Synchronous Use

WebClient can be used in synchronous style by blocking at the end for the result:

Java
Kotlin

Person person = client.get().uri("/person/{id}", i).retrieve()
	.bodyToMono(Person.class)
	.block();

List<Person> persons = client.get().uri("/persons").retrieve()
	.bodyToFlux(Person.class)
	.collectList()
	.block();

val person = runBlocking {
	client.get().uri("/person/{id}", i).retrieve()
			.awaitBody<Person>()
}

val persons = runBlocking {
	client.get().uri("/persons").retrieve()
			.bodyToFlow<Person>()
			.toList()
}

However if multiple calls need to be made, it’s more efficient to avoid blocking on each response individually, and instead wait for the combined result:

Java
Kotlin

Mono<Person> personMono = client.get().uri("/person/{id}", personId)
		.retrieve().bodyToMono(Person.class);

Mono<List<Hobby>> hobbiesMono = client.get().uri("/person/{id}/hobbies", personId)
		.retrieve().bodyToFlux(Hobby.class).collectList();

Map<String, Object> data = Mono.zip(personMono, hobbiesMono, (person, hobbies) -> {
			Map<String, String> map = new LinkedHashMap<>();
			map.put("person", person);
			map.put("hobbies", hobbies);
			return map;
		})
		.block();

val data = runBlocking {
		val personDeferred = async {
			client.get().uri("/person/{id}", personId)
					.retrieve().awaitBody<Person>()
		}

		val hobbiesDeferred = async {
			client.get().uri("/person/{id}/hobbies", personId)
					.retrieve().bodyToFlow<Hobby>().toList()
		}

		mapOf("person" to personDeferred.await(), "hobbies" to hobbiesDeferred.await())
	}

The above is merely one example. There are lots of other patterns and operators for putting together a reactive pipeline that makes many remote calls, potentially some nested, interdependent, without ever blocking until the end.

With Flux or Mono, you should never have to block in a Spring MVC or Spring WebFlux controller. Simply return the resulting reactive type from the controller method. The same principle apply to Kotlin Coroutines and Spring WebFlux, just use suspending function or return Flow in your controller method .