Mockito + Spy: How to gather return values

Question

I got a class using a factory for creating some object. In my unit test I would like to access the return value of the factory. Since the factory is directly passed to the class and no getter for the created object is provided I need to intercept returning the object from the factory.

RealFactory factory     = new RealFactory();
RealFactory spy         = spy(factory);
TestedClass testedClass = new TestedClass(factory);

// At this point I would like to get a reference to the object created
// and returned by the factory.

Is there a possibility to access the return value of the factory? Probably using the spy?
The only way I can see is to mock the factory create method.

Why should `TestedClass` take in the factory as the dependency. Shouldn't it just ask for the actual class created by the factory. ?(Law of Demeter) — Ajay George, Aug 17 '11 at 16:12
The `TestedClass` is an OSGi-Component. A method of the component requires each invocation a new object created by the factory. I refactored the object creation out into a factory class to provide better testability. Since the created object gets initialized depending on the method parameters, there is no way to simply pass in the created objects instead of the factory. — Marc-Christian Schulze, Aug 17 '11 at 16:37

score 77 · Accepted Answer · answered Sep 05 '14 at 21:31

77

First thing, you should be passing spy in as the constructor argument.

That aside, here's how you could do it.

public class ResultCaptor<T> implements Answer {
    private T result = null;
    public T getResult() {
        return result;
    }

    @Override
    public T answer(InvocationOnMock invocationOnMock) throws Throwable {
        result = (T) invocationOnMock.callRealMethod();
        return result;
    }
}

Intended usage:

RealFactory factory     = new RealFactory();
RealFactory spy         = spy(factory);
TestedClass testedClass = new TestedClass(spy);

// At this point I would like to get a reference to the object created
// and returned by the factory.


// let's capture the return values from spy.create()
ResultCaptor<RealThing> resultCaptor = new ResultCaptor<>();
doAnswer(resultCaptor).when(spy).create();

// do something that will trigger a call to the factory
testedClass.doSomething();

// validate the return object
assertThat(resultCaptor.getResult())
        .isNotNull()
        .isInstanceOf(RealThing.class);

answered Sep 05 '14 at 21:31

Jeff Fairley

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Thanks for sharing. IMHO: This should be the accepted answer. – BetaRide Dec 01 '16 at 08:46
Does this return a copy of the result or the same instance? – nahzor Feb 13 '19 at 18:45
it'll return the same instance – Jeff Fairley Feb 14 '19 at 23:52
Thanks Jeff, this was really helpful – timon_the_destroyer Mar 31 '20 at 18:12
This seems like something I'd expect to be part of Mockito. It works, but maybe it's a bad design if we have to test like this? spy() doc hinted me in this direction. Anyone has any ideas? – Ev0oD Nov 25 '20 at 15:08

score 4 · Answer 2 · answered Aug 17 '11 at 16:08

The standard mocking approach would be to:

Pre-create the object you want the factory to return in the test case
Create a mock (or spy) of the factory
Prescribe the mock factory to return your pre-created object.

If you really want to have the RealFactory create the object on the fly, you can subclass it and override the factory method to call super.create(...), then save the reference to a field accessible by the test class, and then return the created object.

Mockito + Spy: How to gather return values

2 Answers2

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