Kotlin/Mockito: How to mock "annotationClass" from kotlin.Annotation

I would like to mock annotations to check the good behavior of a class that returns a result according to the given annotation.

Here is one of my tests that check the good behavior for with the OneToOne annotation:

@Test
fun <T> supports_when_field_has_OneToOne_annotation_with_eager_fetch_type() {
    val myHandler = MyHandler<T>()

    val joinAnnotation = mock<OneToOne> {
        on { this.fetch }.thenReturn(FetchType.EAGER)
        onGeneric { this.annotationClass }.thenReturn(OneToOne::class)
    }
    val supports = myHandler.supports(joinAnnotation)

    assertThat(supports).isTrue()
}

When I run my test, I have the following error:

org.mockito.exceptions.misusing.WrongTypeOfReturnValue: KClassImpl cannot be returned by annotationType() annotationType() should return Class

---

If you're unsure why you're getting above error read on. Due to the nature of the syntax above problem might occur because:

1. This exception might occur in wrongly written multi-threaded tests. Please refer to Mockito FAQ on limitations of concurrency testing.
2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies -
   • with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.

This error occurs when mockito calls the following code:

onGeneric { this.annotationClass }.thenReturn(OneToOne::class)

If I remove this line, I have no problem to mock the Annotation. (mocking 'fetch' attribute works perfectly), but my test doesn't pass because I need to mock 'annotationClass'

I don't understand why I have an error, and why the error is related on annotationType() (java annotation method) ?

Does anyone have an idea how to solve this?

I found a solution to fix my problem.

kotlin.Annotation.annotationClass is kotlin extension function:

/**
 * Returns a [KClass] instance corresponding to the annotation type of this annotation.
 */
public val <T : Annotation> T.annotationClass: KClass<out T>
    get() = (this as java.lang.annotation.Annotation).annotationType().kotlin as KClass<out T>

This function calls java.lang.annotation.Annotation.annotationType()

I can't mock a extension function because kotlin converts extension functions into static methods and Mockito cannot stub static methods.

So I mocked directly java.lang.annotation.Annotation.annotationType().

Here my new code:

@Test
fun <T> supports_when_field_has_OneToOne_annotation_with_eager_fetch_type() {
    val myHandler = MyHandler<T>()

    val joinAnnotation = mock<OneToOne> {
        on { this.fetch }.thenReturn(FetchType.EAGER)
        on { (this as java.lang.annotation.Annotation).annotationType() }.thenReturn(OneToOne::class.java)
    }
    val supports = myHandler.supports(joinAnnotation)

    assertThat(supports).isTrue()
}

There are alternatives to mock static methods (PowerMock, MockK), but I didn't tested.

If anyone has a better suggestion, I would be interested in seeing.