JMock and Kotlin

JMock may have lost the mocking war, but for the London crowd it's still the go-to mocking tool. It turns out that a little Kotlin fairy-dust can make it even more expressive.

In my last post we saw a pretty vanilla use of JMock. In fact there was one nice Kotlin'ism, defining an extension method on Mockery. So where in Java we would write

mockery.checking(new Expectations() { {
    oneOf(progress).reset(2);
    oneOf(indexer).createIndex();
} })

in Kotlin, by defining

fun Mockery.expecting(block: MyExpectations.() -> Unit) {
   this.checking(MyExpectations().apply(block))
}

we can write

mockery.expecting {
    oneOf(progress).reset(2)
    oneOf(indexer).createIndex()
}

In addition, in the name of more readable tests, I had added a subclass of Expectations to allow me to write, for example

mockery.expecting {
   givenActiveJournalIds(
       "1" to throwException(RuntimeException("oops")),
       "2" to returnValue(journal2))
   }
   ...

This was achieved with

class MyExpectations : Expectations() {
   fun givenActiveJournalIds(vararg idResultPairs: Pair<String, Action>) {
       allowing(journals).loadActiveIds()
       will(returnValue(idResultPairs.map { it.first }))
       idResultPairs.forEach { pair ->
           allowing(journals).loadJournalWithArticles(pair.first, 99)
           will(pair.second)
       }
   }
}

which is tighter in Kotlin, but nothing that you couldn't do in Java.

Looking at a one of the tests, this plugs together into something like

@Test fun reports_exceptions_and_continues() {
   mockery.expecting {
       val x = RuntimeException("oops")

       givenActiveJournalIds(
           "1" to throwException(x),
           "2" to returnValue(journal2))

       oneOf(progress).reset(2)
       oneOf(indexer).createIndex()

       never(indexer).index(JournalJson(journal1))
       oneOf(progress).exception("1", x)

       oneOf(indexer).index(JournalJson(journal2))
       oneOf(progress).indexed(journal2)
   }
   refresher.refresh(journals, indexer, emptySet())
}

This isn't bad, but one of JMock's problems is that it doesn't differentiate between interactions that are queries, and those that are operations. In this case the refresher queries the journals to find what needs to be indexed, then operates on the indexer to add them. Introducing givenActiveJournalIds gives a clue, but it would be nice to see the split more formally.

What I'd like to see is

@Test fun reports_exceptions_and_continues() {
    val x = RuntimeException("oops")
    mockery.given {
        activeJournalIds(
            "1" to throwException(x),
            "2" to returnValue(journal2))
    }.whenRunning {
        refresher.refresh(journals, indexer, emptySet())
    }.thenExpect {
        oneOf(progress).reset(2)
        oneOf(indexer).createIndex()

        never(indexer).index(JournalJson(journal1))
        oneOf(progress).exception("1", x)

        oneOf(indexer).index(JournalJson(journal2))
        oneOf(progress).indexed(journal2)
    }
}

This can be achieved through 2 extension methods and a little class -

fun Mockery.expecting(expectations: Expektations.() -> Unit): Mockery {
    this.checking(Expektations().apply(expectations))
    return this
}

fun Mockery.whenRunning(block: () -> Unit) = ThenClause(this, block)

class ThenClause(private val mockery: Mockery, private val block: () -> Unit) {
    fun thenExpect(expectations: Expektations.() -> Unit) {
        mockery.expecting(expectations)
        block()
        mockery.assertIsSatisfied()
    }
}

Actually that trick can be played in Java 8 too.

If you're really eagle-eyed, you may have noticed Expektations pop up in that example. That's to support my final trick, an extension method on Nothing. Why?

Well what if we wanted to simulate an exception in the indexing?

@Test fun `reports exceptions in indexing and continues`() {
    val x = RuntimeException("oops")
    mockery.given {
        activeJournalIds(
            "1" to returnValue(journal1),
            "2" to returnValue(journal2))
    }.whenRunning {
        refresher.refresh(journals, indexer, emptySet())
        executor.runUntilIdle()
    }.thenExpect {
        oneOf(progress).reset(2)
        oneOf(indexer).createIndex()

        oneOf(indexer).index(JournalJson(journal1))
        will(throwException(x))
        oneOf(progress).exception("1", x)

        oneOf(indexer).index(JournalJson(journal2))
        oneOf(progress).indexed(journal2)
    }
}

That will() as a separate statement has always bothered me about JMock - it should bind to the previous statement, but can't because indexer.index(...) returns Unit (void in Java). In Kotlin we can define

class Expektations: Expectations() {
    fun Any?.will(action: Action) = super.will(action)
    // fun Nothing.will(action: Action) = super.will(action) Update 2016-05-17 - looks like this isn't needed, as Unit extends Any
}

and now within our expectation blocks we can write

{
    oneOf(indexer).index(JournalJson(journal1)).will(throwException(x))
    // or
    allowing(journals).loadJournalWithArticles("1", 99).will(returnValue(journal1))
}

With a bit of work I think that would allow a typed JMock Action so that you could only returnValue with the correct type, but I haven't pulled on that thread yet.

The final bonus marks in this post go for spotting

@Test fun `reports exceptions in indexing and continues`()

Ignoring the fact that it breaks Prism's Kotlin highlighter, this hack was pointed out by Nat Pryce - backticks allow spaces in method names, giving beautifully readable specs in code and test runners.