How to Create REST API in Spring Boot: Simple Guide

To create a REST API in Spring Boot, define a class annotated with @RestController and map HTTP requests using @GetMapping, @PostMapping, etc. Use Spring Boot's embedded server to run your application and handle requests easily.

Syntax

Here is the basic syntax to create a REST API endpoint in Spring Boot:

• @RestController: Marks the class as a REST controller to handle HTTP requests.
• @RequestMapping or specific method mappings like @GetMapping, @PostMapping: Define the URL path and HTTP method.
• Method returns data (usually JSON) directly to the client.

java

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

@RestController
public class MyController {

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

Example

This example shows a complete Spring Boot application with a REST API endpoint that returns a greeting message when accessed via HTTP GET at /hello.

java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class RestApiApplication {

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

    @RestController
    static class GreetingController {

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

Output

When you run the application and visit http://localhost:8080/hello in a browser or HTTP client, it returns:
Hello, World!

Common Pitfalls

• Forgetting @RestController causes Spring to treat the class as a regular controller, which may try to resolve views instead of returning data.
• Not specifying the correct HTTP method annotation like @GetMapping or @PostMapping can lead to 404 errors.
• Missing @SpringBootApplication on the main class prevents the app from starting properly.
• Returning complex objects without proper getters or without @ResponseBody (if not using @RestController) can cause serialization issues.

java

/* Wrong way: Missing @RestController, returns view name instead of data */
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;

@Controller
public class WrongController {

    @GetMapping("/wrong")
    public String wrong() {
        return "Hello"; // Spring looks for a view named 'Hello'
    }
}

/* Right way: Use @RestController to return data directly */
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.GetMapping;

@RestController
public class RightController {

    @GetMapping("/right")
    public String right() {
        return "Hello"; // Returns string as response body
    }
}

Quick Reference

Here are quick tips for creating REST APIs in Spring Boot:

• Use @RestController for classes handling REST requests.
• Map URLs with @GetMapping, @PostMapping, @PutMapping, @DeleteMapping.
• Return Java objects or strings; Spring Boot auto-converts to JSON.
• Run your app with SpringApplication.run() in the main class annotated with @SpringBootApplication.
• Test endpoints with browser or tools like Postman or curl.

Key Takeaways

Annotate your class with @RestController to create REST endpoints.

Use @GetMapping, @PostMapping, etc., to map HTTP methods and URLs.

Return data directly from methods; Spring Boot handles JSON conversion.

Ensure your main class has @SpringBootApplication and runs SpringApplication.run().

Test your API endpoints with a browser or HTTP client tools.