What is REST API? How do you build it using Spring Boot?

September 01, 2025

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🔹 What is a REST API?

A REST API (Representational State Transfer API) is an architectural style for designing web services that allow different systems to communicate over HTTP. It uses standard HTTP methods (GET, POST, PUT, DELETE) to perform operations on resources, which are usually represented in JSON or XML format.

✨ Key Principles of REST:

Stateless → Each request is independent; no session is stored on the server.
Client-Server → Client (frontend) and server (backend) are separate.
Uniform Interface → Standard HTTP methods (GET, POST, PUT, DELETE).
Resource-Based → Everything is treated as a resource (e.g., /users, /products).

🔹 Building a REST API with Spring Boot

Spring Boot makes creating REST APIs very simple by handling setup and providing ready-to-use components.

✅ Step 1: Create a Spring Boot Project

Use Spring Initializr or IDE (IntelliJ, Eclipse, STS). Add dependencies:

spring-boot-starter-web (for REST APIs)
spring-boot-starter-data-jpa (for DB access, optional)
spring-boot-starter-test (for testing)

✅ Step 2: Create a Model (Entity)


public class User {
    private Long id;
    private String name;
    private String email;

    // Getters & Setters
}

✅ Step 3: Create a REST Controller


import org.springframework.web.bind.annotation.*;
import java.util.*;

@RestController
@RequestMapping("/users")
public class UserController {

    private List<User> users = new ArrayList<>();

    // GET - fetch all users
    @GetMapping
    public List<User> getUsers() {
        return users;
    }

    // POST - add a new user
    @PostMapping
    public User addUser(@RequestBody User user) {
        users.add(user);
        return user;
    }

    // GET - fetch a user by id
    @GetMapping("/{id}")
    public User getUserById(@PathVariable Long id) {
        return users.stream().filter(u -> u.getId().equals(id)).findFirst().orElse(null);
    }

    // PUT - update a user
    @PutMapping("/{id}")
    public User updateUser(@PathVariable Long id, @RequestBody User updatedUser) {
        for (User u : users) {
            if (u.getId().equals(id)) {
                u.setName(updatedUser.getName());
                u.setEmail(updatedUser.getEmail());
                return u;
            }
        }
        return null;
    }

    // DELETE - delete a user
    @DeleteMapping("/{id}")
    public String deleteUser(@PathVariable Long id) {
        users.removeIf(u -> u.getId().equals(id));
        return "User deleted with id " + id;
    }
}

✅ Step 4: Run the Application

Run your Spring Boot app (@SpringBootApplication class).

GET /users → fetch all users
POST /users → add a new user
GET /users/1 → fetch user with ID 1
PUT /users/1 → update user with ID 1
DELETE /users/1 → delete user with ID 1

👉 In short, a REST API in Spring Boot is built using @RestController, @RequestMapping, and HTTP method annotations (@GetMapping, @PostMapping, etc.).

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