Spring Boot Annotations – Detailed Explanation

Spring Boot annotations are metadata that provide instructions to the Spring framework on how to configure, initialize, and manage application components. They minimize configuration effort and improve development speed.

1. Core Spring Boot Annotations

1.1 @SpringBootApplication

This is the main annotation used to mark a Spring Boot application.

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

Internally it combines:

• @Configuration
• @EnableAutoConfiguration
• @ComponentScan

1.2 @EnableAutoConfiguration

Automatically configures beans based on:

• Classpath dependencies
• Existing beans
• Application properties

Example: If spring-boot-starter-web is present, Spring Boot automatically configures Tomcat and DispatcherServlet.

1.3 @ComponentScan

Used to scan and register Spring components.

    @ComponentScan(basePackages = "com.example")
    

It detects:

• @Component
• @Service
• @Repository
• @Controller

2. Stereotype Annotations

2.1 @Component

A generic Spring-managed bean.

    @Component
    public class EmailUtil {
    }
    

2.2 @Service

Represents the service layer where business logic is written.

    @Service
    public class OrderService {
    }
    

2.3 @Repository

Used for DAO layer and database interaction.

    @Repository
    public class UserRepository {
    }
    

It translates database-specific exceptions into Spring’s DataAccessException.

2.4 @Controller

Used in Spring MVC applications to return views.

    @Controller
    public class PageController {
    }
    

2.5 @RestController

Used for RESTful web services.

    @RestController
    public class UserController {
    }
    

It is a combination of @Controller and @ResponseBody.

3. Dependency Injection Annotations

3.1 @Autowired

Injects dependent objects automatically.

    @Autowired
    private UserService userService;
    

Types of injection:

• Field Injection
• Constructor Injection (Recommended)
• Setter Injection

3.2 @Qualifier

Used when multiple beans of the same type exist.

    @Autowired
    @Qualifier("mysqlDB")
    private DatabaseService dbService;
    

3.3 @Primary

Marks a default bean when multiple beans are available.

    @Primary
    @Service
    public class OracleDBService implements DatabaseService {
    }
    

4. Configuration & Property Annotations

4.1 @Configuration

Defines a configuration class.

    @Configuration
    public class AppConfig {
    }
    

4.2 @Bean

Used to create and configure a Spring bean manually.

    @Bean
    public ModelMapper modelMapper() {
        return new ModelMapper();
    }
    

4.3 @Value

Injects values from application.properties.

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

4.4 @ConfigurationProperties

Used to bind a group of related properties.

    @ConfigurationProperties(prefix = "db")
    public class DBConfig {
        private String url;
        private String username;
    }
    

5. Web & REST Annotations

5.1 @RequestMapping

Maps HTTP requests to handler methods.

    @RequestMapping("/api/users")
    

5.2 HTTP Method Annotations

    @GetMapping("/users")
    @PostMapping("/users")
    @PutMapping("/users/{id}")
    @DeleteMapping("/users/{id}")
    

5.3 @PathVariable

Extracts values from the URL path.

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

5.4 @RequestParam

Extracts query parameters from URL.

    @GetMapping("/users")
    public List getUsers(@RequestParam int page) {
    }
    

5.5 @RequestBody

Maps HTTP request body to a Java object.

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

5.6 @ResponseBody

Returns data directly in the response body.

    @ResponseBody
    public String hello() {
        return "Hello";
    }
    

6. JPA & Database Annotations

@Entity

    @Entity
    public class User {
    }
    

@Id & @GeneratedValue

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

@ManyToOne

    @ManyToOne
    @JoinColumn(name = "dept_id")
    private Department department;
    

7. Transaction & AOP Annotations

@Transactional

Manages database transactions automatically.

    @Transactional
    public void saveOrder() {
    }
    

8. Validation Annotations

    @NotNull
    @NotEmpty
    @NotBlank
    @Size(min = 5, max = 10)
    @Email
    

9. Scheduling & Async Annotations

@Scheduled

    @Scheduled(fixedRate = 5000)
    public void runTask() {
    }
    

@Async

    @Async
    public void sendEmail() {
    }
    

10. Testing Annotations

@SpringBootTest

Loads full application context for integration testing.

@MockBean

    @MockBean
    private UserService userService;
    

Summary