Day-100,101 19-Essential Spring Boot Annotations

1.@SpringBootApplication

• Marks the main class of a Spring Boot application.
• Enables auto-configuration,component scanning, and configuration setup.

@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }
}

2.@RestController

• Marks a class as a REST API controller.
• Combines @Controller + @ResponseBody, so responses are returned as JSON.

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello Spring Boot!";
    }
}

3.@Controller

• Marks a class as a Spring MVC controller.
• Used for serving web pages or templates (like Thymeleaf).

@Controller
public class PageController {
    @GetMapping("/home")
    public String homePage() {
        return "home"; // returns home.html
    }
}

4.@GetMapping

• Maps HTTP GET requests to a controller method.

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

5.@PostMapping

• Maps HTTP POST requests to a method.

@PostMapping("/users")
public User createUser(@RequestBody User user) { ... }

6.@PutMapping

• Maps HTTP PUT requests to a method for updating resources.

@PutMapping("/users/{id}")
public User updateUser(@PathVariable Long id, @RequestBody User user) { ... }

7.@DeleteMapping

• Maps HTTP DELETE requests to a method for deleting resources.

@DeleteMapping("/users/{id}")
public void deleteUser(@PathVariable Long id) { ... }

8.@Autowired

• Automatically injects Spring-managed beans.

@Autowired
private UserService userService;

9.@Service

• Marks a class as a service layer bean containing business logic.

@Service
public class UserService { ... }

10.@Repository

• Marks a class as a data access layer (DAO).
• Supports exception translation for database operations.

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

11.@Component

• Generic annotation for any Spring-managed bean.

@Component
public class Utility { ... }

12.@Entity

• Marks a class as a JPA entity mapping to a database table.

@Entity
public class User { ... }

13.@ Id

• Marks the primary key in a JPA entity.

@Id
private Long id;

14.@GeneratedValue

• Configures automatic generation of primary key values (e.g., auto-increment).

@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

15.@Configuration

• Marks a class as a source of bean definitions.

@Configuration
public class AppConfig { ... }

16.@ Bean

• Declares a bean inside a configuration class; managed by Spring.

@Bean
public RestTemplate restTemplate() {
    return new RestTemplate();
}

17.@RequestBody

• Binds HTTP request body (JSON) to a Java object.

@PostMapping("/users")
public User createUser(@RequestBody User user) { ... }

18.@PathVariable

• Binds a URI path variable to a method parameter.

@GetMapping("/users/{id}")
public User getUser(@PathVariable Long id) { ... }

19.@RequestParam

• Binds query parameters or form data to a method parameter.

@GetMapping("/users")
public List<User> getUsers(@RequestParam int page, @RequestParam int size) { ... }

Conclusion

For freshers, mastering these 19 annotations is enough to handle most Spring Boot interview questions. They cover:

• Application setup
• REST controllers
• Dependency injection
• Service & repository layers
• JPA / database mapping
• Request handling