🌱 Spring Boot REST API – Complete Notes

1️⃣ What is a REST API?

REST (Representational State Transfer) is an architectural style for building web services that communicate over HTTP.

A Spring Boot REST API allows you to:

Expose backend functionality via HTTP endpoints
Send/receive JSON data
Build scalable microservices
Integrate frontend (React/Angular) or mobile apps

2️⃣ REST Architecture Principles

Principle	Meaning
Stateless	Each request is independent
Client-Server	UI and backend separated
Uniform Interface	Standard HTTP methods
Resource-Based	Everything is a resource (User, Order, Product)
Cacheable	Responses can be cached

3️⃣ HTTP Methods in REST

HTTP Method	Purpose	Example
GET	Fetch data	Get all users
POST	Create data	Create user
PUT	Update entire resource	Update user
PATCH	Partial update	Update email only
DELETE	Remove resource	Delete user

4️⃣ REST API Flow in Spring Boot

Client (Browser / Postman)
        ↓
@RestController
        ↓
@Service
        ↓
@Repository (JPA)
        ↓
Database

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Stateless vs Stateful in REST

1️⃣ What Does Stateless Mean?

In REST, stateless means:

Each HTTP request from the client must contain all the information required to process it.
The server does NOT store client session data between requests.
Every request is independent.

Example (Stateless REST API)

GET /api/users/1
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

Even if the same client sends another request, it must again send authentication details.

Why REST is Stateless?

HTTP protocol itself is stateless.
No session memory is stored on server.
Authentication tokens (JWT) are sent with every request.

Advantages of Stateless REST

✔ Easy horizontal scaling
✔ No session synchronization required
✔ Better performance in distributed systems
✔ Suitable for microservices

2️⃣ What is Stateful?

Stateful systems store client session information on the server.

Example (Stateful Application)

Client logs in → Server creates session
Server stores: SessionID = ABC123
Client sends only SessionID next time
Server retrieves session data from memory

Where Stateful is Common?

Traditional Servlet-based applications
Banking portals with server sessions
Shopping carts stored in HTTP session

3️⃣ Stateless vs Stateful Comparison

Feature	Stateless (REST)	Stateful
Session Storage	No server session	Server stores session
Scalability	High	Limited
Load Balancing	Simple	Requires sticky sessions
Memory Usage	Low	Higher (stores sessions)
Best For	Microservices, APIs	Traditional web apps

4️⃣ Visual Diagram

STATELESS REST

Client → Request (with Token) → Server
Client → Request (with Token) → Server
Client → Request (with Token) → Server

(No memory stored between requests)


STATEFUL SYSTEM

Client → Login → Server stores session
Client → Request → Server checks session memory
Client → Request → Server reuses stored session

5️⃣ Important Interview Point

REST is stateless because each request contains complete context, and the server does not rely on stored session information.

RequestParam in Spring Boot

1️⃣ What is @RequestParam?

@RequestParam is used to extract query parameters from the URL in a REST API.

Example URL:

GET /api/users?id=10

The value 10 is captured using @RequestParam.

2️⃣ Basic Example

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

    @GetMapping("/user")
    public String getUser(@RequestParam int id) {
        return "User ID: " + id;
    }

}

Request:

GET /api/user?id=5

Response:

User ID: 5

3️⃣ @RequestParam Attributes

Attribute	Purpose
name / value	Name of query parameter
required	Whether parameter must be present
defaultValue	Default value if parameter missing

4️⃣ defaultValue Example

If the parameter is not provided, Spring automatically uses the default value.

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

Requests:

GET /greet?name=John

Response:

Hello John

If parameter is missing:

GET /greet

Response:

Hello Guest

Important: When defaultValue is used, required=false automatically.

5️⃣ Optional Request Parameter (required=false)

By default, parameters are required. If missing, Spring returns 400 Bad Request.

@GetMapping("/search")
public String search(@RequestParam(required = false) String keyword) {

    if(keyword == null){
        return "No keyword provided";
    }

    return "Searching for: " + keyword;
}

Request:

GET /search?keyword=laptop

Response:

Searching for: laptop

If missing:

GET /search

Response:

No keyword provided

6️⃣ Optional<T> Approach

@GetMapping("/filter")
public String filter(@RequestParam Optional<String> category) {

    return category
            .map(c -> "Filtering category: " + c)
            .orElse("No category provided");

}

7️⃣ RequestParam Examples

URL	Method Parameter	Result
/users?id=10	@RequestParam int id	id=10
/users	required param	400 Error
/users	required=false	null
/users	defaultValue="1"	id=1

8️⃣ Multiple Request Parameters

@GetMapping("/products")
public String getProducts(
        @RequestParam String category,
        @RequestParam(defaultValue = "1") int page,
        @RequestParam(required = false) String sort) {

    return "Category: " + category +
           ", Page: " + page +
           ", Sort: " + sort;
}

⚠️ Important Notes

@RequestParam is used for query parameters: /api/users?id=10
@PathVariable is used for URL path values: /api/users/10
Using defaultValue automatically makes the parameter optional
Missing required parameter results in 400 Bad Request