Spring Boot Annotations

1. Introduction

Annotations in Spring Boot are used to configure applications directly in Java code.

They help to:

Reduce XML configuration
Improve readability
Enable auto-configuration
Simplify dependency injection

2. Core Spring Boot Annotations

@SpringBootApplication

Main annotation for Spring Boot application
Combines:
- @SpringBootConfiguration
- @EnableAutoConfiguration
- @ComponentScan

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@SpringBootApplication
public class DemoApplication {

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

@SpringBootConfiguration

Used for configuration class
Similar to @Configuration

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@SpringBootConfiguration
public class AppConfig {
}

@EnableAutoConfiguration

Enables auto configuration
Automatically configures beans

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@EnableAutoConfiguration
public class AppConfig {
}

@ComponentScan

Scans package for components

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@ComponentScan("com.example")
public class AppConfig {
}

3. Spring Bean Annotations

@Component

Marks class as Spring bean

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@Component
public class EmailService {
}

@Service

Used in service layer

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@Service
public class UserService {
}

@Repository

Used for database layer

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@Repository
public class UserRepository {
}

@Configuration

Used for configuration class

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@Configuration
public class AppConfig {
}

@Bean

Used to create bean manually

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@Configuration
public class AppConfig {

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

4. Dependency Injection Annotations

@Autowired

Automatically injects dependency

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@Autowired
private UserService userService;

@Qualifier

Specifies which bean to inject

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@Autowired
@Qualifier("emailService")
private NotificationService service;

@Primary

Default bean when multiple exist

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@Primary
@Component
public class SmsService implements NotificationService {
}

5. Web and REST API Annotations

@RestController

Used to create REST APIs

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@RestController
public class HelloController {
}

@RequestMapping

Maps URL path

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@RequestMapping("/api")
public class ApiController {
}

HTTP Method Annotations

@GetMapping → Fetch data
@PostMapping → Create data
@PutMapping → Update data
@DeleteMapping → Delete data

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

@PathVariable

Get value from URL

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@GetMapping("/users/{id}")
public User getUser(@PathVariable int id) {
    return userService.getUser(id);
}

@RequestParam

Read query parameters

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@GetMapping("/search")
public String search(@RequestParam String keyword) {
    return keyword;
}

@RequestBody

Read request body

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

6. Configuration Annotations

@Value

Inject value from properties

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@Value("${server.port}")
private String port;

@ConfigurationProperties

Bind properties to class

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@ConfigurationProperties(prefix = "app")
public class AppConfig {
    private String name;
}

7. Validation Annotations

@NotNull → Cannot be null
@NotBlank → Cannot be empty
@Email → Valid email
@Size → Size limit

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public class User {

    @NotBlank
    private String username;

    @Email
    private String email;
}

8. Exception Handling Annotations

@ExceptionHandler

Handle specific exception

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@ExceptionHandler(Exception.class)
public String handleException() {
    return "Error occurred";
}

@ControllerAdvice

Global exception handling

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@ControllerAdvice
public class GlobalExceptionHandler {
}

9. JPA Annotations

@Entity → Define entity class
@Table → Map table
@Id → Primary key
@GeneratedValue → Auto generate ID
@Column → Map column

10. Conclusion

Annotations reduce configuration complexity
Widely used in Spring Boot applications
Improve code readability and maintainability