Reactive Programming in the context of Spring WebFlux ( AI Generated )

 Sure! Let's dive deep into Reactive Programming in the context of Spring WebFlux, especially as it relates to Java development and interviews.

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⚡ What is Reactive Programming?

Reactive Programming is a programming paradigm focused on:

• Asynchronous data streams

• Non-blocking behavior

• Backpressure handling (flow control)

Rather than processing one request at a time, it reacts to events/data as they arrive, making it ideal for handling high-concurrency, I/O-bound, or real-time applications.

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🌐 What is Spring WebFlux?

Spring WebFlux is a reactive web framework introduced in Spring 5 and available via Spring Boot 2.x and 3.x.

🔑 Key Characteristics:

Feature	Description
Reactive Stack	Built on Project Reactor
Non-blocking I/O	Uses Netty (or can run on Tomcat/Jetty/Undertow in non-blocking mode)
Alternative to MVC	WebFlux is to Reactive what Spring MVC is to traditional
Functional or Annotated	Supports both annotation-based (@RestController) and functional-style routing

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🧠 Why Use WebFlux?

Scenario	Why WebFlux Helps
Handling thousands of concurrent users	Event-driven, async, scalable
Building real-time applications (chat, streaming)	Handles streams with minimal threads
Making non-blocking HTTP calls (e.g., calling other microservices)	Uses reactive WebClient instead of RestTemplate
Saving resources under high load	No thread-per-request overhead

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🔩 Core Concepts in Spring WebFlux

1. Mono and Flux (from Project Reactor)

Type	Description
Mono<T>	Emits 0 or 1 item
Flux<T>	Emits 0 to N items (stream of values)

These are the reactive equivalents to Optional<T> and List<T> but asynchronous.

@GetMapping("/user/{id}")
public Mono<User> getUser(@PathVariable String id) {
    return userService.findById(id); // Non-blocking
}

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2. WebClient (Non-blocking HTTP client)

WebClient client = WebClient.create("https://api.example.com");

Mono<String> response = client.get()
    .uri("/data")
    .retrieve()
    .bodyToMono(String.class);

Unlike RestTemplate, WebClient is non-blocking, async, and reactive.

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3. Functional Routing Style (Optional Alternative)

@Bean
public RouterFunction<ServerResponse> route(UserHandler handler) {
    return RouterFunctions
        .route(GET("/user/{id}"), handler::getUser);
}

This replaces @Controller and is more aligned with functional programming paradigms.

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🆚 WebFlux vs Spring MVC

Feature	WebFlux	Spring MVC
I/O model	Non-blocking, async	Blocking, servlet-based
Backpressure	Yes	No
Performance	Better under high concurrency	Better for simple, quick apps
Thread model	Few threads	One thread per request
Use case	Real-time, async microservices	Traditional web apps

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🧪 Testing in WebFlux

WebFlux supports reactive testing using StepVerifier.

@Test
public void testReactiveMono() {
    Mono<String> mono = Mono.just("Spring");

    StepVerifier.create(mono)
        .expectNext("Spring")
        .verifyComplete();
}

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🎯 Interview Focus Points

Here are common interview questions:

1. What is Reactive Programming?

2. What is the difference between Flux and Mono?

3. When would you use WebFlux over Spring MVC?

4. How does WebClient differ from RestTemplate?

5. Explain backpressure and how WebFlux handles it.

6. How do you test reactive streams?

7. Does WebFlux only work with Netty? (No — it can work with servlet containers too.)

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Would you like a visual diagram or code examples comparing WebFlux and Spring MVC side by side?

( Generated by Chatgpt)