How to mock void methods with Mockito

user_115890 0
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How to mock methods with void return type?

I implemented an observer pattern but I can't mock it with Mockito because I don't know how.

And I tried to find an example on the Internet but didn't succeed.

My class looks like this:

public class World {

    List<Listener> listeners;

    void addListener(Listener item) {
        listeners.add(item);
    }

    void doAction(Action goal,Object obj) {
        setState("i received");
        goal.doAction(obj);
        setState("i finished");
    }

    private string state;
    //setter getter state
} 

public class WorldTest implements Listener {

    @Test public void word{
    World  w= mock(World.class);
    w.addListener(this);
    ...
    ...

    }
}

interface Listener {
    void doAction();
}

The system is not triggered with mock.

I want to show the above-mentioned system state. And make assertions according to them.

void mockito mocking unit-testing java
11 Answers

Take a look at the Mockito API docs. As the linked document mentions (Point # 12) you can use any of the doThrow(),doAnswer(),doNothing(),doReturn() family of methods from Mockito framework to mock void methods.

For example,

Mockito.doThrow(new Exception()).when(instance).methodName();

or if you want to combine it with follow-up behavior,

Mockito.doThrow(new Exception()).doNothing().when(instance).methodName();

Presuming that you are looking at mocking the setter setState(String s) in the class World below is the code uses doAnswer method to mock the setState.

World mockWorld = mock(World.class); 
doAnswer(new Answer<Void>() {
    public Void answer(InvocationOnMock invocation) {
      Object[] args = invocation.getArguments();
      System.out.println("called with arguments: " + Arrays.toString(args));
      return null;
    }
}).when(mockWorld).setState(anyString());
user_582200

The solution of so-called problem is to use a spy Mockito.spy(...) instead of a mock Mockito.mock(..).

Spy enables us to partial mocking. Mockito is good at this matter. Because you have class which is not complete, in this way you mock some required place in this class.

user_1158900

If you need to do some operations in the mocked void method, and you need to manipulate the argument that sent to void method; you can combine Mockito.doAnswer with ArgumentCaptor.capture method.

Let's say you have SpaceService that autowires a GalaxyService, which has a void method called someServiceMethod.

You want to write test for one of your method in SpaceService that calls GalaxyService's void method. Your planet is also generated inside SpaceService. So you don't have any chance to mock that.

Here is your sample SpaceService class that you want to write tests for.

class SpaceService {
    @Autowired
    private GalaxyService galaxyService;

    public Date someCoolSpaceServiceMethod() {
        // does something

        Planet planet = new World();
        galaxyService.someServiceMethod(planet); //Planet updated in this method.

        return planet.getCurrentTime();
    }
}

The GalaxyService.someServiceMethod method expects a planet argument. Does some stuff in the method. See :

GalaxyService {
    public void someServiceMethod(Planet planet) {
        //do fancy stuff here. about solar system etc.

        planet.setTime(someCalculatedTime); // the thing that we want to test.

        // some more stuff.
    }
}

And you want to test this feature.

Here is an example :

ArgumentCaptor<World> worldCaptor = ArgumentCaptor.forClass(World.class);
Date testDate = new Date();

Mockito.doAnswer(mocked-> {
    World capturedWorld = worldCaptor.getValue();
    world.updateTime(testDate);
    return null;
}).when(galaxyService.someServiceMethod(worldCaptor.capture());

Date result = spaceService.someCoolSpaceServiceMethod();

assertEquals(result, testDate);
user_33781920

In your example you should mock Listener item and use Mockito.verify to check interactions with it

user_69123720
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