Building a Hello World REST API with Spring Boot

Spring Boot makes it incredibly easy to create production-ready REST APIs. In this tutorial, we’ll build a simple “Hello World” REST API from scratch.

Prerequisites

Before we start, make sure you have:

• Java 17 or higher installed
• Maven or Gradle
• Your favorite IDE (IntelliJ IDEA, Eclipse, or VS Code)

Step 1: Create a New Spring Boot Project

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

1. Go to start.spring.io
2. Choose Maven Project
3. Select Java 17
4. Add dependency: Spring Web
5. Click Generate to download the project

Alternatively, you can use the Spring Boot CLI or create it directly in your IDE.

Step 2: Project Structure

After extracting the downloaded zip, your project structure should look like this:

hello-world-api/
├── src/
│   └── main/
│       └── java/
│           └── com/example/demo/
│               └── DemoApplication.java
├── pom.xml
└── application.properties

Step 3: Create a REST Controller

Create a new file called HelloController.java in the same package as your main application class:

package com.example.demo;

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

@RestController
@RequestMapping("/api")
public class HelloController {
    
    @GetMapping("/hello")
    public String sayHello() {
        return "Hello, World!";
    }
    
    @GetMapping("/greet")
    public Greeting greet() {
        return new Greeting("Hello from Spring Boot!");
    }
}

Step 4: Create a Response Model

Let’s create a simple model class for structured responses:

package com.example.demo;

public class Greeting {
    private String message;
    private long timestamp;
    
    public Greeting(String message) {
        this.message = message;
        this.timestamp = System.currentTimeMillis();
    }
    
    // Getters and setters
    public String getMessage() {
        return message;
    }
    
    public void setMessage(String message) {
        this.message = message;
    }
    
    public long getTimestamp() {
        return timestamp;
    }
    
    public void setTimestamp(long timestamp) {
        this.timestamp = timestamp;
    }
}

Step 5: Run Your Application

Run your Spring Boot application:

mvn spring-boot:run

Or if you’re using Gradle:

./gradlew bootRun

Your application will start on port 8080 by default.

Step 6: Test Your API

Open your browser or use curl to test the endpoints:

Simple String Response

curl http://localhost:8080/api/hello

Response:

Hello, World!

JSON Response

curl http://localhost:8080/api/greet

Response:

{
  "message": "Hello from Spring Boot!",
  "timestamp": 1700000000000
}

Understanding the Annotations

Let’s break down what each annotation does:

• @RestController: Combines @Controller and @ResponseBody, indicating this class handles REST requests
• @RequestMapping("/api"): Sets the base path for all endpoints in this controller
• @GetMapping("/hello"): Maps HTTP GET requests to the specified method

Adding More Endpoints

You can easily add more endpoints with different HTTP methods:

@PostMapping("/create")
public String create(@RequestBody String data) {
    return "Created: " + data;
}

@PutMapping("/update/{id}")
public String update(@PathVariable Long id) {
    return "Updated item: " + id;
}

@DeleteMapping("/delete/{id}")
public String delete(@PathVariable Long id) {
    return "Deleted item: " + id;
}

Customizing the Port

If you want to run on a different port, add this to application.properties:

server.port=9090

What’s Next?

Now that you have a basic REST API running, you can:

• Add a database with Spring Data JPA
• Implement proper error handling
• Add validation to your endpoints
• Secure your API with Spring Security
• Write unit tests for your controllers

Conclusion

Spring Boot makes it incredibly simple to create REST APIs. With just a few lines of code, you have a fully functional web service. The framework handles all the boilerplate, letting you focus on your business logic.

Happy coding!