@MockBean for mocking dependencies in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mock a dependency using @MockBean in a Spring Boot test.

Spring Boot

@SpringBootTest
public class UserServiceTest {

    @[1]
    private UserRepository userRepository;

    @Autowired
    private UserService userService;

}

Drag options to blanks, or click blank then click option'

AService

BAutowired

CInjectMocks

DMockBean

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Autowired instead of @MockBean for mocking.

Confusing @InjectMocks with Spring annotations.

The @MockBean annotation is used to create and inject a mock of the UserRepository into the Spring test context.

2fill in blank

medium

Complete the code to define behavior for the mocked repository method.

Spring Boot

when(userRepository.[1](anyLong())).thenReturn(Optional.of(new User()));

Drag options to blanks, or click blank then click option'

Asave

BfindAll

CfindById

DdeleteById

Attempts:

3 left

💡 Hint

Common Mistakes

Using save or deleteById which do not return Optional.

Using findAll which returns a list, not Optional.

The findById method is used to fetch a user by ID, which is commonly mocked in tests.

3fill in blank

hard

Fix the error in the test annotation to enable Mockito support.

Spring Boot

@SpringBootTest
@[1]
public class ProductServiceTest {

    @MockBean
    private ProductRepository productRepository;

}

Drag options to blanks, or click blank then click option'

ARunWith(SpringRunner.class)

BExtendWith(MockitoExtension.class)

CEnableMocking

DMockitoTest

Attempts:

3 left

💡 Hint

Common Mistakes

Using MockitoExtension which is for pure Mockito tests, not Spring Boot tests.

Using non-existent annotations like EnableMocking.

The @RunWith(SpringRunner.class) annotation integrates Spring TestContext Framework with JUnit and enables Mockito support.

4fill in blank

hard

Fill both blanks to mock a service bean and inject it into the test class.

Spring Boot

@SpringBootTest
public class OrderServiceTest {

    @[1]
    private PaymentService paymentService;

    @Autowired
    private OrderService orderService;

    @Test
    public void testOrder() {
        when(paymentService.[2](anyDouble())).thenReturn(true);
    }
}

Drag options to blanks, or click blank then click option'

AMockBean

BAutowired

CprocessPayment

DcancelPayment

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Autowired instead of @MockBean for mocking.

Mocking the wrong method name.

Use @MockBean to mock PaymentService and define behavior for processPayment method.

5fill in blank

hard

Fill all three blanks to mock a repository, define a method behavior, and verify interaction.

Spring Boot

@SpringBootTest
public class CustomerServiceTest {

    @[1]
    private CustomerRepository customerRepository;

    @Autowired
    private CustomerService customerService;

    @Test
    public void testFindCustomer() {
        when(customerRepository.[2](anyString())).thenReturn(Optional.of(new Customer()));
        customerService.findCustomerByEmail("[e-mail]");
        verify(customerRepository).[3]("[e-mail]");
    }
}

Drag options to blanks, or click blank then click option'

AMockBean

BfindByEmail

Dsave

Attempts:

3 left

💡 Hint

Common Mistakes

Using save instead of findByEmail for mocking and verification.

Not using @MockBean for mocking the repository.

Use @MockBean to mock the repository, mock findByEmail method, and verify it was called with the email.

Practice

1. What is the main purpose of using @MockBean in a Spring Boot test?

easy

A. To replace a real Spring bean with a mock for isolated testing

B. To create a new Spring bean in the application context

C. To automatically inject real dependencies into the test

D. To configure database connections for integration tests

Solution

1. Step 1: Understand the role of @MockBean

   @MockBean is used to replace a real bean in the Spring context with a mock version during tests.

2. Step 2: Identify the testing benefit

   This replacement helps isolate the component under test by faking dependencies, avoiding real implementations.

3. Final Answer:

   To replace a real Spring bean with a mock for isolated testing -> Option A

4. Quick Check:

   @MockBean replaces real beans [OK]

Hint: Remember: @MockBean fakes dependencies for focused tests [OK]

Common Mistakes:

• Thinking @MockBean creates new beans instead of replacing
• Confusing @MockBean with @Autowired injection
• Using @MockBean to configure real database connections

2. Which of the following is the correct way to declare a mock bean for a service named UserService in a Spring Boot test?

easy

A. @MockBean UserService userService = new UserService();

B. @MockBean private UserService userService;

C. @MockBean public void userService();

D. @MockBean UserService userService();

Solution

1. Step 1: Recall @MockBean declaration syntax

   @MockBean is used as an annotation on a field to create a mock instance injected by Spring.

2. Step 2: Check each option

   @MockBean private UserService userService; correctly declares a private field annotated with @MockBean. Other options use invalid syntax such as instantiating manually with new or declaring as methods.

3. Final Answer:

   @MockBean private UserService userService; -> Option B

4. Quick Check:

   @MockBean on private field [OK]

Hint: Use @MockBean on private fields only [OK]

Common Mistakes:

• Trying to instantiate mock manually with new
• Declaring @MockBean on methods instead of fields
• Omitting access modifier or using invalid syntax

3. Given this test snippet:

@MockBean
private PaymentService paymentService;

@Test
void testProcess() {
  when(paymentService.pay()).thenReturn(true);
  boolean result = paymentService.pay();
  System.out.println(result);
}

What will be printed when the test runs?

medium

A. null

B. false

C. true

D. Exception thrown

Solution

1. Step 1: Understand mocking behavior

   The mock paymentService is told to return true when pay() is called using when().thenReturn(true).

2. Step 2: Trace the method call

   Calling paymentService.pay() returns true as configured, so result is true and printed.

3. Final Answer:

   true -> Option C

4. Quick Check:

   Mock returns configured value = true [OK]

Hint: Mock returns what you tell it with when().thenReturn() [OK]

Common Mistakes:

• Expecting default false instead of configured true
• Confusing mock with real implementation output
• Not setting up when().thenReturn() before call

4. Consider this test code snippet:

@MockBean
private OrderService orderService;

@Test
void testOrder() {
  orderService.placeOrder();
  verify(orderService).placeOrder();
}

When running this test, it fails with a NullPointerException. What is the most likely cause?

medium

A. The orderService is not mocked properly because it is private

B. The verify method is called before placeOrder()

C. The placeOrder() method is final and cannot be mocked

D. The @MockBean annotation was not processed because the test class lacks @SpringBootTest

Solution

1. Step 1: Check test setup requirements

   @MockBean requires Spring Boot test context to process annotations, usually via @SpringBootTest or similar.

2. Step 2: Identify cause of NullPointerException

   If @MockBean is not processed, orderService remains null, causing NPE when calling placeOrder().

3. Final Answer:

   The @MockBean annotation was not processed because the test class lacks @SpringBootTest -> Option D

4. Quick Check:

   @MockBean needs Spring context [OK]

Hint: Always use @SpringBootTest to enable @MockBean processing [OK]

Common Mistakes:

• Assuming private fields can't be mocked
• Misordering verify and method calls
• Not realizing final methods can't be mocked (but not cause NPE here)

5. You have a Spring Boot service NotificationService that depends on EmailSender. You want to test NotificationService without sending real emails. Which setup correctly uses @MockBean to mock EmailSender and verify sendEmail() was called once during the test?

hard

A. Use @MockBean EmailSender emailSender; in the test class, call notificationService.notify(), then verify with verify(emailSender, times(1)).sendEmail();

B. Use @Autowired EmailSender emailSender; and call sendEmail() directly in the test

C. Create a new EmailSender instance manually and inject it into NotificationService

D. Use @MockBean NotificationService notificationService; and verify sendEmail() on it

Solution

1. Step 1: Mock the dependency EmailSender

   Use @MockBean on EmailSender to replace the real bean with a mock in the test context.

2. Step 2: Call the method under test and verify interaction

   Call notificationService.notify() which uses EmailSender internally, then verify sendEmail() was called once on the mock.

3. Final Answer:

   Use @MockBean EmailSender emailSender; call notificationService.notify(); verify sendEmail() called once -> Option A

4. Quick Check:

   Mock dependency, call service, verify mock call [OK]

Hint: Mock dependencies, test service, verify mock interactions [OK]

Common Mistakes:

• Mocking the service instead of its dependency
• Manually creating dependency instead of using @MockBean
• Not verifying the mock method call properly