Learning Terms in Java

What is Reactive Programming?

Reactive Programming = "Don't wait. React when data arrives."

In normal (blocking) programming:

• You ask for data
• Your code waits
• When data comes, the code continues

In Reactive Programming:

• You say: "Hi, tell me when data is ready"
• Your code does not wait
• When data arrives, your code reacts

Think of it like:

• Normal: You call someone and wait on the line
• Reactive: You leave your number and get a callback

This is great for:

• Web APIs
• WebSockets
• Streaming data
• High-traffic systems

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What is Reactor?

Reactor = a Java library that implements reactive programming

• Made by the Spring team
• Used in Spring WebFlux
• Handles async, non-blocking streams

So:

• Reactive Programming -> concept
• Reactor -> Tool to use that concept in Java

Reactor mainly gives you:

• Mono
• Flux

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What is Publisher?

Publisher = something that sends data
This comes from the Reactive Streams specification.

A Publisher:

• Produces data
• Sends data to subscribers
• Can send:
  • 0 items
  • 1 item
  • many items
  • or an error

You usually don't implement Publisher itself.
Reactor already does that.

Mono and Flux are Publishers

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What is Mono?

Mono = a Publisher that emits at most ONE value

Possible results:

• 1 value
• no value
• error

Examples:

• Get one user by ID
• Get account balance
• Call an external API that returns one result

Example:

Mono<String> mono = Mono.just("Hello");

Think of Mono as:

"I promise to give you one result later (or nothing)."

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What is Flux?

Just for completeness:
Flux = 0 to many values

Example:

• Price stream
• WebSocket messages
• List of users

Flux<Integer> flux = Flux.just(1, ,2, 3);