Article 4: Building Your First REST API with Spring Boot

1. Introduction

This is the moment where Spring Boot starts to feel real.

In this article, you will:

• Create your first REST API
• Understand how HTTP requests are handled
• Return text and JSON responses
• Test APIs like a backend developer

By the end, you’ll have a working API running locally.

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2. What Is a REST API? (Very Brief)

A REST API:

• Exposes data or functionality over HTTP
• Uses standard HTTP methods:
  • GET (read)
  • POST (create)
  • PUT (update)
  • DELETE (delete)

Spring Boot makes creating REST APIs extremely simple.

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3. Creating Your First Controller

3.1 Create Controller Package

Inside src/main/java, create:

com.example.demo.controller

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3.2 Create a Controller Class

package com.example.demo.controller;

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 from Spring Boot!";
    }
}

What Just Happened?

• @RestController → Marks class as REST controller
• @GetMapping → Maps HTTP GET requests
• Spring automatically converts response to HTTP output

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4. Run and Test the API

Start your application and open browser:

http://localhost:8080/hello

Expected output:

Hello from Spring Boot!

🎉 You’ve built your first REST API.

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5. Returning JSON Instead of String

5.1 Create a Simple POJO

package com.example.demo.model;

public class Message {

    private String text;

    public Message(String text) {
        this.text = text;
    }

    public String getText() {
        return text;
    }
}

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5.2 Return JSON from Controller

@GetMapping("/hello-json")
public Message helloJson() {
    return new Message("Hello JSON response");
}

Spring Boot automatically:

• Converts object → JSON
• Sets correct HTTP headers

Output:

{
  "text": "Hello JSON response"
}

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6. Using @RequestMapping at Class Level

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

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

Endpoint becomes:

/api/hello

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7. Using Path Variables

@GetMapping("/hello/{name}")
public String greet(@PathVariable String name) {
    return "Hello " + name;
}

Test:

/api/hello/Bharat

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8. Using Request Parameters

@GetMapping("/greet")
public String greetUser(@RequestParam String name) {
    return "Hello " + name;
}

Test:

/api/greet?name=Bharat

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9. HTTP Status Codes (Basic)

@GetMapping("/status")
public ResponseEntity<String> status() {
    return ResponseEntity.ok("Everything is OK");
}

Spring Boot encourages proper HTTP responses.

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10. Testing APIs Properly

Recommended tools:

• Browser (simple GETs)
• Postman
• curl

Example:

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

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11. Common Beginner Mistakes

❌ Forgetting @RestController
❌ Wrong URL path
❌ Port mismatch
❌ Class outside base package

✔ Keep controllers under main package

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12. What You Should Understand Before Moving On

You should now know:

• What a controller is
• How @GetMapping works
• How to return JSON
• How to test APIs locally

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13. What’s Next?

➡ Article 5: Dependency Injection – The Heart of Spring

• @Component
• @Service
• @Autowired
• Constructor injection

Type Next when you’re ready.