentMatchers._
class Test1 extends AnyFunSuite {
#Mock
val action = Action()
when(action.process(anyInt())).thenReturn("x")
...
}
I get this exception on the "when" line:
[info] Test1 *** ABORTED ***
[info] org.mockito.exceptions.misusing.InvalidUseOfMatchersException: Misplaced or misused argument matcher detected here:
[info]
[info] -> at Test1.<init>(Test1.scala:17)
[info]
[info] You cannot use argument matchers outside of verification or stubbing.
[info] Examples of correct usage of argument matchers:
[info] when(mock.get(anyInt())).thenReturn(null);
[info] doThrow(new RuntimeException()).when(mock).someVoidMethod(anyObject());
[info] verify(mock).someMethod(contains("foo"))
[info]
[info] This message may appear after an NullPointerException if the last matcher is returning an object
[info] like any() but the stubbed method signature expect a primitive argument, in this case,
[info] use primitive alternatives.
[info] when(mock.get(any())); // bad use, will raise NPE
[info] when(mock.get(anyInt())); // correct usage use
[info]
[info] Also, this error might show up because you use argument matchers with methods that cannot be mocked.
[info] Following methods *cannot* be stubbed/verified: final/private/equals()/hashCode().
[info] Mocking methods declared on non-public parent classes is not supported.
[info] at Test1.<init>(Test1.scala:17)
[info] at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
[info] at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
[info] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
[info] at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
[info] at java.lang.Class.newInstance(Class.java:442)
[info] at org.scalatest.tools.Framework$ScalaTestTask.execute(Framework.scala:450)
[info] at sbt.TestRunner.runTest$1(TestFramework.scala:140)
[info] at sbt.TestRunner.run(TestFramework.scala:155)
[info] at sbt.TestFramework$$anon$3$$anonfun$$lessinit$greater$1.$anonfun$apply$1(TestFramework.scala:318)
[info] ...

Answer 1

Personally, I would avoid mocking altogether, but if you actually need it, in Scala I would use something like ScalaMock which cooperates better with the language.
class Test1 extends AnyFunSuite {
val action = mock[Action]
(action.process _).expects(*).returning("x")
...
}
From what I see #Mock annotation relies on some custom JUnit runner which injects logic into test class by combining Guice/Spring-like dependency injection with mocking. But you are using it wrong:
// Here you would have to configure JUnit runner
// (but Scala test libraries are not really JUnit-based)
class Test1 extends AnyFunSuite {
// Here you would have to configure action as var
// and not initialize it yourself, so that runner could
// create the mocks and put them here using reflection
// after instance initialization
#Mock
var action: Action = _
// Here you could not have the test logic be implemented
// as vals or vars, because it would be executed in object's
// constructor - when action is still unititialized
// (therefore null). JUnit doesn't show this issue as it
// uses #Mock on class attributes, and put #Test logic in
// methods - called only after constructor is completed
// and JUnitRunner injects additional logic
// ...
}
You could totally do it like:
// use JUnit instead of Scalatest
#RunWith(classOf[MockitoJUnitRunner])
class Test1 {
#Mock
var action: Action = _
#Test
def example(): Unit = {
// ...
}
}
so it is possible to make it work in Scala, just not with the non-JUnit-based testing libraries. If you are interested in Mockito conventions in ScalaTest see a dedicated documentation page.