Reactive Spring: Webflux Multipart File Upload

A clear, simple example of multipart file upload using Reactive Spring

Spring Boot v2.0.3.RELEASE
Coding using reactive functional style (as opposed to annotation-based)
Spring Initializr Dependencies: “Reactive Web”
Language: Kotlin

Notes follow after the code:

	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) })

	}

	}

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DefaultRouter.kt

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	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)
	}

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FileUploadExample.kt

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	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"))
	}
	}
	}

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HomeHandler.kt

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Notes:

Spring Webflux makes it extremely easy to build reactive web services – but it takes a while to get used to the terms used to enable effective research and documentation lookup
The spring reactive framework is powered by project reactor – your first stop should be the documentation there.
Spring includes a number of really helpful “BodyExtractors” and “BodyInserters“. The former allows for easy extraction of information from the client request while the latter helps to build the server response. The hardest part of this exercise was realizing that “BodyExtractors.toMultipartData()” exists!
Casting the variable “filePart” as FilePart in line 28 of HomeHandler.kt is essential to expose the “transferTo()” method which makes it easy to save the file to disk
Sample curl command to use the above:

curl -F "file=@/home/dvas0004/Downloads/demo.zip" localhost:8080/upload/123

Note the “file” before the equals sign must match the map index used in line 28 of HomeHandler.kt