So far, we’ve understood Spring Boot concepts and core features.
Now it’s time to build a real application and see everything in action.

In this hands-on post, we’ll create a basic Spring Boot application, add a simple REST endpoint, and run it locally.

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1. What We Are Going to Build

We’ll build a simple Spring Boot application that:

• Starts using Spring Boot defaults
• Exposes a REST endpoint
• Returns a simple response
• Runs as an executable JAR

No database, no external services — just core Spring Boot.

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2. Creating the Project

The easiest way to create a Spring Boot project is using Spring Initializr.

Project configuration:

• Project: Maven
• Language: Java
• Spring Boot: Latest stable
• Group: com.example
• Artifact: demo
• Packaging: JAR
• Java: 17+
• Dependencies:
  • Spring Web

Generate and open the project in your IDE.

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3. Project Structure Overview

Once created, your project structure looks like this:

src
 └── main
     ├── java
     │   └── com.example.demo
     │       └── DemoApplication.java
     └── resources
         └── application.properties

This structure is standard and recommended.

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4. Understanding the main() Method

Open DemoApplication.java:

@SpringBootApplication
public class DemoApplication {

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

This method:

• Bootstraps the application
• Creates the ApplicationContext
• Triggers auto-configuration
• Starts the embedded server

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5. Creating a Simple REST Controller

Let’s create a REST endpoint.

@RestController
@RequestMapping("/api")
public class HelloController {

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

What’s happening:

• @RestController marks this as a REST controller
• @GetMapping maps HTTP GET requests
• The method returns a simple String response

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6. Running the Application

You can run the application in multiple ways.

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6.1 Run from IDE

• Right-click DemoApplication
• Click Run

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6.2 Run using Maven

mvn spring-boot:run

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6.3 Run as Executable JAR

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

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7. Testing the Endpoint

Open a browser or Postman and hit:

http://localhost:8080/api/hello

Response:

Hello, Spring Boot!

Congratulations — your Spring Boot application is running.

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8. Understanding What Just Happened

Behind the scenes, Spring Boot:

• Started the embedded Tomcat server
• Registered DispatcherServlet
• Mapped your controller
• Converted the response automatically
• Returned HTTP response to client

All with minimal configuration.

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9. Common Beginner Mistakes

• Placing controllers outside base package
• Forgetting @RestController
• Port conflicts (8080 already in use)
• Missing dependencies

Spring Boot error messages usually guide you clearly.

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10. Best Practices

• Keep main class in root package
• Follow layered structure as app grows
• Use REST controllers for APIs
• Keep application small and incremental

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11. Summary

• Creating a Spring Boot app is quick and simple
• Spring Initializr sets up everything
• Minimal configuration is needed
• Embedded server runs out of the box
• Ideal for rapid development

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What's Next?

Next post:

28 - Embedded Web Servers (Tomcat, Jetty, Netty)

We’ll explore how Spring Boot embeds web servers and how to customize them.