Explore how Spring Boot’s run() method works internally. Learn what happens behind the scenes when a Spring Boot application starts.

Ramesh Fadatare

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3 min read

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Mar 25, 2025

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Previous Chapter:

Introduction

Inside the main entry point class of Spring Boot, you can see the run() method:

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

Spring Boot applications always start with one familiar line:

SpringApplication.run(MyApp.class, args);

It looks simple, but behind the scenes, this line does a lot of heavy lifting.

In this chapter, we’ll explore:

• 🔍 What the run() method really does
• 🧩 Step-by-step flow inside SpringApplication
• 🛠️ Internal components involved in the startup process

📌 Basic Example

@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }
}

✅ This starts your entire application — web server, context, configurations, beans — everything.

But how? Let’s break it down.

🛠️ Step-by-Step: What Happens Inside SpringApplication.run()

The run() method internally does the following:

🔹 Step 1: Create a SpringApplication Instance

SpringApplication app = new SpringApplication(MyApp.class);

✅ This step initializes the SpringApplication object with the primary source (your main class).

It also:

• Detects application type: Servlet, Reactive, or None
• Sets up environment and default listeners