What is Inversion of control in Spring boot. Explain with example

Inversion of Control (IoC) is a design pattern used in software development, including in the popular Spring Boot framework. At its core, IoC is a way of structuring code so that the control of object creation and management is passed to an external component, known as a container. In Spring Boot, this container is called the Spring container or ApplicationContext.

The Spring container is responsible for creating, configuring, and managing objects, known as beans, which are the building blocks of a Spring Boot application. By delegating the responsibility of object creation and management to the Spring container, the codebase becomes more modular, testable, and maintainable.

IoC is implemented in Spring Boot through a technique called Dependency Injection (DI). DI is a way of injecting the dependencies of an object, such as other objects or resources, into it at runtime, rather than the object creating and managing its own dependencies.

Let’s take an example to understand how IoC and DI work in Spring Boot. Suppose we have a simple web application that displays a list of books. The application has a BookService class that provides a list of books to the BookController class, which then renders the list on a web page.

In a traditional programming approach, the BookService would create its own dependencies, such as a BookRepository, to fetch the list of books. The BookController would then create a new instance of the BookService and call its methods to get the list of books. This approach tightly couples the classes together, making the code hard to test and maintain.

With IoC and DI, the Spring container manages the dependencies of the BookService and injects them when the BookService is created. The BookController is also created by the Spring container and is injected with a BookService instance. This approach is more flexible, testable, and maintainable than the previous approach.

To implement IoC and DI in Spring Boot, we can use annotations such as @Autowired, @Component, @Service, and @Controller to identify and configure beans. The @Autowired annotation tells the Spring container to inject a bean’s dependencies, while the @Component, @Service, and @Controller annotations tell the container which classes should be instantiated and managed.

Here’s an example of how this can be achieved in Spring Boot:

javaCopy code@Service
public class BookService {
  private final BookRepository bookRepository;

  public BookService(BookRepository bookRepository) {
    this.bookRepository = bookRepository;

  public List<Book> getBooks() {
    return bookRepository.findAll();

public class BookController {
  private final BookService bookService;

  public BookController(BookService bookService) {
    this.bookService = bookService;

  public String getAllBooks(Model model) {
    List<Book> books = bookService.getBooks();
    model.addAttribute("books", books);
    return "books";

In the above code, the @Service and @Controller annotations tell Spring that these classes should be managed by the Spring container. The @Autowired annotations in the constructors tell Spring to inject the required dependencies into the beans at runtime.

In conclusion, IoC and DI are powerful techniques that make Spring Boot applications more modular, testable, and maintainable. By delegating the responsibility of object creation and management to the Spring container, we can focus on writing clean, concise, and efficient code that meets our business requirements.

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