Feature: mocking native methods alike System.currentTimeMillis()

Closed stale thread - I know - but it still comes up in search results. I have a workaround that is not clearly expressed elsewhere:

You can’t mockkstatic System but you can mockkstatic Calendar. So in your production code use:

Calendar.getInstance().timeInMillis

instead of:

System.currentTimeMillis()

then in your tests you can do something like:

        val calendar = mock<Calendar>()
        mockkStatic(Calendar::class)
        every { Calendar.getInstance() } returns calendar
        whenever(calendar.timeInMillis).thenReturn(System.currentTimeMillis())

It beats the hell out of messing around with DI.

In case you don’t want to create a new calendar instance everytime (by calling Calendar.getInstance().timeInMillis) here is another workaround:

object TimeHelper {
    fun getNow() = System.currentTimeMillis()
}

in production replace System call with the helper

//val now = System.currentTimeMillis()
val now = TimeHelper.getNow()

in test:

@Test
fun `whatever you want to test`() = mockkObject(TimeHelper) {
    every { TimeHelper.getNow() } returns 1234
    ...
}