This episode story is simple. If we receive response that is list type, we can use flatMapIterable operator(finally return type is Flux). Sometimes, I found the Mono<List<T>> type.

public Flux<String> getData() {
    return this.webClient.get()
        .uri(uriBuilder -> uriBuilder
            .path("/data")
            .build()
        )
        .retrieve()
        .bodyToMono(DataApiResponse.class)
        .flatMapIterable(DataApiResponse::getContent);
}

@Value
public class DataApiResponse {
    List<String> content;
}

Afterwards, If we use getData() method in the business layer, we can change Flux to Stream using toStream() operator and then change Stream to List using collect operator again.

List<String> data = apiClient.getData()
    .toStream()
    .collect(toUnmodifiableList());