spring-boot-rest/spring-5/src/main/java/com/baeldung/functional/FormHandler.java

package com.baeldung.functional;

import org.springframework.util.MultiValueMap;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import reactor.core.publisher.Mono;

import java.util.concurrent.atomic.AtomicLong;

import static org.springframework.web.reactive.function.BodyExtractors.toDataBuffers;
import static org.springframework.web.reactive.function.BodyExtractors.toFormData;
import static org.springframework.web.reactive.function.BodyInserters.fromObject;
import static org.springframework.web.reactive.function.server.ServerResponse.ok;

public class FormHandler {

    Mono<ServerResponse> handleLogin(ServerRequest request) {
        return request
          .body(toFormData())
          .map(MultiValueMap::toSingleValueMap)
          .filter(formData -> "baeldung".equals(formData.get("user")))
          .filter(formData -> "you_know_what_to_do".equals(formData.get("token")))
          .flatMap(formData -> ok().body(Mono.just("welcome back!"), String.class))
          .or(ServerResponse.badRequest().build());
    }

    Mono<ServerResponse> handleUpload(ServerRequest request) {
        return request
          .body(toDataBuffers())
          .collectList()
          .flatMap(dataBuffers -> {
              AtomicLong atomicLong = new AtomicLong(0);
              dataBuffers.forEach(d -> atomicLong.addAndGet(d
                .asByteBuffer()
                .array().length));

              return ok()
                .body(fromObject(atomicLong.toString()));
          });
    }
}