Spring Stereotype Annotations

1. Overview

In Spring Framework, the following annotations are used to define beans and represent different layers:

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

All are specializations of @Component.

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2. @Component

Generic annotation to declare a Spring-managed bean.

Usage:

Used when class does not belong to a specific layer.

@Component
public class UtilityClass { }

Key Points:

• Base annotation
• Enables component scanning
• No special behavior

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3. @Service

Represents business logic layer.

@Service
public class PaymentService { }

Key Points:

• Used for business logic
• Supports AOP (transactions, etc.)
• Improves readability

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4. @Repository

Represents persistence layer (database interaction).

@Repository
public class UserRepository { }

Key Points:

• Used with JPA/JDBC
• Provides exception translation

Exception Translation:

• Converts SQL exceptions into Spring exceptions
• Example: SQLException → DataAccessException

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5. @Controller

Used in Spring MVC to handle HTTP requests.

@Controller
public class UserController {

    @GetMapping("/users")
    public String getUsers() {
        return "users";
    }
}

Key Points:

• Returns view (HTML/JSP)
• Used in web layer

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6. @RestController

Combination of @Controller and @ResponseBody.

@RestController
public class UserController {

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

Key Points:

• Returns JSON response
• Used in REST APIs

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7. Internal Hierarchy

@Component ├── @Service ├── @Repository └── @Controller

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8. Key Differences

Annotation	Layer	Purpose	Special Feature
@Component	Generic	Any bean	No special behavior
@Service	Business	Business logic	AOP support
@Repository	Persistence	DB access	Exception translation
@Controller	Web	HTTP handling	MVC support
@RestController	Web API	REST APIs	JSON response

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9. Real-World Flow

Controller → Service → Repository → Database

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10. Why Not Use Only @Component?

Problems:

• No clear architecture
• Hard to maintain
• Lose special features

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11. Best Practices

• Use layer-specific annotations
• Keep separation of concerns
• Apply transactions in service layer
• Use repository only for DB operations

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Summary

All annotations are specializations of @Component used to represent different layers in an application.

• @Component → Generic bean
• @Service → Business logic
• @Repository → Database layer
• @Controller → Web layer
• @RestController → REST APIs

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