Understanding the Internal Working of Spring Boot

Spring Boot is built on top of the Spring Framework. It is a Java-based framework that makes it easy to create standalone, production-ready applications with minimal configuration.

Spring Boot has revolutionized how Java applications are developed by simplifying configuration. It is widely used for building Microservices, REST APIs, and enterprise-grade applications. But how does Spring Boot work internally to provide such a seamless development experience? This article takes a deep dive into the internal working of Spring Boot.

The Core Building Blocks of Spring Boot#

If you have worked with Spring Boot before, you’ve probably noticed how little setup is required to get things running. You create a project, add a few annotations, maybe change some properties, and boom, your API is live.

But this simplicity doesn't happen automatically. Behind the scenes, Spring Boot performs a lot of smart configurations to hide complexity from developers. Let’s break down the key components working inside Spring Boot that make development so smooth.

1. Auto-Configuration#

This is one of the main reasons Spring Boot feels so easy to use. Auto-Configuration is a powerful feature that tries to automatically set up your application based on the dependencies present in the classpath.

Examples:

1. If you add Spring Web, Spring Boot automatically sets up an embedded web server (like Tomcat) and configures request handling.
2. If you include Spring Data JPA, it prepares database connections, scans entities, and configures repositories.

How It Works:

• Spring Boot scans the classpath for libraries and annotations.
• It matches these against a predefined set of auto-configuration classes.
• These classes are conditionally activated based on what’s present in your project (for example, if spring-boot-starter-web is present, Spring Boot configures a web application with an embedded server).
• Auto-Configuration classes are marked with @Configuration and use conditional annotations like @ConditionalOnClass to enable/disable certain features.

2. Spring Boot Starters Dependencies#

One of the biggest pain points in Spring framework was managing dependencies. Each module (like Spring MVC, Spring Data JPA, or security) required manual dependency management and this often leading to version conflicts and bloated configurations.

Spring Boot solves this problem with starter dependencies that are predefined sets of dependencies bundled for specific functionalities.

How It Works:

• Starters (e.g., spring-boot-starter-web, spring-boot-starter-data-jpa) group related dependencies together.
• Instead of adding individual libraries (like Tomcat, Jackson, Hibernate), you include a single starter.
• Spring Boot ensures all included libraries versions are compatible with each-other.

3. Embedded Servers#

Traditional Java web applications require deploying WAR files to external servers like Tomcat or Jetty But Spring Boot allows you to run your application as a standalone application without needing a separate server.

How It Works:

• The spring-boot-starter-web includes an embedded Tomcat server by default.
• When you include a web starter, During the application startup Spring Boot automatically bundles an embedded Tomcat server.
• The server is started when the SpringApplication.run() method is called.

4. Externalized Configuration (properties/yml)#

Hardcoding configuration details like database passwords, API keys, or server ports directly in your code is risky and inflexible. So Spring Boot fixes this problem with externalized configuration with a way to store settings outside your code, making your app more secure and easier to manage.

How It Works:

• Configurations can be set in application.properties  or application.yml.
• You can override these properties depending on the environment by creating different profiles (e.g., application-dev.properties for the development environment).

Here is an example of application.properties file:

spring.datasource.url=jdbc:mysql://localhost:3306/mydb  
spring.datasource.username=admin  
spring.datasource.password=secret123

4. SpringApplication.run() - The Entry Point#

Every Spring Boot application starts with a main() method calling SpringApplication.run(). But what happens behind this simple line?

Step-by-Step Breakdown:

The main() method in your class annotated with @SpringBootApplication is executed.

Internally, this calls the run() method of the SpringApplication class.

run() performs the following:

• Creates and initializes the Application-Context.
• Performs component scanning (@Component, @Service, @Repository, etc.).
• Applies auto-configuration based on the dependencies and classes on the classpath.
• Starts the embedded server (like Tomcat, Jetty, etc. — if a web starter is present).

Application Ready

After the embedded server starts and the context is initialized, the application is ready.

Conclusion:#

Spring Boot simplifies development by handling most of the configurations for you. With features like Auto-Configuration, Starter Dependencies, Embedded Servers and Externalized Configurations helps to build Spring Boot applications fast and hassle-free.