Spring boot annotation

 A Spring Boot application consists of several key annotations that configure components, enable auto-configuration, and manage dependency injection. Below is a comprehensive list of Spring Boot annotations categorized by their purpose.

1. Core Annotations (Bootstrapping & Configuration)

Annotation	Description
@SpringBootApplication	Main entry point for a Spring Boot application. Combines @Configuration, @EnableAutoConfiguration, and @ComponentScan.
@Configuration	Marks a class as a Spring configuration class (equivalent to XML config).
@ComponentScan	Enables scanning for components (@Component, @Service, @Repository, etc.) in the package and sub-packages.
@EnableAutoConfiguration	Automatically configures Spring beans based on dependencies. (Part of @SpringBootApplication).

Example

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication

public class MySpringBootApp {

    public static void main(String[] args) {

        SpringApplication.run(MySpringBootApp.class, args);

    }

}

2. Component Scanning & Stereotypes

Annotation	Description
@Component	Generic annotation for Spring-managed beans.
@Service	Specialized @Component for business logic/service layer.
@Repository	Specialized @Component for data persistence (adds exception translation).
@Controller	Specialized @Component for Spring MVC controllers.
@RestController	Combination of @Controller and @ResponseBody, returning JSON/XML responses.

Example

import org.springframework.stereotype.Service;

@Service

public class MyService {

    public String getMessage() {

        return "Hello from MyService!";

    }

}

3. Dependency Injection (DI)

Annotation	Description
@Autowired	Injects dependencies automatically by type.
@Qualifier	Specifies which bean to inject when multiple candidates exist.
@Primary	Marks a bean as the default when multiple beans of the same type exist.
@Bean	Defines a Spring-managed bean inside a @Configuration class.

Example: Dependency Injection

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.stereotype.Component;

@Component

public class MyController {

    private final MyService myService;

    @Autowired  // Injecting MyService bean

    public MyController(MyService myService) {

        this.myService = myService;

    }

}

4. Bean Scope Management

Annotation	Description
@Scope("singleton")	Default scope; only one instance is created.
@Scope("prototype")	A new instance is created for every request.
@Scope("request")	A new bean instance per HTTP request (for web apps).
@Scope("session")	A new bean per HTTP session (for web apps).

Example

import org.springframework.context.annotation.Scope;

import org.springframework.stereotype.Service;

@Service

@Scope("prototype")  // A new instance each time it is requested

public class PrototypeService { }

5. Spring MVC & REST API Annotations

Annotation	Description
@RequestMapping	Maps HTTP requests to handler methods.
@GetMapping, @PostMapping, @PutMapping, @DeleteMapping, @PatchMapping	Specialized versions of @RequestMapping for specific HTTP methods.
@RequestBody	Maps JSON/XML request body to a Java object.
@ResponseBody	Directly returns data (JSON/XML) instead of a view.
@PathVariable	Extracts values from the URL path.
@RequestParam	Extracts query parameters from a request.

Example

import org.springframework.web.bind.annotation.*;

@RestController

@RequestMapping("/api")

public class MyRestController {

    @GetMapping("/hello/{name}")

    public String sayHello(@PathVariable String name) {

        return "Hello, " + name;

    }

}

6. Exception Handling

Annotation	Description
@ExceptionHandler	Handles exceptions at the controller level.
@RestControllerAdvice	Global exception handling for all controllers.
@ResponseStatus	Specifies the HTTP status for an exception.

Example

import org.springframework.web.bind.annotation.*;

@RestControllerAdvice

public class GlobalExceptionHandler {

    @ExceptionHandler(Exception.class)

    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)

    public String handleException(Exception ex) {

        return "Error: " + ex.getMessage();

    }

}

7. Transaction Management

Annotation	Description
@Transactional	Marks a method or class as transactional (commits or rolls back transactions).

Example

import org.springframework.stereotype.Service;

import org.springframework.transaction.annotation.Transactional;

@Service

public class TransactionalService {

    @Transactional

    public void performTransaction() {

        // Database operations here

    }

}

8. Caching

Annotation	Description
@EnableCaching	Enables Spring’s caching mechanism.
@Cacheable	Caches method results to improve performance.
@CacheEvict	Removes an entry from the cache.

Example

import org.springframework.cache.annotation.Cacheable;

import org.springframework.stereotype.Service;

@Service

public class CacheService {

    @Cacheable("items")

    public String getData(String key) {

        return "Data for " + key;

    }

}

9. Spring Security Annotations

Annotation	Description
@EnableWebSecurity	Enables Spring Security.
@Secured	Restricts access based on roles.
@PreAuthorize	More advanced access control before method execution.

Example

import org.springframework.security.access.prepost.PreAuthorize;

import org.springframework.stereotype.Service;

@Service

public class SecureService {

    @PreAuthorize("hasRole('ADMIN')")

    public String adminAccess() {

        return "Admin Only";

    }

}

10. Spring Boot Actuator Annotations

Annotation	Description
@EnableAutoConfiguration	Enables Spring Boot’s auto-configuration features.
@ConditionalOnProperty	Enables a bean based on a property in application.properties.
@ConditionalOnClass	Enables a bean if a specific class is present in the classpath.

Example

import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;

import org.springframework.stereotype.Component;

@Component

@ConditionalOnProperty(name = "feature.enabled", havingValue = "true")

public class FeatureBean { }

Conclusion

These are the most commonly used annotations in Spring Boot, covering configuration, dependency injection, REST APIs, transactions, security, caching, and more.