Spring Boot Architecture Explained

Spring Boot is one of the most popular Java frameworks used to build modern web applications and microservices.

However, many beginners find it difficult to understand Spring Boot architecture — especially what happens behind the scenes and how different components work together.

In this blog, we will explain Spring Boot architecture in a simple, clear, and easy-to-understand way, without unnecessary complexity.

What is Spring Boot Architecture?

Spring Boot Architecture describes how different components of a Spring Boot application interact with each other to build, run, and manage an application efficiently.

Spring Boot follows a layered architecture and internally uses the Spring Framework, but it simplifies development by providing:

• Auto Configuration
• Embedded Servers
• Opinionated Defaults
• Starter Dependencies

These features help developers focus more on business logic instead of configuration.

High-Level Spring Boot Architecture (Conceptual Diagram)

Client (Browser / Mobile / API Tool)
        ↓
Controller Layer
        ↓
Service Layer
        ↓
Repository Layer
        ↓
Database

👉 Behind the scenes, Spring Boot Core manages everything using the IoC Container.

Core Components of Spring Boot Architecture

Let’s understand each component step by step in simple terms.

1. Client Layer

The Client layer represents the user or system that sends requests to the application.

Examples of Clients:

• Web Browser
• Mobile Application
• Postman
• Frontend Frameworks (React / Angular)

Clients send HTTP requests such as:

GET /users
POST /login

2. Controller Layer

The Controller layer is responsible for handling incoming HTTP requests.

Responsibilities of Controller:

• Accepts HTTP requests
• Calls appropriate service methods
• Returns responses (JSON / HTML)

Common Controller Annotations:

• @RestController
• @Controller
• @RequestMapping
• @GetMapping, @PostMapping

Example:

@RestController
@RequestMapping("/users")
publicclassUserController {

@GetMapping
public StringgetUsers() {
return"List of users";
    }
}

➡️ Important: Controller should not contain business logic.

3. Service Layer

The Service layer contains the business logic of the application.

Responsibilities of Service:

• Processes data
• Applies business rules
• Acts as a bridge between Controller and Repository

Annotation:

• @Service

Example:

@Service
publicclassUserService {

public StringprocessUser() {
return"User processed";
    }
}

➡️ This layer keeps the application clean, scalable, and maintainable.

4. Repository Layer (Data Access Layer)

The Repository layer communicates directly with the database.

Responsibilities:

• Performs CRUD operations
• Uses JPA / Hibernate to access data

Common Annotations:

• @Repository
• JpaRepository

Example:

@Repository
publicinterfaceUserRepositoryextendsJpaRepository<User, Long> {
}

➡️ Spring Data JPA automatically generates queries, so no need to write SQL for basic operations.

5. Database Layer

The Database stores application data permanently.

Supported Databases:

• MySQL
• PostgreSQL
• MongoDB
• Oracle
• H2 (In-Memory Database)

Spring Boot configures the database automatically using properties defined in

application.properties or application.yml.

6. Spring Boot Core (Heart of the Architecture)

This is the most important part of Spring Boot architecture.

🔹 IoC Container (ApplicationContext)

• Creates and manages Spring Beans
• Handles the complete lifecycle of objects
• Manages Dependency Injection

🔹 Spring Beans

• Objects managed by the Spring container
• Created using annotations like:
  • @Component
  • @Service
  • @Repository

🔹 Dependency Injection (DI)

Dependency Injection allows Spring to inject required objects automatically, reducing tight coupling.

Example:

@Service
publicclassOrderService {

privatefinal PaymentService paymentService;

publicOrderService(PaymentService paymentService) {
this.paymentService = paymentService;
    }
}

➡️ This improves testability and flexibility.

7. Auto Configuration

Auto Configuration is one of the most powerful features of Spring Boot.

What Auto Configuration Does:

• Automatically configures beans
• Works based on classpath dependencies
• Uses smart default settings

Example:

• Add spring-boot-starter-web
• Spring Boot auto-configures:
  • Embedded Tomcat
  • Spring MVC
  • Jackson (JSON)

➡️ No XML configuration required.

8. Starter Dependencies

Spring Boot provides starter dependencies to simplify dependency management.

Common Starters:

• spring-boot-starter-web
• spring-boot-starter-data-jpa
• spring-boot-starter-security

➡️ One starter includes multiple related libraries.

9. Embedded Server

Spring Boot comes with embedded servers, so no external server is needed.

Supported Embedded Servers:

• Tomcat (default)
• Jetty
• Undertow

Run the application using:

java -jar application.jar

➡️ No WAR deployment required.

10. application.properties / application.yml

These files are used to configure the application.

Example:

server.port=8081
spring.datasource.url=jdbc:mysql://localhost:3306/test

➡️ Centralized configuration makes applications easier to manage.

Request Flow in Spring Boot Architecture

Understanding the request flow helps you clearly see how data moves inside a Spring Boot application.

Step-by-Step Request Flow:

1. Client sends an HTTP request

The request comes from a browser, mobile app, Postman, or frontend application.

2. Controller receives the request

The controller maps the request URL and HTTP method using annotations like @GetMapping or @PostMapping.

3. Controller calls the Service layer

The controller forwards the request to the service for processing.

4. Service layer applies business logic

All validations, calculations, and business rules are executed here.

5. Service calls the Repository layer

The service requests data access or persistence operations.

6. Repository interacts with the Database

The repository communicates with the database using JPA/Hibernate.

7. Response flows back to the Client

The result travels back from Repository → Service → Controller → Client as JSON or HTML.

Why Spring Boot Architecture is Powerful?

Spring Boot architecture is powerful because it simplifies development, reduces configuration effort, and follows best design practices.

Key Advantages of Spring Boot Architecture:

• Clean Layered Architecture: Separates Controller, Service, and Repository layers, making code easy to understand and maintain.
• Loose Coupling: Uses Dependency Injection to reduce dependency between components, improving flexibility.
• Easy Testing: Each layer can be tested independently using unit and integration tests.
• Faster Development: Auto configuration and starter dependencies save development time.
• Production-Ready Features: Built-in support for monitoring, security, and configuration management.
• Scalable & Maintainable: Ideal for building scalable applications and microservices.

Spring Boot vs Traditional Spring

Spring Boot Architecture – Frequently Asked Questions (FAQs)

1. Is Spring Boot based on MVC architecture?

👉 Yes, Spring Boot internally uses Spring MVC architecture to handle web requests and responses in web applications.

2. What is the role of the IoC Container in Spring Boot?

👉 The IoC (Inversion of Control) Container creates, manages, and injects Spring Beans.

It controls the entire bean lifecycle and enables Dependency Injection.

3. What is Auto Configuration in Spring Boot?

👉 Auto Configuration automatically configures application components based on:

• Classpath dependencies
• Application properties
• Default Spring Boot settings

This significantly reduces manual configuration.

4. Why is Spring Boot faster than traditional Spring?

👉 Spring Boot speeds up development by:

• Providing auto configuration
• Using starter dependencies
• Eliminating XML configuration
• Offering embedded servers

5. What is the difference between @Controller and @RestController?

👉 @Controller is mainly used for returning views (HTML), while @RestController is used for REST APIs and returns JSON/XML responses.

6. Is Spring Boot suitable for microservices?

👉 Yes, Spring Boot is highly suitable for building scalable, lightweight, and production-ready microservices.

Conclusion

Spring Boot architecture is designed to:

• Simplify application development
• Reduce configuration effort
• Improve developer productivity

Once you understand Controllers, Services, Repositories, and Spring Core concepts, Spring Boot becomes very easy to learn and master.