@MockBean for mocking dependencies in Spring Boot - Mini Project: Build & Apply

Using @MockBean to Mock Dependencies in Spring Boot Tests

📖 Scenario: You are building a simple Spring Boot application that manages books. You want to write a test for the BookService class, but it depends on BookRepository which accesses the database. To test BookService without using the real database, you will use @MockBean to create a fake BookRepository.

🎯 Goal: Write a Spring Boot test class that uses @MockBean to mock the BookRepository dependency. Then write a test method that verifies BookService returns the expected book title.

📋 What You'll Learn

Create a Book class with id and title fields

Create a BookRepository interface with a method findById(Long id)

Create a BookService class that uses BookRepository to get a book by id

Write a test class using @SpringBootTest and @MockBean to mock BookRepository

Write a test method that mocks findById to return a sample book and asserts the service returns the correct title

💡 Why This Matters

🌍 Real World

Mocking dependencies in tests helps you isolate the code you want to test. This avoids using real databases or external services, making tests faster and more reliable.

💼 Career

Understanding @MockBean is essential for writing effective unit and integration tests in Spring Boot applications, a common requirement in Java backend development jobs.

1

Create the Book class and BookRepository interface

Create a class called Book with Long id and String title fields and their getters. Then create an interface called BookRepository with a method Optional findById(Long id).

Spring Boot

// Create Book class with id and title fields
// Create BookRepository interface with findById method

Need a hint?

Use a simple Java class for Book with constructor and getters. The repository is an interface with one method returning Optional.

2

Create the BookService class with a dependency on BookRepository

Create a class called BookService with a private final BookRepository bookRepository field. Add a constructor to inject BookRepository. Add a method String getBookTitle(Long id) that returns the book title by calling bookRepository.findById(id) and returning the title or "Not Found" if empty.

Spring Boot

import java.util.Optional;

// Create BookService class with BookRepository dependency
// Add getBookTitle method

Need a hint?

Inject BookRepository via constructor. Use Optional.map() to get title or return "Not Found".

3

Create the test class and mock BookRepository with @MockBean

Create a test class called BookServiceTest annotated with @SpringBootTest. Inside, declare a @MockBean field of type BookRepository named bookRepository. Also declare an @Autowired field of type BookService named bookService. Write a test method testGetBookTitle() annotated with @Test.

Spring Boot

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

// Create BookServiceTest class annotated with @SpringBootTest
// Add @MockBean BookRepository bookRepository
// Add @Autowired BookService bookService
// Add test method testGetBookTitle

Need a hint?

Use @MockBean to create a fake BookRepository and @Autowired to inject BookService. Add a test method annotated with @Test.

4

Mock findById and assert getBookTitle returns expected title

Inside the testGetBookTitle() method, use when(bookRepository.findById(1L)) to return Optional.of(new Book(1L, "Spring in Action")). Then call bookService.getBookTitle(1L) and assert it equals "Spring in Action" using assertEquals.

Spring Boot

    @Test
    public void testGetBookTitle() {
        // Mock bookRepository.findById to return a Book with id 1L and title "Spring in Action"
        // Call bookService.getBookTitle(1L) and assert it equals "Spring in Action"
    }

Need a hint?

Use Mockito's when(...).thenReturn(...) to mock the repository method. Use assertEquals to check the service returns the mocked title.