Using the @Autowired annotation to inject beans

This project is based on chapter 3.2. Using the @Autowired annotation to inject beans from book Spring Starts here (2021) by Laurentiu Spilca.

Class diagram

Create Maven project with Intellij Idea

File > New project > Java

Add Spring Context dependency

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>6.1.10</version>
</dependency>

Create entities

public class Book {
    private final String title;
    private final Author author;

    public Book(String title, Author author) {
        this.title = title;
        this.author = author;
    }

    public String getTitle() {
        return title;
    }

    public Author getAuthor() {
        return author;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        Book book = (Book) obj;
        return title.equals(book.title) && author.equals(book.author);
    }

    @Override
    public int hashCode() {
        return title.hashCode() + author.hashCode();
    }
}

public class Author {
    private final String name;

    public Author(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        Author author = (Author) obj;
        return name.equals(author.name);
    }

    @Override
    public int hashCode() {
        return name.hashCode();
    }
}

Create repositories

• use @Repository annotation to mark classes as bean

@Repository
public class AuthorRepository {
    public Author getAuthor() {
        return new Author("F. Scott Fitzgerald");
    }
}

@Repository
public class BookRepository {
    public Book getBook(Author author) {
        return new Book("The Great Gatsby", author);
    }
}

Create service

• use @Autowired annotation to inject BookRepository through the class field
• use @Autowired annotation to inject AuthorRepository through the setter method
• use @Service annotation to mark class as bean

@Service
public class BookService {
    @Autowired
    private BookRepository bookRepository;
    private AuthorRepository authorRepository;

    @Autowired
    public void setAuthorRepository(AuthorRepository authorRepository) {
        this.authorRepository = authorRepository;
    }

    public Book getBook() {
        final Author author = authorRepository.getAuthor();
        return bookRepository.getBook(author);
    }
}

Create controller

• use @Autowired annotation to inject BookService through the constructor parameter
• use @Controller annotation to mark class as bean

@Controller
public class BookController {
    private final BookService bookService;

    @Autowired
    public BookController(BookService bookService) {
        this.bookService = bookService;
    }

    public Book getBook() {
        return bookService.getBook();
    }
}

Create configuration class

• use @ComponentScan annotation to configure component scan

@Configuration
@ComponentScan(basePackages = "org.example")
public class ApplicationConfiguration {
}

Create Spring context

ApplicationContext context = new AnnotationConfigApplicationContext(ApplicationConfiguration.class);

Get bean from Spring context

Book book = context.getBean(BookController.class).getBook();
System.out.println("The book is: " + book.getTitle() + " by " + book.getAuthor().getName());

Create Tests

Create unit tests for repositories

• AuthorRepository
• BookRepository

Add dependency for JUnit

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.11.0-M2</version>
    <scope>test</scope>
</dependency>

Create test

• use annotations @Test and @DisplayName to mark test methods
• use assertEquals method to compare expected and actual values

class AuthorRepositoryTest {

    @Test
    @DisplayName("Fetches the author")
    void getAuthor() {
        AuthorRepository authorRepository = new AuthorRepository();

        Author actual = authorRepository.getAuthor();
        Author expected = new Author("F. Scott Fitzgerald");

        assertEquals(expected, actual);
    }
}

Create unit test for service and controller with mocked dependencies

Add dependency for Mockito framework

• allows creating mock objects

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

Add dependency for Mockito JUnit 5 support

• allows to initialize mocks in JUnit 5 tests

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

Create test

• use @ExtendWith(MockitoExtension.class) annotation to add Mockito JUnit 5 support
• use @Mock annotation to mark fields that should be mocked

@ExtendWith(MockitoExtension.class)
class BookServiceTest {
    @Mock
    private BookRepository bookRepository;

    @Mock
    private AuthorRepository authorRepository;

    @Test
    @DisplayName("Combines the author and book to get the book")
    void getBook() {
        given(authorRepository.getAuthor()).willReturn(new Author("author"));
        given(bookRepository.getBook(new Author("author"))).willReturn(new Book("book", new Author("author")));

        BookService bookService = new BookService(bookRepository, authorRepository);

        Book actual = bookService.getBook();
        Book expected = new Book("book", new Author("author"));

        assertEquals(expected, actual);
    }
}

Create integration test for controller

Add dependency for Spring TestContext Framework

Spring TestContext Framework provides support for loading Spring ApplicationContext and beans in tests.

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-test</artifactId>
    <version>6.1.10</version>
    <scope>test</scope>
</dependency>

Create test

• use @ExtendWith(SpringExtension.class) annotation to add Spring TestContext Framework support
• use @ContextConfiguration annotation to specify the configuration class

@ExtendWith(SpringExtension.class)
@ContextConfiguration(classes = ApplicationConfiguration.class)
public class IntegrationTest {
    @Autowired
    private BookController bookController;

    @Test
    void fetchesBook() {
        Book actual = bookController.getBook();
        Book expected = new Book("The Great Gatsby", new Author("F. Scott Fitzgerald"));

        assertEquals(expected, actual);
    }
}