Spring Boot is not just a collection of annotations — it is a well-designed architecture built on top of the Spring Framework to simplify application development.

Understanding Spring Boot’s architecture helps you:

• Debug startup issues
• Customize auto-configuration
• Design scalable applications
• Use Spring Boot confidently in production

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1. High-Level Spring Boot Architecture

At a high level, Spring Boot consists of:

• Spring Core & IoC Container
• Auto-Configuration Engine
• Starter Dependencies
• Embedded Web Server
• Externalized Configuration
• Production-ready features (Actuator)

All these components work together to minimize boilerplate and maximize productivity.

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2. Spring Boot Built on Spring Framework

Spring Boot does not replace Spring.

It uses:

• Spring Core (IoC, DI, AOP)
• Spring MVC
• Spring Data
• Spring Security
• Spring Context

Spring Boot simply adds smart defaults and automation on top.

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3. Auto-Configuration: The Heart of Spring Boot

Auto-configuration automatically configures beans based on:

• Classpath dependencies
• Application properties
• Environment
• Conditional annotations

Example:

@ConditionalOnClass(DataSource.class)
@ConditionalOnMissingBean(DataSource.class)
public class DataSourceAutoConfiguration { }

Spring Boot configures components only if needed.

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4. Starter Dependencies

Starters bundle commonly used dependencies.

Example:

spring-boot-starter-web

Includes:

• Spring MVC
• Embedded Tomcat
• Jackson
• Validation

Starters ensure compatible versions and reduce dependency conflicts.

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5. Embedded Web Server

Spring Boot embeds servers like:

• Tomcat (default)
• Jetty
• Netty

Benefits:

• No external server setup
• Easy deployment
• Executable JARs

Application runs using:

java -jar app.jar

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6. Externalized Configuration

Spring Boot supports configuration from:

• application.properties / application.yml
• Environment variables
• Command-line arguments
• Profiles

This enables environment-specific behavior without code changes.

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7. Application Startup Flow

1. main() method runs
2. SpringApplication created
3. ApplicationContext initialized
4. Auto-configuration applied
5. Beans created and wired
6. Embedded server starts
7. Application ready

Spring Boot logs reflect this entire process.

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8. Production-Ready Features

Spring Boot provides built-in production tools:

• Actuator endpoints
• Health checks
• Metrics
• Info endpoints

These features are critical for monitoring and operations.

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9. Customizing Spring Boot Architecture

You can customize by:

• Excluding auto-configurations
• Overriding beans
• Custom starters
• Profiles

Example:

@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)

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10. Summary

• Spring Boot architecture builds on Spring Framework
• Auto-configuration is the core innovation
• Starters simplify dependency management
• Embedded servers simplify deployment
• Architecture supports scalable, production-ready apps

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What's Next?

Next post:

19 - Key Spring Boot Annotations Explained

We’ll explore the most important annotations that power Spring Boot applications.