Running a Spring Boot application in Spring Boot - Step-by-Step Execution

Concept Flow - Running a Spring Boot application

Start Application

↓

Spring Boot Main Method

↓

SpringApplication.run()

↓

Spring Boot Context Initialized

↓

Embedded Server Starts

↓

Application Ready to Serve Requests

This flow shows how starting a Spring Boot app runs the main method, initializes the context, starts the embedded server, and makes the app ready.

Execution Sample

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

This code starts the Spring Boot application by running the main method which calls SpringApplication.run.

Execution Table

Step	Action	Method Called	Result	Next Step
1	Start main method	main()	Entered main method	Call SpringApplication.run()
2	Run Spring Boot app	SpringApplication.run()	Spring context initializing	Initialize beans
3	Initialize beans	Context initialization	Beans created and wired	Start embedded server
4	Start embedded server	Tomcat/Jetty start	Server listening on port 8080	Application ready
5	Application ready	N/A	App ready to accept requests	Wait for requests

💡 Application runs continuously until stopped by user or error

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Application Context	null	initializing	initialized	initialized	running
Embedded Server	stopped	stopped	starting	running	running

Key Moments - 2 Insights

Why does the application keep running after main method finishes?

Because the embedded server (like Tomcat) runs in the background, keeping the app alive as shown in steps 4 and 5 of the execution_table.

What does SpringApplication.run() actually do?

It initializes the Spring context, creates beans, and starts the embedded server, as detailed from step 2 to step 4 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the Application Context after step 3?

A null

B initialized

C initializing

D running

💡 Hint

Check the variable_tracker row for Application Context after Step 3

At which step does the embedded server start listening on port 8080?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Result' column in execution_table for server listening info

If SpringApplication.run() fails to initialize beans, what step would fail?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Bean creation happens during context initialization in Step 3

Concept Snapshot

Spring Boot apps start by running the main method.
Inside main, SpringApplication.run() initializes the app.
It creates the Spring context and beans.
Then it starts an embedded server (like Tomcat).
The app stays running, ready to handle requests.
Stop the app to end the server and process.

Full Transcript

Running a Spring Boot application begins with the main method. This method calls SpringApplication.run(), which initializes the Spring context and creates all necessary beans. After that, an embedded server such as Tomcat or Jetty starts listening on the default port 8080. The application remains running, ready to serve incoming requests until it is stopped by the user or an error occurs.