Mockito.spy and doReturn() does not work when using Kotlin by lazy

Version 2.18.3

open class Target{
  open val a:Int = 0
  val b:Int by lazy {
    10 + a
  }
}
@RunWith(RobolectricTestRunner::class)
class MyTest {

  @Test
  fun test() {
    val target = spy(Target())
    whenever(target.a).doReturn(10)
    assertEquals(20, target.b)
  }
}

junit.framework.AssertionFailedError: 
Expected :20
Actual   :10
 <Click to see difference>

[skykelsey]

You can’t mock spies like this. You are accessing a on the concrete class. The proper way to mock this is to do:

doReturn(10).whenever(target).a

This way, you are referencing the spy, and not the concrete class.

For this reason, I strongly suggest you stick with the doReturn().when() style syntax instead of whenever().doReturn() style. The latter may work with mocks, but then if you ever switch to a spy, you will see failures even though your test code didn’t change.

[nightfly89]

I can reproduce this as well. Any new information on this?