Mocking an extension function with Mockito - spring-boot

Question

How can I test an extension function with Mockito? It doesn't seem to work nicely.
This is my extension function
fun <T> CrudRepository<T, String>.findOneById(id: String): T? {
val o = findById(id)
return if (o.isPresent) o.get() else null
}
And this is what I'm trying to test
#Test
fun getIslandById() {
//given
BDDMockito.given(islandRepository.findOneById("islandId1"))
.willReturn(IslandEntity(tileList, "1", "islandId1")) //findOneById is my extension function
//when
val island = islandService.getIslandById("islandId1")
//then
Assertions.assertThat(island?.id).isEqualTo("islandId1")
}
But the preceeding test throws the following error
org.mockito.exceptions.misusing.WrongTypeOfReturnValue:
IslandEntity cannot be returned by findById()
findById() should return Optional
Any ideas?

Answer 1

Instance extension functions can be mocked like this with a little help of mockito-kotlin:
data class Bar(thing: Int)
class Foo {
fun Bar.bla(anotherThing: Int): Int { ... }
}
val bar = Bar(thing = 1)
val foo = mock<Foo>()
with(foo) {
whenever(any<Bar>().bla(any()).doReturn(3)
}
verify(foo).apply {
bar.bla(anotherThing = 2)
}

Answer 2

As I said in a comment above, in the bytecode extension functions are nothing more than static functions which accept receiver as a first argument. Therefore you can't mock an extension function with a Mockito since it is not able to mock static functions.
What you can do, in case that findById(id) is implemented by Repository and not another extension function, is next:
Mock return value of findById(id) instead.
Take a look at the sample code below:
#Test
fun getIslandById() {
//given
BDDMockito.given(islandRepository.findById("islandId1"))
.willReturn(Optional.of(IslandEntity(tileList, "1", "islandId1"))) //mock findById function
//when
val island = islandService.getIslandById("islandId1")
//then
Assertions.assertThat(island?.id).isEqualTo("islandId1")
}
This way you are indirectly mocking your extension by providing it a mocked value that you want it to operate on.
Note: Error that you posted above says that your findById(id) should return an optional. So, wrap your return value of findById(id) function with an optional by calling Optional.of(result).