what does Mono.defer() do?

I've come across Mono.defer() in some Spring webflux code

I looked up the method in the docs but don't understand the explanation:

"Create a Mono provider that will supply a target Mono to subscribe to for each Subscriber downstream"

please could I have an explanation and an example. Is there a place with a bunch of Reactor example code (their unit tests?) that I might reference.

thanks

It is a bit of an oversimplification but conceptually Reactor sources are either lazy or eager. More advanced ones, like an HTTP request, are expected to be lazily evaluated. On the other side the most simple ones like Mono.just or Flux.fromIterable are eager.

By that, I mean that calling Mono.just(System.currentTimeMillis()) will immediately invoke the currentTimeMillis() method and capture the result. Said result is only emitted by the Mono once it is subscribed to. Subscribing multiple times doesn't change the value either:

Mono<Long> clock = Mono.just(System.currentTimeMillis());
//time == t0

Thread.sleep(10_000);
//time == t10
clock.block(); //we use block for demonstration purposes, returns t0

Thread.sleep(7_000);
//time == t17
clock.block(); //we re-subscribe to clock, still returns t0

The defer operator is there to make this source lazy, re-evaluating the content of the lambda each time there is a new subscriber:

Mono<Long> clock = Mono.defer(() -> Mono.just(System.currentTimeMillis()));
//time == t0

Thread.sleep(10_000);
//time == t10
clock.block(); //invoked currentTimeMillis() here and returns t10

Thread.sleep(7_000);
//time == t17
clock.block(); //invoke currentTimeMillis() once again here and returns t17

with simple words if you see in the first view it is like Mono.just() but is not. when you run Mono.just() it creates immediately an Observable(Mono)and reuses it but when you use defer it doesn't create it immediately it creates a new Observable in every subscribe.

One use case to see the difference

    int a = 5;
@Override
public void run(String... args) throws Exception {

    Mono<Integer> monoJust = Mono.just(a);
    Mono<Integer> monoDefer = Mono.defer(() -> Mono.just(a));

    monoJust.subscribe(integer1 -> System.out.println(integer1));
    monoDefer.subscribe(integer1 -> System.out.println(integer1));

    a = 7;
    monoJust.subscribe(integer1 -> System.out.println(integer1));
    monoDefer.subscribe(integer1 -> System.out.println(integer1));
}

print:

5
5
5
7

if you see mono.just has created the observable immediately and it doesn't change even if the value has changed but the defer create the observable in subscribe so you will work with the current onSubscribe value

I was attempting defer for a different use case. Wrote the below code to check and sharing as it might help others. My use case was to chain two Monos and ensure that the first one is complete before the second one is taken up. And second one contained a blocking call whose result is used to respond a Mono either with empty or error response. Without defer, my blocking call is executed irrespective of the result of first one. But while using defer the blocking call is executed only when the first Mono completes. Code below:

public static void main(String[] args) {
    long cur = System.currentTimeMillis();
    boolean succeed = true;

    Mono<Integer> monoJust = Mono.create(consumer -> {
        System.out.println("MonoJust inside " + (System.currentTimeMillis() - cur));
        if (succeed) {
            consumer.success(1);
        } else {
            consumer.error(new RuntimeException("aaa"));
        }
    });

    Mono<String> monoJustStr = Mono.create(consumer -> {
        System.out.println("MonoJustStr inside: " + (System.currentTimeMillis() - cur));
        consumer.success("one");
    });

    System.out.println("##1##: Begin");
    monoJust.then(evaluator() ? Mono.empty() : monoJustStr).subscribe(d -> System.out.println("##1##: "+d), e-> System.err.println(e));
    System.out.println("##1##: Done: "+(System.currentTimeMillis() - cur));

    System.out.println("\n\n\n##2##: Begin");
    monoJust.then(Mono.defer(() -> evaluator() ? Mono.empty() : monoJustStr)).subscribe(d -> System.out.println("##2##: "+d), e-> System.err.println(e));
    System.out.println("##2##: Done: " + (System.currentTimeMillis() - cur));

}

private static boolean evaluator() {
    System.out.println("Inside Evaluator");
    return false;
}

Output with succeed=true - Observe the sequence of "Inside Evaluator" and "MonoJust inside"

##1##: Begin
Inside Evaluator
MonoJust inside 540
MonoJustStr inside: 542
##1##: one
##1##: Done: 542



##2##: Begin
MonoJust inside 544
Inside Evaluator
MonoJustStr inside: 544
##2##: one
##2##: Done: 544

Below is output with succeed = false - Note that evaluator is not called.

##1##: Begin
Inside Evaluator
MonoJust inside 565
java.lang.RuntimeException: aaa
##1##: Done: 567



##2##: Begin
MonoJust inside 569
java.lang.RuntimeException: aaa
##2##: Done: 569

Simple answer for beginners:

When calling subscribe on monoJust variable it will print a single random integer thrice. But on calling subscribe on monoDefer variable it can print a random number everytime.

   Mono<Integer> justMono = Mono.just((new Random()).nextInt(10));

    //this will print same random number thrice
    for(int i=0;i<3;i++)
        justMono.subscribe(x -> {System.out.println("Just Mono: " + x);});

    Mono<Integer> deferMono = Mono.defer(() -> Mono.just((new Random()).nextInt(10)));

    //this might print three different random numbers
    for(int i=0;i<3;i++)
        deferMono.subscribe(x -> {System.out.println("Defer Mono: " + x);});

In Mono.just() instantiation happens only once when the first subscribe happens. In Mono.defer() instantiation happens everytime a subscribe is called.

For more reference check this out: https://www.youtube.com/watch?v=eupNfdKMFL4&t=381s at 3:15 mins