A clear, simple example of multipart file upload using Reactive Spring (http://blog.davidvassallo.me/2018/07/09/reactive-spring-webflux-multipart-file-upload/)

DefaultRouter.kt

package com.example.FileUploadExample

import com.example.FileUploadExample.HomeHandler
import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration
import org.springframework.http.MediaType
import org.springframework.web.reactive.function.server.HandlerFunction
import org.springframework.web.reactive.function.server.RequestPredicates
import org.springframework.web.reactive.function.server.RouterFunction
import org.springframework.web.reactive.function.server.RouterFunctions
import org.springframework.web.reactive.function.server.ServerResponse


@Configuration
class DefaultRouter{

    @Bean
    fun route(homeHandler: HomeHandler): RouterFunction<ServerResponse> {

        return RouterFunctions
                .route(RequestPredicates.POST("/upload/{id}").and(RequestPredicates.accept(MediaType.MULTIPART_FORM_DATA)),
                        HandlerFunction<ServerResponse> { homeHandler.upload(it) })

    }
  
}

FileUploadExample.kt

package com.example.FileUploadExample

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class FileUploadExample

fun main(args: Array<String>) {
    runApplication<FileUploadExample>(*args)
}

HomeHandler.kt

package com.example.FileUploadExample

import org.springframework.beans.factory.annotation.Autowired
import org.springframework.http.MediaType
import org.springframework.http.codec.multipart.FilePart
import org.springframework.http.codec.multipart.Part
import org.springframework.stereotype.Component
import org.springframework.web.reactive.function.BodyExtractors
import org.springframework.web.reactive.function.BodyInserters
import org.springframework.web.reactive.function.server.ServerRequest
import org.springframework.web.reactive.function.server.ServerResponse
import org.springframework.web.reactive.function.server.bodyToServerSentEvents

import reactor.core.publisher.Mono
import java.io.File


@Component
class HomeHandler {

    fun upload(request: ServerRequest): Mono<ServerResponse> {

        // Not used in this example, but useful for logging, etc
        val id = request.pathVariable("id")

        return request.body(BodyExtractors.toMultipartData()).flatMap { parts ->
            val map: Map<String, Part> = parts.toSingleValueMap()
            val filePart : FilePart = map["file"]!! as FilePart
            // Note cast to "FilePart" above

            // Save file to disk - in this example, in the "tmp" folder of a *nix system
            val fileName = filePart.filename()
            filePart.transferTo( File("/tmp/$fileName"))

            ServerResponse.ok().body(BodyInserters.fromObject("OK"))
        }
    }
}