Spring Boot Annotation Guide

🌱 Spring Boot Core Annotations Guide

This guide explains the most commonly used annotations in Spring Boot. Each annotation is presented with a clear definition, usage example, explanation, and tips on when to use it.

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🔧 @Bean

📝 Definition:

Marks a method that returns a Spring-managed object (bean), also known as a Spring Managed Bean — an object created, configured, and managed by the Spring IoC container.

📌 Usage:

Used inside a @Configuration class to define custom beans manually.

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

💡 Explanation:

The @Bean tells Spring:

• “Please call this method, and register the returned object (RestTemplate) as a bean in the application context.”
• Anywhere you @Autowired RestTemplate, Spring will inject this same instance.

🧠 Why use it?

Use @Bean when:

• You want full control over how the object is created.
• You need to configure third-party classes (e.g., RestTemplate, ObjectMapper, etc.) that you can’t annotate with @Component.

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🏗️ @Configuration

📝 Definition:

Marks a class that defines one or more @Bean methods. It tells Spring this class contains bean definitions to be managed by the Spring container.

📌 Usage:

Used to centralize bean creation using methods annotated with @Bean.

@Configuration
public class AppConfig {
    @Bean
    public MyService myService() {
        return new MyService();
    }
}

💡 Explanation:

• Acts as a configuration blueprint for Spring.
• Helps Spring manage lifecycle and dependencies of beans.

🧠 Why use it?

Use @Configuration when:

• You want to define multiple beans manually.
• You prefer Java-based configuration over XML.
• You need conditional or advanced bean setup logic.

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🧱 @Component

📝 Definition:

Generic annotation to mark a class as a Spring-managed bean.

📌 Usage:

Used on utility or helper classes that don’t fit into other stereotypes like @Service or @Repository.

@Component
public class EmailValidator {
    public boolean isValid(String email) {
        return email.contains("@");
    }
}

💡 Explanation:

• Automatically detected by Spring during component scanning.
• Registered as a singleton bean in the context.

🧠 Why use it?

Use @Component when:

• You want Spring to detect and manage a class automatically.
• It’s a utility, config helper, or shared logic class.

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🧠 @Service

📝 Definition:

Specialization of @Component used to mark service-layer classes that contain business logic.

📌 Usage:

Used for classes that implement business operations or domain logic.

@Service
public class PaymentService {
    public void processPayment() {
        // business logic
    }
}

💡 Explanation:

• Discovered by Spring during scanning.
• Injected where needed for business operations.

🧠 Why use it?

Use @Service when:

• The class holds core business logic.
• You want clear separation between business and infrastructure layers.

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🗄️ @Repository

📝 Definition:

Specialization of @Component for data access logic (DAO layer). Enables Spring's automatic exception translation.

📌 Usage:

Used on classes or interfaces that interact with the database.

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {}

💡 Explanation:

• Handles low-level persistence exceptions.
• Translates JDBC/ORM exceptions to Spring's DataAccessException.

🧠 Why use it?

Use @Repository when:

• You interact with the database.
• You want Spring to handle exception translation.

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🌐 @Controller

📝 Definition:

Marks a class as a Spring MVC controller responsible for handling web requests and returning views.

📌 Usage:

Used in MVC-based applications to return HTML (e.g., Thymeleaf, JSP).

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

💡 Explanation:

• Maps HTTP requests to methods.
• Returns view names for rendering.

🧠 Why use it?

Use @Controller when:

• Building a server-side rendered web app.
• Returning HTML templates.

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📡 @RestController

📝 Definition:

A convenience annotation that combines @Controller and @ResponseBody. Returns data (e.g., JSON) instead of views.

📌 Usage:

Used for building RESTful API endpoints.

@RestController
public class ApiController {
    @GetMapping("/status")
    public String status() {
        return "OK";
    }
}

💡 Explanation:

• Treats all methods as if they had @ResponseBody.
• Automatically serializes return values to JSON or XML.

🧠 Why use it?

Use @RestController when:

• Building REST APIs.
• Returning data instead of HTML.

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🧾 Tip: @Component, @Service, @Repository, and @Controller are auto-detected via component scanning.

@Bean and @Configuration are used for manual bean definitions.

📋 Spring Boot Annotations Summary

Annotation	Layer/Type	Purpose	Auto-Detected	Returns Data	Usage Context
@Bean	Configuration	Declares a manually created Spring bean	❌ No	✅ Yes	Used in @Configuration classes
@Configuration	Configuration	Declares a class containing @Bean definitions	✅ Yes	❌ No	Java-based configuration
@Component	Generic Bean	Generic stereotype for any Spring-managed class	✅ Yes	❌ No	Utility classes, general-purpose
@Service	Service Layer	Marks a class that holds business logic	✅ Yes	❌ No	Business logic layer
@Repository	Data Access	DAO layer, adds exception translation for persistence	✅ Yes	❌ No	Database access classes
@Controller	Web MVC	Handles web requests and returns HTML views	✅ Yes	❌ No	Server-side rendered web apps
@RestController	Web API (REST)	Combines @Controller + @ResponseBody to return JSON/XML	✅ Yes	✅ Yes	REST API endpoints