1. Overview

In this tutorial, we'll illustrate how to make the most out of spies in Mockito.

We will talk about the @Spy annotation, how to stub a spy and, finally – we will go into the difference between Mock and Spy.

And of course, for more Mockito goodness, have a look at the series here.

2. Simple Spy Example

Let's start with a simple example of how to use a spy.

Simply put, the API is Mockito.spy() – to spy on a real object.

This will allow us to call all the normal methods of the object while still tracking every interaction, just as we would with a mock.

OK, let's do a quick example where we'll spy on an existing ArrayList object:

@Test public void whenSpyingOnList_thenCorrect() { List<String> list = new ArrayList<String>(); List<String> spyList = Mockito.spy(list); spyList.add("one"); spyList.add("two"); Mockito.verify(spyList).add("one"); Mockito.verify(spyList).add("two"); assertEquals(2, spyList.size()); }

Note how the real method add() is actually called and how the size of spyList becomes 2.

3. The @Spy Annotation

Next – let's see how to use the @Spy annotation. We can use @Spy annotation instead of spy() as in the following example:

@Spy List<String> spyList = new ArrayList<String>(); @Test public void whenUsingTheSpyAnnotation_thenObjectIsSpied() { spyList.add("one"); spyList.add("two"); Mockito.verify(spyList).add("one"); Mockito.verify(spyList).add("two"); assertEquals(2, spyList.size()); }

In order to enable Mockito annotation (such as @Spy, @Mock, … ) – we need to do one of the following:

Call the method MockitoAnnotations.initMocks(this) to initialize annotated fields

Use the built-in runner @RunWith(MockitoJUnitRunner.class)

4. Stubbing a Spy

Now – Let's see how to stub a Spy. We can configure/override the behavior of a method using the same syntax we would use with a mock.

In the following example – we use doReturn() to override the size() method:

@Test public void whenStubASpy_thenStubbed() { List<String> list = new ArrayList<String>(); List<String> spyList = Mockito.spy(list); assertEquals(0, spyList.size()); Mockito.doReturn(100).when(spyList).size(); assertEquals(100, spyList.size()); }

5. Mock vs. Spy in Mockito

Now – let's discuss the difference between Mock and Spy in Mockito – not the theoretical differences between the two concepts, just how they differ within Mockito itself.

When Mockito creates a mock – it does so from the Class of a Type, not from an actual instance. The mock simply creates a bare-bones shell instance of the Class, entirely instrumented to track interactions with it.

On the other hand, the spy will wrap an existing instance. It will still behave in the same way as the normal instance – the only difference is that it will also be instrumented to track all the interactions with it.

In the following example – we create a mock of the ArrayList class:

@Test public void whenCreateMock_thenCreated() { List mockedList = Mockito.mock(ArrayList.class); mockedList.add("one"); Mockito.verify(mockedList).add("one"); assertEquals(0, mockedList.size()); }

As we can see – adding an element into the mocked list doesn't actually add anything – it just calls the method with no other side-effect.

A spy on the other hand will behave differently – it will actually call the real implementation of the add method and add the element to the underlying list:

@Test public void whenCreateSpy_thenCreate() { List spyList = Mockito.spy(new ArrayList()); spyList.add("one"); Mockito.verify(spyList).add("one"); assertEquals(1, spyList.size()); }

6. Understanding the Mockito NotAMockException

In this final section, we'll learn about the Mockito NotAMockException. This exception is one of the common exceptions we will likely encounter when misusing mocks or spies.

Let's start by seeing under what circumstance this exception can occur:

List<String> list = new ArrayList<String>(); Mockito.doReturn(100).when(list).size(); assertEquals("Size should be 100: ", 100, list.size());

When we run this code snippet, we'll get the following error:

org.mockito.exceptions.misusing.NotAMockException: Argument passed to when() is not a mock! Example of correct stubbing: doThrow(new RuntimeException()).when(mock).someMethod();

Thankfully it is quite clear from the Mockito error message what the problem is here. In our example, the list object is not a mock. The Mockito when() method expects a mock or spy object as the argument.

As we can also see the Exception message even describes what a correct invocation should look like. Now that we have a better understanding of what the problem is, let's fix it following the recommendation:

final List<String> spyList = Mockito.spy(new ArrayList<String>()); Mockito.doReturn(100).when(spyList).size(); assertEquals("Size should be 100: ", 100, spyList.size());

Our example now behaves as expected and we no longer see the Mockito NotAMockException.

7. Conclusion

In this quick article, we discussed the most useful examples of using Mockito spies.

We learned how to create a spy, how to use @Spy annotation, how to stub a spy and, finally – the difference between Mock and Spy.

The implementation of all these examples can be found over on GitHub.

This is a Maven project, so it should be easy to import and run as it is.

And of course, for more Mockito goodness, have a look at the series here.