@Component, @Service, @Repository, @Controller in Spring Boot - Mini Project: Build & Apply

Using @Component, @Service, @Repository, and @Controller in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage books in a library. You want to organize your code using Spring stereotypes to clearly separate responsibilities.

🎯 Goal: Learn how to use @Component, @Service, @Repository, and @Controller annotations to create a well-structured Spring Boot app.

📋 What You'll Learn

Create a class annotated with @Component for a utility helper

Create a class annotated with @Service to handle business logic

Create a class annotated with @Repository to simulate data access

Create a class annotated with @Controller to handle web requests

💡 Why This Matters

🌍 Real World

Organizing code in Spring Boot applications using stereotypes helps keep code clean and maintainable, making it easier to work on large projects with teams.

💼 Career

Understanding these annotations is essential for Java developers working with Spring Boot, as they are fundamental to building scalable and well-structured backend applications.

1

Create a utility class with @Component

Create a class called BookHelper annotated with @Component inside package com.example.library.util. This class will have a method formatTitle(String title) that returns the title in uppercase.

Spring Boot

package com.example.library.util;

import org.springframework.stereotype.Component;

// Your code here

Hint

Use @Component above the class declaration to mark it as a Spring component.

2

Create a service class with @Service

Create a class called BookService annotated with @Service inside package com.example.library.service. Add a method getBookCount() that returns the integer 5.

Spring Boot

package com.example.library.service;

import org.springframework.stereotype.Service;

// Your code here

Hint

Use @Service above the class declaration to mark it as a service layer component.

3

Create a repository class with @Repository

Create a class called BookRepository annotated with @Repository inside package com.example.library.repository. Add a method findAll() that returns a List<String> with these exact book titles: "Spring Basics", "Java Fundamentals".

Spring Boot

package com.example.library.repository;

import org.springframework.stereotype.Repository;
import java.util.List;
import java.util.Arrays;

// Your code here

Hint

Use @Repository above the class declaration to mark it as a data access component.

4

Create a controller class with @Controller

Create a class called BookController annotated with @Controller inside package com.example.library.controller. Add a method showBooks() annotated with @GetMapping("/books") that returns the string "books".

Spring Boot

package com.example.library.controller;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;

// Your code here

Hint

Use @Controller above the class and @GetMapping("/books") above the method to handle web requests.

Practice

1. Which annotation is the most general one used to mark a Spring bean in a Spring Boot application?

easy

A. @Service

B. @Component

C. @Repository

D. @Controller

Solution

1. Step 1: Understand the purpose of each annotation

   @Component is the base annotation for any Spring-managed bean. @Service, @Repository, and @Controller are specialized stereotypes built on top of @Component.

2. Step 2: Identify the most general annotation

   Since @Component is the general-purpose annotation for any Spring bean, it is the correct answer.

3. Final Answer:

   @Component -> Option B

4. Quick Check:

   General Spring bean marker = @Component [OK]

Hint: General bean annotation is @Component, others are special cases [OK]

Common Mistakes:

• Confusing @Service as the general bean annotation
• Thinking @Repository is for all beans
• Assuming @Controller is the general annotation

2. Which annotation should you use to mark a class that handles HTTP requests in a Spring Boot application?

easy

A. @Repository

B. @Service

C. @Controller

D. @Component

Solution

1. Step 1: Recall the role of each annotation

   @Controller is specifically designed for classes that handle web requests and return views or data.

2. Step 2: Match the annotation to the web controller role

   Since the question asks about handling HTTP requests, @Controller is the correct annotation.

3. Final Answer:

   @Controller -> Option C

4. Quick Check:

   Web request handler = @Controller [OK]

Hint: Web controllers use @Controller annotation [OK]

Common Mistakes:

• Using @Service for web controllers
• Confusing @Repository with @Controller
• Using @Component instead of @Controller for HTTP handling

3. Consider the following Spring Boot class:

@Service
public class UserService {
    public String getUser() {
        return "Alice";
    }
}

What is the role of this class in the application?

medium

A. Contains business logic related to users

B. Accesses the database to retrieve user data

C. General Spring bean without specific role

D. Handles HTTP requests and responses

Solution

1. Step 1: Identify the annotation and its meaning

   The class is annotated with @Service, which is used for business logic components in Spring.

2. Step 2: Understand the method and class purpose

   The method getUser() returns a user name, indicating business logic rather than database access or HTTP handling.

3. Final Answer:

   Contains business logic related to users -> Option A

4. Quick Check:

   @Service = business logic class [OK]

Hint: @Service means business logic, not web or data access [OK]

Common Mistakes:

• Thinking @Service accesses the database directly
• Confusing @Service with @Controller
• Assuming @Service is a generic bean without role

4. What is wrong with this Spring Boot repository class?

@Repository
public class UserRepository {
    public void saveUser(String user) {
        // save logic
    }
}

Options: A) Class should be an interface extending JpaRepository B) Missing @Service annotation C) @Repository cannot be used on classes D) Method saveUser must return a value

medium

A. Class should be an interface extending JpaRepository

B. Missing @Service annotation

C. @Repository cannot be used on classes

D. Method saveUser must return a value

Solution

1. Step 1: Understand typical repository pattern in Spring Boot

   Spring Data JPA repositories are usually interfaces extending JpaRepository or CrudRepository to get built-in methods.

2. Step 2: Identify the issue with the class declaration

   The class is a concrete class, not an interface extending JpaRepository, so it misses Spring Data JPA features.

3. Final Answer:

   Class should be an interface extending JpaRepository -> Option A

4. Quick Check:

   Repositories usually extend JpaRepository interface [OK]

Hint: Repositories are interfaces extending JpaRepository, not plain classes [OK]

Common Mistakes:

• Thinking @Repository cannot be on classes
• Assuming @Service is needed for repositories
• Believing saveUser must return a value

5. You have a Spring Boot application with these classes:

@Component
public class UtilityHelper { }

@Service
public class OrderService { }

@Repository
public interface OrderRepository extends JpaRepository { }

@Controller
public class OrderController { }

Which statement correctly describes the roles and how Spring manages these beans?

hard

A. @Component manages data access; @Service handles web requests; @Repository is for business logic; @Controller is a utility bean

B. @Component handles web requests; @Service manages data access; @Repository is for utility beans; @Controller handles business logic

C. All annotations are interchangeable and have the same role in Spring

D. @Component is for utility beans; @Service handles business logic; @Repository manages data access; @Controller handles web requests

Solution

1. Step 1: Recall the purpose of each annotation

   @Component is a general-purpose bean, often used for utilities. @Service is for business logic. @Repository is for data access layers. @Controller is for handling web requests.

2. Step 2: Match each class role to its annotation

   UtilityHelper is a utility bean, so @Component fits. OrderService contains business logic, so @Service fits. OrderRepository accesses data, so @Repository fits. OrderController handles HTTP requests, so @Controller fits.

3. Final Answer:

   @Component is for utility beans; @Service handles business logic; @Repository manages data access; @Controller handles web requests -> Option D

4. Quick Check:

   Annotations match roles: Component-utility, Service-logic, Repository-data, Controller-web [OK]

Hint: Match annotation to role: Component-utility, Service-logic, Repository-data, Controller-web [OK]

Common Mistakes:

• Mixing up @Service and @Controller roles
• Thinking all annotations are interchangeable
• Confusing @Component with @Repository