Spring Reactor Merge vs Concat

问题

I´m playing with Spring reactor, and I cannot see any differences between concat and merge operator

Here's my example

    @Test
    public void merge() {
        Flux<String> flux1 = Flux.just("hello").doOnNext(value -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux2 = Flux.just("reactive").doOnNext(value -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux3 = Flux.just("world");
        Flux.merge(flux1, flux2, flux3)
                .map(String::toUpperCase)
                .subscribe(System.out::println);
    }

    @Test
    public void concat() {
        Flux<String> flux1 = Flux.just("hello").doOnNext(value -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux2 = Flux.just("reactive").doOnNext(value -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux3 = Flux.just("world");
        Flux.concat(flux1, flux2, flux3)
                .map(String::toUpperCase)
                .subscribe(System.out::println);    
}

Both behave exactly the same. Can someone explain the difference between the two operations?

回答1:

The essential difference between merge and concat is that in merge, both streams are live. In case of concat, first stream is terminated and then the other stream is concatenated to it.

Concat

Merge

回答2:

The difference is already mentioned in the API docs that while concat first reads one flux completely and then appends the second flux to that, merge operator doesn't guarantee the sequence between the two flux.

In order to see the difference, modify your merge() code as below.

e.g. sample code below

//Flux with Delay
@Test
public void merge() {
    Flux<String> flux1 = Flux.just("Hello", "Vikram");

    flux1 = Flux.interval(Duration.ofMillis(3000))
    .zipWith(flux1, (i, msg) -> msg);


    Flux<String> flux2 = Flux.just("reactive").doOnNext(value -> {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
    Flux<String> flux3 = Flux.just("world");
    Flux.merge(flux1, flux2, flux3)
            .subscribe(System.out::println);

    try {
        Thread.sleep(5000);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

As you modify the Flux.interval duration, currently set as 3000 milliseconds you will see that the output with merge() keeps changing. But with concat(), the output will be always same.

来源：https://stackoverflow.com/questions/48478420/spring-reactor-merge-vs-concat

标签

java

Spring

flux

Reactor