How to mock void methods with Mockito

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());

I think I've found a simpler answer to that question, to call the real method for just one method (even if it has a void return) you can do this:

Mockito.doCallRealMethod().when(<objectInstance>).<method>();
<objectInstance>.<method>();

Or, you could call the real method for all methods of that class, doing this:

<Object> <objectInstance> = mock(<Object>.class, Mockito.CALLS_REAL_METHODS);

Adding to what @sateesh said, when you just want to mock a void method in order to prevent the test from calling it, you could use a Spy this way:

World world = new World();
World spy = Mockito.spy(world);
Mockito.doNothing().when(spy).methodToMock();

When you want to run your test, make sure you call the method in test on the spy object and not on the world object. For example:

assertEquals(0, spy.methodToTestThatShouldReturnZero());

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.

First of all: you should always import mockito static, this way the code will be much more readable (and intuitive):

import static org.mockito.Mockito.*;

For partial mocking and still keeping original functionality on the rest mockito offers "Spy".

You can use it as follows:

private World world = spy(new World());

To eliminate a method from being executed you could use something like this:

doNothing().when(someObject).someMethod(anyObject());

to give some custom behaviour to a method use "when" with an "thenReturn":

doReturn("something").when(this.world).someMethod(anyObject());

For more examples please find the excellent mockito samples in the doc.

How to mock void methods with mockito - there are two options:

doAnswer - If we want our mocked void method to do something (mock the behavior despite being void). doThrow - Then there is Mockito.doThrow() if you want to throw an exception from the mocked void method.

Following is an example of how to use it (not an ideal usecase but just wanted to illustrate the basic usage).

@Test
public void testUpdate() {

    doAnswer(new Answer<Void>() {

        @Override
        public Void answer(InvocationOnMock invocation) throws Throwable {
            Object[] arguments = invocation.getArguments();
            if (arguments != null && arguments.length > 1 && arguments[0] != null && arguments[1] != null) {

                Customer customer = (Customer) arguments[0];
                String email = (String) arguments[1];
                customer.setEmail(email);

            }
            return null;
        }
    }).when(daoMock).updateEmail(any(Customer.class), any(String.class));

    // calling the method under test
    Customer customer = service.changeEmail("old@test.com", "new@test.com");

    //some asserts
    assertThat(customer, is(notNullValue()));
    assertThat(customer.getEmail(), is(equalTo("new@test.com")));

}

@Test(expected = RuntimeException.class)
public void testUpdate_throwsException() {

    doThrow(RuntimeException.class).when(daoMock).updateEmail(any(Customer.class), any(String.class));

    // calling the method under test
    Customer customer = service.changeEmail("old@test.com", "new@test.com");

}
}

You could find more details on how to mock and test void methods with Mockito in my post How to mock with Mockito (A comprehensive guide with examples)

In Java 8 this can be made a little cleaner, assuming you have a static import for org.mockito.Mockito.doAnswer:

doAnswer(i -> {
  // Do stuff with i.getArguments() here
  return null;
}).when(*mock*).*method*(*methodArguments*);

The return null; is important and without it the compile will fail with some fairly obscure errors as it won't be able to find a suitable override for doAnswer.

For example an ExecutorService that just immediately executes any Runnable passed to execute() could be implemented using:

doAnswer(i -> {
  ((Runnable) i.getArguments()[0]).run();
  return null;
}).when(executor).execute(any());

Adding another answer to the bunch (no pun intended)...

You do need to call the doAnswer method if you can't\don't want to use spy's. However, you don't necessarily need to roll your own Answer. There are several default implementations. Notably, CallsRealMethods.

In practice, it looks something like this:

doAnswer(new CallsRealMethods()).when(mock)
        .voidMethod(any(SomeParamClass.class));

Or:

doAnswer(Answers.CALLS_REAL_METHODS.get()).when(mock)
        .voidMethod(any(SomeParamClass.class));

I think your problems are due to your test structure. I've found it difficult to mix mocking with the traditional method of implementing interfaces in the test class (as you've done here).

If you implement the listener as a Mock you can then verify the interaction.

Listener listener = mock(Listener.class);
w.addListener(listener);
world.doAction(..);
verify(listener).doAction();

This should satisfy you that the 'World' is doing the right thing.

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);

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