How can Mockito mock a Kotlin `use` (aka try-with-resource)?

Question

If I were trying to add mocks to the following code (where the MyTypeBuilder is mocked to return a mocked MyType - which implements Closeable):

myTypeBuilder.build(myTypeConfiguration).use { myType ->
    myType.callMyMethod()
}

Then trying to verify interactions with myType.callMethod() something like:

myType: MyType = mock()
myTypeBuilder: MyTypeBuilder = mock()
whenever(myTypeBuilder.build(any())).thenReturn(myType)
doMethodCall()
verify(myType, times(1)).callMyMethod()

I'm getting errors:

Wanted but not invoked:
myType.callMethod()
-> at package.me.MyType.callMyMethod(MyType.kt:123)

However, there was exactly 1 interaction with this mock:
myType.close()
-> at kotlin.io.CloseableKt.closeFinally(Closeable.kt:57)

So it appears that I need to add a whenever to execute the use block, but I'm not sure what that should look like. Alternatively, the use should act like a Mockito spy rather than a mock, but then allow mocking on the other methods.

Answer 1

I tried to reconstruct the error by writing the following code which is basically what you wrote in your question plus some println statements and some boilerplate to make it runnable:

import org.junit.jupiter.api.Test
import org.mockito.Mockito.*
import org.mockito.kotlin.any
import org.mockito.kotlin.mock
import org.mockito.kotlin.whenever
import java.io.Closeable

open class MyTypeBuilder {
    open fun build(config: Any): MyType {
        println("build")
        return MyType()
    }

}

open class MyType : Closeable {

    fun callMyMethod() {
        println("callMyMethod")
    }

    override fun close() {
        println("close")
    }
}

val myTypeConfiguration: Any = "heyho"

fun call(myTypeBuilder: MyTypeBuilder) {
    myTypeBuilder.build(myTypeConfiguration).use { myType ->
        println("call")
        myType.callMyMethod()
    }
}

class MockAndUseTest {

    @Test
    fun test() {
        val myType: MyType = mock()
        val myTypeBuilder: MyTypeBuilder = mock()
        whenever(myTypeBuilder.build(any())).thenReturn(myType)
        call(myTypeBuilder)
        verify(myType, times(1)).callMyMethod()
    }
}

When I run the test case test, it succeeds and is green.

So, unfortunately whatever may cause your problem, it is not contained in the details given by your question.