Testing ExecutorService with Mockito

There are smiliar questions to this but they don't quite cover the answer I need, so could anyone help a real noob to Java and Mockito? I have code as part of a larger method in a class like so:

private final ExecutorService executorService; //(also declared in constructor)


executorService.submit(() -> {
      someCool.stuffInHere
});

And I would like to unit test it with something along the lines of

@Mock
ExecutorService executorServiceMock

doAnswer(invocationOnMock -> {
    testSomeCoolStuffInHere;
}).when(executorServiceMock).submit(() -> any())

Is that a correct way to go about it? I'm trying that but I keep getting an error along the lines of:

executorServiceMock.submit(CLassName$$LongLambdaAddress) at ClassName
has following stubbings with different arguments

1. executorServiceMock.submit(CLassName$$ADifferentLongLambdaAddress)

Thanks for looking. hopefully there's enough info there. I'm hoping it's something simple.

Solution

You are not submitting () -> any(). You are submitting a Callable<T> or a Runnable (the latter in your case, because you don't return a value).

So if you really want to stub the calls on the executor service (instead of using one of the existing implementations that help with testing, e.g. Runnable::run is a handy Executor to use in a test), you must use the correct argument matcher.

Your code is effectively equivalent to:

doAnswer(invocationOnMock -> {
    testSomeCoolStuffInHere;
}).when(executorServiceMock).submit(eq(() -> null))

Instead, you want to match a runnable:

doAnswer(invocationOnMock -> {
    testSomeCoolStuffInHere;
}).when(executorServiceMock).submit(any(Runnable.class))

(or any<Runnable>() – this will match null as well)