You can mock the template but it's better to mock the interface.

    Sender sender = new Sender();
    KafkaOperations template = mock(KafkaOperations.class);
    SettableListenableFuture<SendResult<String, String>> future = new SettableListenableFuture<>();
    when(template.send(anyString(), any(Message.class))).thenReturn(future);
    sender.setTemplate(template);
    sender.send(...);

    future.set(new SendResult<>(...));

    ...or...

    future.setException(...

You can't do this with Mockito, but Apache Commons actually has a way to do this using one of its built in utilities. You can put this in a function in JUnit that is run after Mockito injects the rest of the mocks but before your test cases run, like this:

@InjectMocks
MyClass myClass;

@Before
public void before() throws Exception {
    FieldUtils.writeField(myClass, "fieldName", fieldValue, true);
}

Generally, don't mock the class under test. If your test is for a Person, you shouldn't ever see Mockito.mock(Person.class) in it, as that's a pretty clear sign that you're testing the mocking framework instead of the system-under-test.

Instead, you may want to create a spy(new Person()), which will create a real Person implementation using a real constructor and then copy its data to a Mockito-generated proxy. You can use MockingDetails.getInvocations() to reflectively check that every getter was called.

// This code is untested, but should get the point across. Edits welcome.
// 2016-01-20: Integrated feedback from Georgios Stathis. Thanks Georgios!

@Test
public void callAllGetters() throws Exception {
  Person personSpy = spy(new Person());
  personSpy.getFirstname();
  personSpy.getLastname();

  assertAllGettersCalled(personSpy, Person.class);
}

private static void assertAllGettersCalled(Object spy, Class<?> clazz) {
  BeanInfo beanInfo = Introspector.getBeanInfo(clazz);
  Set<Method> setOfDescriptors = beanInfo.getPropertyDescriptors()
      .stream()
      .map(PropertyDescriptor::getReadMethod)
      .filter(p -> !p.getName().contains("getClass"))
      .collect(Collectors.toSet());
  MockingDetails details = Mockito.mockingDetails(spy);
  Set<Method> setOfTestedMethods = details.getInvocations()
      .stream()
      .map(InvocationOnMock::getMethod)
      .collect(Collectors.toSet());
  setOfDescriptors.removeAll(setOfTestedMethods);
  // The only remaining descriptors are untested.
  assertThat(setOfDescriptors).isEmpty();
}

There might be a way to call verify and invoke on the Mockito-generated spy, but that seems very fragile, and very dependent on Mockito internals.

As an aside, testing bean-style getters seems like an odd use of time/effort. In general focus on testing implementations that are likely to change or break.

PowerMockRunner is part of powermock-module-junit4.jar. You need to explicity import this jar

You could specify the dependency for this jar as per your requirement. Refer to this link.

First Mock your Worker with Mockito (just as you did). Also mock your ChildService. Then:

@Test
public void shouldUseChildService() {
  ArgumentCaptor<Callable> callableCaptor = ArgumentCaptor.forClass(Callable.class);
  ArgumentCaptor<Consumer> consumerCaptor = ArgumentCaptor.forClass(Consumer.class);

  parentService.doAction();

  verify(worker).execute(callableCaptor.capture(), consumerCaptor.capture());
  callableCaptor.getValue().call(); //this will execute whatever was captured
  consumerCaptor.getValue().accept(null);//this will execute whatever was captured

  // now verify that childService::listClients and childService::refreshObjects have been called
}

After a lot more hunting around and looking at all the options Mockito/Powermock had to offer, I found the solution (which I will share in case others run into this same issue).

When you have private member variables that are never initialized (and just assumed created in other places), you can use the @InjectMocks annotation to "inject" Mocks you want into your class you are testing.

1. Add a variable in your test class for the class you are testing, and give it the annotation @InjectMocks (org.Mockito.InjectMocks).
2. Use @Mock annotations to setup the mocks you want to inject. Use the @Mock (name = "privateVariableNameHere") name property to map the Mock object to the private variable inside your class you are testing.
3. In either a setup function or before you call your class, initialize the mocks. The easiest way I have found is to use a "setup" method with the @Before annotation. Then inside there call MockitoAnnotations.initMocks(this); to quickly initialize anything with the @Mock annotation.
4. Define your Mock functionality in your test method (before calling the method you are testing).
5. Using the @InjectMock object, call your method you are testing... the mocks SHOULD be hooked in and working as defined in the earlier steps.

So for the example class I use above, the code to test/mock would have Connection returned as a mock which you can do whatever with. Based on the example above in my question, this is what the code would look like:

@RunWith(PowerMockRunner.class)
@PrepareForTest({/* Static Classes I am Mocking */})
public class ExampleTest {
  @Mock (name = "queueFactory") //same name as private var.
  QueueConnectionFactory queueFactoryMock;
  @Mock
  Connection connectionMock; //the object we want returned
  @InjectMocks
  Example exampleTester; //the class to test

  @Before
  public void setup(){
    MockitoAnnotations.initMocks(this); // initialize all the @Mock objects
    // Setup other Static Mocks
  }

  @Test
  public void testTestMe(){
    //Mock your objects like other "normally" mocked objects
    PowerMockito.when(queueFactoryMock.createConnection()).thenReturn(connectionMock);
    //...Mock ConnectionMock functionality...
    exampleTester.testMe();
  }
}

For Java users

Based on the WireMock docs.

There are 3 possibilities to use WireMock in your tests :

1. If you are using Wiremock as JUnit 4 rule to configure the port use :

import static com.github.tomakehurst.wiremock.core.WireMockConfiguration.wireMockConfig;

...

@Rule
public WireMockRule wireMockRule = new WireMockRule(wireMockConfig().port(8888));

2. If you are using new instance and start it from your Test class (for example @Before) :

import static com.github.tomakehurst.wiremock.core.WireMockConfiguration.options;

...

public class Test {

    WireMockServer wm;

    @BeforeEach
    void setUp() {
        wm = new WireMockServer(options().port(8888));
        wm.start();
    }

    @Test
    void test() {
        wm.stubFor(...);
    }
}

3. With static configuration of default instance (not using new instance in your test) :

WireMock.configureFor(8888);

For Kotlin users

If you are using kotlin you can add actual wiremock instance to stubFor and verify calls like wm.stubFor() and configure the port like in option 3 of this answer.

Since Mockito 1.9.5+ and Java 8+

You can use a lambda expression, like:

when(myMock.myFunction(anyString())).thenAnswer(i -> i.getArguments()[0]);

Where i is an instance of InvocationOnMock.

For older versions

You can create an Answer in Mockito. Let's assume, we have an interface named MyInterface with a method myFunction.

public interface MyInterface {
    public String myFunction(String abc);
}

Here is the test method with a Mockito answer:

public void testMyFunction() throws Exception {
    MyInterface mock = mock(MyInterface.class);
    when(mock.myFunction(anyString())).thenAnswer(new Answer<String>() {
    @Override
    public String answer(InvocationOnMock invocation) throws Throwable {
        Object[] args = invocation.getArguments();
        return (String) args[0];
    }
    });

    assertEquals("someString",mock.myFunction("someString"));
    assertEquals("anotherString",mock.myFunction("anotherString"));
}

The problem is on your test class Before method you are instantiating new controller

@Before
    public void setup() {
        categoryController = new CategoryController();
        mockMvc = MockMvcBuilders.standaloneSetup(categoryController).build();
    }

Here is how I do test for Controller

Controller Class :

@Controller
public class CategoryController {

    private SimpleCategoryDAO simpleCategoryDAO;
    private SimpleGifDAO simpleGifDAO;

    @Autowired
    public void setSimpleGifDAO(SimpleGifDAO simpleGifDAO) {
        this.simpleGifDAO = simpleGifDAO;
    }

    @Autowired
    public void setSimpleCategoryDAO(SimpleCategoryDAO simpleCategoryDAO) {
        this.simpleCategoryDAO = simpleCategoryDAO;
    }

    @RequestMapping("/categories")
    public String getAllCategories(ModelMap modelMap) {
        modelMap.put("categories", simpleCategoryDAO.getAllCategories());
        return "categories";
    }

    @RequestMapping("/category/{categoryID}")
    public String getGifsByCategoryID(@PathVariable int categoryID, ModelMap modelMap) {
        modelMap.put("gifs", simpleGifDAO.findGifsByCategoryID(categoryID));
        modelMap.put("category", simpleCategoryDAO.getCategoryByID(categoryID));
        return "category";
    }
}

Notice I'm using setter injection here not field injection. You can also use constructor injection ( preferred way for me).

In you test class

@RunWith(MockitoJUnitRunner.class)
public class CategoryControllerTest {

    private MockMvc mockMvc;

    @Mock
    private SimpleCategoryDAO simpleCategoryDAO;

    @Before
    public void setup() {
        final CategoryController categoryController = new CategoryController();

        //notice here I'm setting the mocked dao here
        // if you didn't use @RunWith(MockitoJUnitRunner.class)
        // you can do: simpleCategoryDAO = Mockito.mock(SimpleCategoryDAO.class);

        categoryController.setSimpleCategoryDAO(simpleCategoryDAO);

        mockMvc = MockMvcBuilders.standaloneSetup(categoryController).build();
    }

    @Test
    public void categories_ShouldRenderCategoriesView() throws Exception {
        List<Category> ALL_CATEGORIES = Arrays.asList(
                new Category(1, "Funny"),
                new Category(2, "JoyFul")
        );
        Mockito.when(simpleCategoryDAO.getAllCategories()).thenReturn(ALL_CATEGORIES);

        mockMvc.perform(get("/categories"))
                //.andExpect((MockMvcResultMatchers.model()).attribute("categories",ALL_CATEGORIES));
                .andExpect(MockMvcResultMatchers.view().name("categories"));
    }
}

Take a look at Before method on test. I'm setting the mocked DAO on the new instance of controller that I've created and then I'm creating the MockMvc using same instance of controller.