🚀 What Happens When You Run a Spring Boot Application? (High-Level Theory)

Spring Boot is designed to remove the heavy lifting involved in configuring and running Java applications. As a developer, you often just write a main() method, annotate your class with @SpringBootApplication, and run the app. Behind the scenes, however, a lot of intelligent orchestration happens inside the JVM.

Understanding this internal flow is extremely useful for interviews, debugging, performance tuning, and real-world system design. Let’s break down what actually happens when you run a Spring Boot application, step by step, at a high theoretical level.

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1️⃣ JVM Starts Execution

Everything begins with the Java Virtual Machine (JVM).

When you run your Spring Boot application:

• The JVM is started by the operating system
• The JVM loads the main class into memory
• The public static void main(String[] args) method is invoked

Inside this main() method, Spring Boot calls:

SpringApplication.run(Application.class, args);

This single line is the entry point to the entire Spring Boot startup process.

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2️⃣ SpringApplication Bootstraps the Application

SpringApplication is the core class responsible for bootstrapping the application. At this stage, Spring Boot performs several critical tasks:

🔹 Application Type Detection

Spring Boot automatically determines whether your application is:

• Servlet-based web application (Spring MVC)
• Reactive web application (Spring WebFlux)
• Non-web application

This decision is based on classpath dependencies (for example, whether spring-webmvc or spring-webflux is present).

🔹 Loading Initializers and Listeners

Spring loads:

• ApplicationContextInitializers
• ApplicationListeners

These hooks allow developers and frameworks to customize behavior before and during context startup.

🔹 Environment Preparation

Spring Boot prepares the Environment, which includes:

• application.properties or application.yml
• Active profiles (dev, test, prod)
• System properties
• Environment variables

At this stage, configuration values become available to the application.

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3️⃣ ApplicationContext Creation

Next, Spring creates the ApplicationContext, which is the heart of the Spring Framework.

Depending on the application type, Spring Boot chooses the appropriate context:

• AnnotationConfigServletWebServerApplicationContext → for web applications
• AnnotationConfigReactiveWebServerApplicationContext → for reactive apps
• AnnotationConfigApplicationContext → for non-web apps

The ApplicationContext:

• Acts as a container for beans
• Manages bean lifecycle
• Handles dependency injection
• Publishes application events

Without this context, Spring as a framework cannot function.

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4️⃣ Auto-Configuration Kicks In

This is where Spring Boot truly shines.

🔹 How Auto-Configuration Works

Spring Boot scans the classpath to understand which libraries are present and automatically configures beans accordingly. This is enabled by @EnableAutoConfiguration, which is part of @SpringBootApplication.

🔹 Examples

• If spring-boot-starter-web is present → Spring configures:
  • DispatcherServlet
  • Embedded Tomcat
  • Default error handling
• If Spring Data JPA is present → Spring configures:
  • EntityManagerFactory
  • DataSource
  • Transaction management
• If no explicit configuration is provided → Spring applies sensible defaults

This mechanism drastically reduces boilerplate and configuration effort.

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5️⃣ Component Scanning

Spring Boot now scans your application packages to find Spring-managed components.

The base package is determined by the location of the class annotated with @SpringBootApplication.

🔹 What Gets Detected?

Classes annotated with:

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

All detected classes are registered as beans inside the ApplicationContext.

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6️⃣ Bean Creation & Dependency Injection

Once all bean definitions are known, Spring begins creating beans.

🔹 Bean Instantiation

• Beans are instantiated based on scope (singleton, prototype, etc.)
• Constructor-based injection is preferred
• Field and setter injection are also supported

🔹 Dependency Injection

Spring resolves dependencies using:

• Constructor injection
• @Autowired
• @Qualifier
• @Value

🔹 Bean Lifecycle

During this phase:

• @PostConstruct methods are executed
• BeanPostProcessors are applied
• Initialization callbacks are triggered

This ensures all beans are fully wired and ready to use.

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7️⃣ Embedded Server Starts

Spring Boot uses embedded servers, which eliminates the need for external application servers.

Depending on dependencies, Spring Boot starts:

• Tomcat (default)
• Jetty
• Netty

The server:

• Binds to the configured port (default: 8080)
• Initializes network connectors
• Prepares to accept HTTP requests

At this point, your application is officially running as a web server.

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8️⃣ DispatcherServlet Initialization

The DispatcherServlet is the front controller of Spring MVC.

🔹 Responsibilities

• Receives incoming HTTP requests
• Delegates requests to appropriate controllers
• Handles request mapping, validation, and response rendering

Spring initializes:

• HandlerMappings
• HandlerAdapters
• ViewResolvers

This setup enables smooth request routing from URLs to controller methods.

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9️⃣ Application Ready State

Finally, Spring publishes key lifecycle events:

• ApplicationStartedEvent
• ApplicationReadyEvent

Once ApplicationReadyEvent is published:

• All beans are fully initialized
• Server is up and running
• Application is ready to serve requests 🚀

You can hook into this phase using:

@EventListener(ApplicationReadyEvent.class)

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🎯 One-Line Interview Answer

When a Spring Boot application runs, it starts the JVM, bootstraps SpringApplication, creates the application context, performs auto-configuration, scans and initializes beans, starts the embedded server, initializes the DispatcherServlet, and finally becomes ready to handle HTTP requests.

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📌 Final Thoughts

Spring Boot hides a complex startup lifecycle behind a simple abstraction. While developers enjoy faster development and minimal configuration, understanding this flow helps you:

• Debug startup issues
• Optimize application performance
• Answer interview questions confidently
• Design scalable Spring applications