1. Overview

In this quick tutorial, we’ll show how to integrate Mockito with the JUnit 5 extension model. To learn more about the JUnit 5 extension model, have a look at this article.

First, we’ll show how to create an extension that automatically creates mock objects for any class attribute or method parameter annotated with @Mock.

Then, we’ll use our Mockito extension in a JUnit 5 test class.

2. Maven Dependencies

Let’s add the JUnit 5 (jupiter) and mockito dependencies to our pom.xml:

<dependency>
 <groupId>org.junit.jupiter</groupId>
 <artifactId>junit-jupiter-engine</artifactId>
 <version>5.10.2</version>
 <scope>test</scope>
</dependency>
<dependency>
 <groupId>org.mockito</groupId>
 <artifactId>mockito-core</artifactId>
 <version>5.11.0</version>
 <scope>test</scope>
</dependency>

<dependency>
 <groupId>org.mockito</groupId>
 <artifactId>mockito-junit-jupiter</artifactId>
 <version>5.12.0</version>
 <scope>test</scope>
</dependency>

4. Building the Test Class

Let’s build our test class and attach the Mockito extension to it:

@ExtendWith(MockitoExtension.class)
class UserServiceUnitTest {

 UserService userService;

 // ...
}

We can use the @Mock annotation to inject a mock for an instance variable that we can use anywhere in the test class:

@Mock UserRepository userRepository;

Also, we can inject mock objects into method parameters:

@BeforeEach
void init(@Mock SettingRepository settingRepository) {
 userService = new DefaultUserService(userRepository, settingRepository, mailClient);
 
 lenient().when(settingRepository.getUserMinAge()).thenReturn(10);
 
 when(settingRepository.getUserNameMinLength()).thenReturn(4);
 
 lenient().when(userRepository.isUsernameAlreadyExists(any(String.class)))
 .thenReturn(false);
}

Please note the use of lenient() here. Mockito throws an UnsupportedStubbingException when an initialized mock is not called by one of the test methods during execution. We can avoid this strict stub checking by using this method when initializing the mocks.

We can even inject a mock object into a test method parameter:

@Test
void givenValidUser_whenSaveUser_thenSucceed(@Mock MailClient mailClient) {
 // Given
 user = new User("Jerry", 12);
 when(userRepository.insert(any(User.class))).then(new Answer<User>() {
 int sequence = 1;
 
 @Override
 public User answer(InvocationOnMock invocation) throws Throwable {
 User user = (User) invocation.getArgument(0);
 user.setId(sequence++);
 return user;
 }
 });

 userService = new DefaultUserService(userRepository, settingRepository, mailClient);

 // When
 User insertedUser = userService.register(user);
 
 // Then
 verify(userRepository).insert(user);
 assertNotNull(user.getId());
 verify(mailClient).sendUserRegistrationMail(insertedUser);
}

Note that the MailClient mock we inject as a test parameter will NOT be the same instance we injected in the init method.

5. Conclusion

Junit 5 has provided a nice model for the extension. We demonstrated a simple Mockito extension that simplified our mock creation logic.