Master Spring Boot Annotations: @SpringBootApplication, @RestController, @Autowired & More

Spring Boot provides many annotations that simplify configuration and development of Java applications. Below is a detailed overview of the most frequently used annotations.

@SpringBootApplication

This annotation marks the entry point of a Spring Boot application. It combines @Configuration, @EnableAutoConfiguration, and @ComponentScan, reducing boilerplate configuration.

@RestController

Indicates that a class is a RESTful controller, equivalent to @Controller combined with @ResponseBody.

@RestController
@RequestMapping("/api")
public class UserController {
    @Autowired
    private UserService userService;

    @GetMapping("/users")
    public List<User> getUsers() {
        return userService.getUsers();
    }

    @PostMapping("/users")
    public User addUser(@RequestBody User user) {
        return userService.addUser(user);
    }
}

@RequestMapping

Used to map HTTP requests to handler methods or classes, specifying path, method, and parameters.

@RestController
@RequestMapping("/api")
public class UserController {
    // ...
    @GetMapping("/users/{id}")
    public User getUserById(@PathVariable("id") Long id) {
        return userService.getUserById(id);
    }
}

@Autowired

Enables automatic injection of beans by type, applicable to constructors, fields, or method parameters.

@Service
public class UserServiceImpl implements UserService {
    @Autowired
    private UserRepository userRepository;

    // ...
}

@Value

Injects values from properties or YAML files, or literal constants, into fields.

@Component
public class AppConfig {
    @Value("${app.name}")
    private String appName;

    @Value("${app.version}")
    private String appVersion;

    // ...
}

@Component

Marks a class as a Spring component, allowing it to be detected and registered in the application context.

@Component
public class MyComponent {
    // ...
}

@Service

Designates a class as a service layer component, typically used together with DAO components.

@Service
public class UserServiceImpl implements UserService {
    // ...
}

@Repository

Indicates a class is a data access component, usually interfacing with the database.

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // ...
}

@Configuration

Marks a class as a source of bean definitions, replacing XML configuration files.

@Configuration
public class AppConfig {
    // ...
}

These annotations together form the core of Spring Boot development, enabling rapid setup and clean code organization.