You have installed Java, Maven, and your IDE. Now it's time to run your first Spring Boot application.

Spring Boot makes this incredibly simple because it provides:

• An embedded server
• Auto-configuration
• Zero setup requirements

In this post, you'll learn what happens when a Spring Boot application starts and how to verify it's working properly.

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1. Create a Spring Boot Project

To run your first Spring Boot application, start by generating a project using Spring Initializr.

Open:
https://start.spring.io

Select:

• Project: Maven
• Language: Java
• Spring Boot Version: 3.x
• Dependencies: Spring Web

Click Generate, then extract the project and open it in your IDE.

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2. Understanding the Project Structure

A fresh Spring Boot project looks like this:

src/
 ├── main/
 │   ├── java/
 │   │   └── com.example.demo/
 │   │       └── DemoApplication.java
 │   └── resources/
 │       ├── application.properties
 │       ├── static/
 │       └── templates/
 └── test/
     └── java/

Let's break it down:

• DemoApplication.java
  The main class that starts the Spring Boot application.

• application.properties
  Where configuration goes (port, DB settings, etc.)

• static/
  Stores static files like CSS, JS, images.

• templates/
  Stores server-side view templates (Thymeleaf, FreeMarker).

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3. The Main Class Explained

Open the main class:

@SpringBootApplication
public class DemoApplication {

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

What does @SpringBootApplication do?

It's a combination of:

• @Configuration
• @EnableAutoConfiguration
• @ComponentScan

This tells Spring Boot to:

1. Configure the app automatically
2. Scan for components
3. Load beans into the IoC container

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4. Run the Spring Boot Application

You can run the project in two ways:

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4.1 Run from IDE

In IntelliJ / STS / Eclipse, click the green Run button next to the main class.

You should see logs like:

Tomcat started on port 8080
Started DemoApplication in 2.345 seconds

This means the application successfully started.

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4.2 Run from Terminal

Navigate to your project folder and run:

mvn spring-boot:run

Or build a jar file:

mvn clean package
java -jar target/demo-0.0.1-SNAPSHOT.jar

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5. Test the Application

Spring Boot starts an embedded server on port 8080 by default.

Open in your browser:

http://localhost:8080

You should see a generic Whitelabel Error Page.

This is expected because you haven't created any endpoints yet.

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6. Create Your First REST Endpoint

Let's add a simple REST Controller.

Create a file:

src/main/java/com/example/demo/HelloController.java

Paste this code:

@RestController
public class HelloController {

    @GetMapping("/hello")
    public String sayHello() {
        return "Hello, Spring Boot!";
    }
}

Restart the application and visit:

http://localhost:8080/hello

You should see:

Hello, Spring Boot!

Congratulations — you've successfully created and run your first Spring Boot application.

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7. What Happens Behind the Scenes?

When the application starts:

1. Spring Boot loads auto-configuration
2. An embedded Tomcat server starts
3. Beans are scanned and initialized
4. Routes are mapped
5. Application logs are printed to the console

All without configuring a single XML file.

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8. Common Issues & Fixes

Problem: java: command not found

Java is not added to PATH.
Fix: Update environment variables.

Problem: Maven build failure

Run:

mvn -version

Ensure Maven is installed and PATH is correctly set.

Problem: Port already in use

Change the port in:

application.properties

server.port=9090

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What's Next?

In the next post:

06 - Inversion of Control (IoC) in Spring

We'll explore the core concept that powers all of Spring and Spring Boot.