How to mock final classes on Kotlin using Mockito 2 (KAD 23)

What is the problem?
 

class ClosedClass {

fun doSomething() {

}

}

And you want to test that its doSomething method is called.
The way to do it would be the following:

@Test fun testClosedClass() {

val c = Mockito.mock(ClosedClass::class.java)

c.doSomething()

verify(c).doSomething()

}

If you use Mockito 1.x, you’ll get the following error:

Mockito cannot mock/spy following:
– final classes
– anonymous classes
– primitive types

Update dependencies to Mockito 2

As we have said, Mockito 2 is able to mock it all, so we’re going to update the dependency. At the time of writing this article the latest version is 2.8.9. But check it out because they are updating very often lately.

testCompile 'org.mockito:mockito-core:2.8.9'

So now we run our code again, but… it fails again!

Mockito cannot mock/spy because :
– final class

We’re no longer limited to mock anonymous classes or primitive types, but it’s not the same for final classes. Why is this?
This option is still a bit experimental, and requires a manual activation.

Enable the option to mock final classes

To do this, you’ll need to create a file in the test/resources/mockito-extensions folder called org.mockito.plugins.MockMaker:

It’s a simple text file, in which you have to write:

mock-maker-inline

Nothing else.
Now you can run the test again, and you’ll see that it runs smoothly. Great!

Mocking Properties

You can also mock properties with no issues. If we change the code of the class to this, for example:

class ClosedClass(val prop: Int) {

fun doSomething() {

}

}

Now let’s mock the value of the property:

@Test fun testClosedClass() {

val c = Mockito.mock(ClosedClass::class.java)

`when`(c.prop).thenReturn(3)

val prop = c.prop

assertEquals(3, prop)

}

I’m asking it to return 3 when it’s called, and later, I check that the value is correct.
You can also check that a property has been called with:

verify(c).prop

Conclusion

As you can see, all the limitations have disappeared thanks to the latest version of the most popular mocking library.
So excuses are over! You can now write all your tests using Kotlin.