Build a Simple “Hello World” API

Spring Boot makes it easy to create REST APIs with minimal configuration. This guide walks you through building a simple Hello World API.

Step 1: Create a Spring Boot Project

Use Spring Initializr to generate a project:

• Project: Maven or Gradle
• Language: Java
• Spring Boot Version: Latest stable (e.g., 3.x)
• Dependencies: Spring Web

Download the project and open it in your IDE (IntelliJ IDEA, Eclipse, or VS Code).

Project Structure:

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

Step 2: Create the Main Application Class

The main class is annotated with @SpringBootApplication and serves as the entry point:

package com.example.demo;

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);
    }
}

Step 3: Create a REST Controller

Controllers handle HTTP requests. Create HelloController.java:

package com.example.demo;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {

    @GetMapping("/hello")
    public String hello() {
        return "Hello World!";
    }
}

• @RestController: Combines @Controller + @ResponseBody.
• @GetMapping("/hello"): Maps HTTP GET requests to /hello.

Step 4: Run the Application

You can run your Spring Boot app in multiple ways:

1. From IDE: Right-click DemoApplication → Run/Debug.

2. Using Maven:

mvn spring-boot:run

3. Using Gradle:

./gradlew bootRun

4. Running the JAR:

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

By default, Spring Boot starts an embedded Tomcat server on port 8080.

Step 5: Test the API

Open your browser or Postman and navigate to:

<http://localhost:8080/hello>

Response:

Hello World!

Step 6: Optional Enhancements

Change Server Port (application.properties):

server.port=8081

Enable Logging (application.properties):

logging.level.com.example.demo=DEBUG

Use Profiles (dev/test/prod):

spring.profiles.active=dev

Conclusion

You now have a fully working “Hello World” REST API with Spring Boot.

Key Points:

• No XML configuration required.
• Embedded server starts automatically.
• Controllers map HTTP requests easily.
• Can be extended with services, repositories, and database integration.
• Serves as a foundation for RESTful APIs and microservices.