Java.Reative.What are the operators in Project Reactor and what are they used for?

🎯 In Project Reactor, operators are the core tools you use to transform, filter, merge, delay, or otherwise manipulate data as it flows through a reactive pipeline.

Think of operators as the “verbs” in your reactive code — each one defines how the data should be handled.

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⚙️ What is an Operator in Project Reactor?

An operator is a method that you chain on a Flux or Mono to define how data should be:

• Transformed
• Filtered
• Delayed
• Combined
• Scheduled
• Handled on error or completion

All operators are lazy — they don’t execute until the Flux or Mono is subscribed to.

🔧 Categories of Common Operators

Let’s break them down with examples:

🔁 Transformation Operators

Operator	Purpose	Example
map	Transform each element	flux.map(i -> i * 2)
flatMap	Async transform + flatten	flux.flatMap(this::callService)
concatMap	Ordered version of flatMap	flux.concatMap(...)
switchMap	Switch to new inner stream	flux.switchMap(...)

🧹 Filtering Operators

Operator	Purpose	Example
filter	Only pass values that match a condition	flux.filter(i -> i % 2 == 0)
distinct	Remove duplicates	flux.distinct()
take(n)	Take only first n elements	flux.take(3)
skip(n)	Skip first n elements	flux.skip(2)

⏱ Timing Operators

Operator	Purpose	Example
delayElements	Delay each item	flux.delayElements(Duration.ofSeconds(1))
timeout	Timeout on slow items	flux.timeout(Duration.ofSeconds(2))
interval	Emit items over time	Flux.interval(Duration.ofSeconds(1))

🔗 Combining Operators

Operator	Purpose	Example
zip	Combine multiple sources element-wise	Flux.zip(f1, f2)
merge	Merge sources concurrently	Flux.merge(f1, f2)
concat	Append one after the other	Flux.concat(f1, f2)
combineLatest	Combine latest values from sources	Flux.combineLatest(...)

🧯 Error Handling Operators

Operator	Purpose	Example
onErrorReturn	Return fallback value	flux.onErrorReturn("fallback")
onErrorResume	Switch to another Publisher	flux.onErrorResume(e -> Flux.just("safe"))
retry	Retry on error	flux.retry(3)

🧼 Lifecycle Hooks

Operator	Purpose	Example
doOnNext	Side-effect per item	flux.doOnNext(System.out::println)
doOnComplete	On successful completion	flux.doOnComplete(...)
doFinally	On any termination	flux.doFinally(...)
doOnError	On error	flux.doOnError(...)

🧵 Threading (Schedulers)

Operator	Purpose	Example
subscribeOn	Control where upstream runs	.subscribeOn(Schedulers.boundedElastic())
publishOn	Control where downstream runs	.publishOn(Schedulers.parallel())

🧪 Quick Example with Operators:

Flux.range(1, 5)
    .filter(i -> i % 2 == 0)           // Keep only even numbers
    .map(i -> i * 10)                  // Multiply each by 10
    .doOnNext(i -> System.out.println("Mapped: " + i))
    .subscribe(System.out::println);

✅ Output:

Mapped: 20
20
Mapped: 40
40

🧠 Pro Tips

• Operators are lazy — they don’t execute until you .subscribe().
• You can chain operators fluently.
• Use .checkpoint() to debug operator chains.
• Be mindful of operator order — it affects performance and behavior.