@MockBean for Spring mocking in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Spring test with @MockBean?

Consider this Spring Boot test class using @MockBean to mock a service. What will be the printed output when the test runs?

JUnit

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import static org.mockito.BDDMockito.given;

@SpringBootTest
class UserServiceTest {

    @MockBean
    private UserRepository userRepository;

    @Autowired
    private UserService userService;

    @Test
    void testGetUserName() {
        given(userRepository.findNameById(1L)).willReturn("Alice");
        System.out.println(userService.getUserName(1L));
    }
}

@Service
class UserService {
    @Autowired
    private UserRepository userRepository;

    public String getUserName(Long id) {
        return userRepository.findNameById(id);
    }
}

interface UserRepository {
    String findNameById(Long id);
}

A null

B Alice

C Throws NullPointerException

D Compilation error due to missing bean

💡 Hint

Think about what @MockBean does and how given(...).willReturn(...) affects the mock.

The @MockBean annotation creates a mock of UserRepository and injects it into the Spring context. The given(...).willReturn("Alice") sets the mock to return "Alice" when findNameById(1L) is called. So, the userService.getUserName(1L) call prints "Alice".

❓ assertion

intermediate

Which assertion correctly verifies a mocked method call with @MockBean?

You have a @MockBean for PaymentService. Which assertion correctly verifies that processPayment() was called exactly once?

JUnit

import static org.mockito.Mockito.*;

@MockBean
PaymentService paymentService;

// In test method:
paymentService.processPayment(100);

A verify(paymentService, times(1)).processPayment(100);

B assertEquals(1, paymentService.processPayment(100));

C assertTrue(paymentService.processPayment(100));

D verify(paymentService).processPayment(anyInt());

💡 Hint

Remember how Mockito verifies method calls on mocks.

The verify() method with times(1) checks that processPayment(100) was called exactly once. Option A does not specify times(1), so it passes if called any number of times. Options A and B are incorrect because they assert return values, not method calls.

🔧 Debug

advanced

Why does this test fail with NullPointerException despite using @MockBean?

Given this test, why does it throw a NullPointerException?

JUnit

import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import static org.mockito.BDDMockito.given;

@SpringBootTest
class OrderServiceTest {

    @MockBean
    private PaymentService paymentService;

    private OrderService orderService = new OrderService();

    @Test
    void testOrder() {
        given(paymentService.pay(50)).willReturn(true);
        boolean result = orderService.placeOrder(50);
        System.out.println(result);
    }
}

class OrderService {
    private PaymentService paymentService;

    public boolean placeOrder(int amount) {
        return paymentService.pay(amount);
    }
}

interface PaymentService {
    boolean pay(int amount);
}

A OrderService.paymentService is null because it is not injected by Spring

B The @MockBean annotation is missing on OrderService

C given() is called on a null object causing NullPointerException

D SpringBootTest does not support @MockBean in this context

💡 Hint

Check how the OrderService instance is created and how dependencies are injected.

The OrderService instance is created manually with new and not managed by Spring. Therefore, its paymentService field is null. The @MockBean creates a mock and injects it into Spring-managed beans only. So calling paymentService.pay() causes NullPointerException.

🧠 Conceptual

advanced

What is the main difference between @Mock and @MockBean in Spring tests?

Choose the best explanation of the difference between @Mock and @MockBean in Spring Boot testing.

A @MockBean automatically verifies method calls; @Mock requires manual verification

B @MockBean is used only for integration tests; @Mock is used only for unit tests

C @MockBean creates a spy object; @Mock creates a pure mock

D @Mock creates a mock without Spring context integration; @MockBean creates a mock and injects it into the Spring context

💡 Hint

Think about how Spring manages beans and how mocks are injected.

@Mock creates a mock object but does not integrate with Spring's application context. @MockBean creates a mock and replaces the bean in the Spring context, so Spring injects the mock wherever needed.

❓ framework

expert

Which Spring test configuration ensures @MockBean mocks are injected properly in nested test classes?

You have nested test classes inside a Spring Boot test. Which configuration ensures @MockBean mocks are injected correctly in all nested classes?

A Use @SpringBootTest only on nested classes and @MockBean only on the outer class

B Annotate each nested test class separately with @SpringBootTest and @MockBean

C Annotate the outer test class with @SpringBootTest and use @TestInstance(Lifecycle.PER_CLASS)

D Use @MockBean only in the outer class and annotate nested classes with @ExtendWith(MockitoExtension.class)

💡 Hint

Consider how Spring manages context and test instance lifecycle.

Annotating the outer class with @SpringBootTest loads the Spring context once. Using @TestInstance(Lifecycle.PER_CLASS) allows nested test classes to share the same test instance and context, so @MockBean mocks are injected properly in nested classes.