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1. Overview

In this tutorial, we’ll cover the annotations of the Mockito library@Mock, @Spy, @Captor, and @InjectMocks.

For more Mockito goodness, have a look at the series here.

Further reading:

Mockito – Using Spies

Making good use of Spies in Mockito, and how spies are different from mocks.

Read more

Mockito vs EasyMock vs JMockit

A quick and practical guide to understanding and comparing Java mocking libraries.

Read more

Injecting Mockito Mocks into Spring Beans

This article will show how to use dependency injection to insert Mockito mocks into Spring Beans for unit testing.

Read more

2. Enable Mockito Annotations

First – let’s see how to enable the use of annotations with Mockito tests.

In order for these annotations to be enabled, we’ll need to annotate the JUnit test with a runnerMockitoJUnitRunner as in the following example:

public class MockitoAnnotationTest {

Alternatively, we can enable these annotations programmatically as well, by invoking MockitoAnnotations.initMocks() as in the following example:

public void init() {

3. @Mock Annotation

The most used widely used annotation in Mockito is @Mock. We can use @Mock to create and inject mocked instances without having to call Mockito.mock manually.

In the following example – we’ll create a mocked ArrayList with the manual way without using @Mock annotation:

public void whenNotUseMockAnnotation_thenCorrect() {
    List mockList = Mockito.mock(ArrayList.class);
    assertEquals(0, mockList.size());

    assertEquals(100, mockList.size());

And now we’ll do the same but we’ll inject the mock using the @Mock annotation:

List<String> mockedList;

public void whenUseMockAnnotation_thenMockIsInjected() {
    assertEquals(0, mockedList.size());

    assertEquals(100, mockedList.size());

Note how – in both examples, we’re interacting with the mock and verifying some of these interactions – just to make sure that the mock is behaving correctly.

4. @Spy Annotation

Now – let’s see how to use @Spy annotation to spy on an existing instance.

In the following example – we create a spy of a List with the old way without using @Spy annotation:

public void whenNotUseSpyAnnotation_thenCorrect() {
    List<String> spyList = Mockito.spy(new ArrayList<String>());


    assertEquals(2, spyList.size());

    assertEquals(100, spyList.size());

Let’s now do the same – spy on the list – but do so using the @Spy annotation:

List<String> spiedList = new ArrayList<String>();

public void whenUseSpyAnnotation_thenSpyIsInjected() {


    assertEquals(2, spiedList.size());

    assertEquals(100, spiedList.size());

Note how, as before – we’re interacting with the spy here to make sure that it behaves correctly. In this example we:

  • Used the real method spiedList.add() to add elements to the spiedList.
  • Stubbed the method spiedList.size() to return 100 instead of 2 using Mockito.doReturn().

5. @Captor Annotation

Next – let’s see how to use the @Captor annotation to create an ArgumentCaptor instance.

In the following example – we create an ArgumentCaptor with the old way without using @Captor annotation:

public void whenNotUseCaptorAnnotation_thenCorrect() {
    List mockList = Mockito.mock(List.class);
    ArgumentCaptor<String> arg = ArgumentCaptor.forClass(String.class);


    assertEquals("one", arg.getValue());

Let’s now make use of @Captor for the same purpose – to create an ArgumentCaptor instance:

List mockedList;

ArgumentCaptor argCaptor;

public void whenUseCaptorAnnotation_thenTheSam() {

    assertEquals("one", argCaptor.getValue());

Notice how the test becomes simpler and more readable when we take out the configuration logic.

6. @InjectMocks Annotation

Now – let’s discuss how to use @InjectMocks annotation – to inject mock fields into the tested object automatically.

In the following example – we use @InjectMocks to inject the mock wordMap into the MyDictionary dic:

Map<String, String> wordMap;

MyDictionary dic = new MyDictionary();

public void whenUseInjectMocksAnnotation_thenCorrect() {

    assertEquals("aMeaning", dic.getMeaning("aWord"));

And here is the class MyDictionary:

public class MyDictionary {
    Map<String, String> wordMap;

    public MyDictionary() {
        wordMap = new HashMap<String, String>();
    public void add(final String word, final String meaning) {
        wordMap.put(word, meaning);
    public String getMeaning(final String word) {
        return wordMap.get(word);

7. Running into NPE while using annotation

Often, we may run into NullPointerException when we try to actually use the instance annotated with @Mockor @Spy, as in the example:

public class NPETest {

    List mockedList;

    public void test() {

Most of the time this happens simply because we forgot to properly enable Mockito annotations.

So we have to keep in mind, that each time we want to use Mockito annotations we must take an extra step and initialize them as we already explained earlier.

8. Notes

Finally – here are some notes about Mockito annotations:

  • Use annotation to minimize repetitive mock creation code
  • Use annotation to make the test more readable
  • Use @InjectMocks to inject both @Spy and @Mock instances

9. Conclusion

In this quick tutorial, we showed the basics of annotations in the Mockito library.

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.

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I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:


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Rafał Borowiec

Simple, concise and useful.

To improve the readability even further I use static imports for Mockito:

import static org.mockito.Mockito.*

So instead of writing: Mockito.verify I simply write: verfiy.

If you are looking for an exercise for Mockito with annotations (@Mock and @InjectMocks) you may want to check how they can be utilized in FizzBuzz problem here:

Eugen Paraschiv

Thanks for the feedback – I was going for simple 🙂
Yeah, that’s what I did on github, in the code. Here, I included the “Mockito.” so that a first time user can see where the API lives.
Finally – I did read that post a few days back when you published it. I’ve been using FizzBuzz for years now in interviews, so it’s a nice exercise to see it tested like that. Cheers,

Ravi Nain
Ravi Nain

Can we use @InjectMocks for private fields?

Grzegorz Piwowarek
Grzegorz Piwowarek

Well, it depends about which use-case you are asking about. This StackOverflow thread should explain it nicely: