Kotlin + Spring Webflux File Upload - Single&Multiple - REST API Example

This article shows you how to upload single|multiple files in the Kotlin + Spring Webflux application. 

More Spring WebFlux practice:

• Kotlin + Spring Webflux File Download - REST API Example
• Spring Webflux File Upload (Single/Multiple)- REST API Example
• Spring Webflux File Download - REST API Example
• Reactive Rest CRUD APIs using Spring Boot, WebFlux, and Reactive Mongo

Technologies Used:

• Kotlin
• Spring Boot 2.5.4
• Spring Webflux
• Java 11
• Gradle

Project Structure:

Project Dependency(build.gradle.kts)

Spring boot dependencies, no need for an extra library for file upload.

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
plugins {
   id("org.springframework.boot") version "2.5.4"
   id("io.spring.dependency-management") version "1.0.11.RELEASE"
   kotlin("jvm") version "1.5.21"
   kotlin("plugin.spring") version "1.5.21"
}
group = "com.knf.dev.demo"
version = "0.0.1-SNAPSHOT"
java.sourceCompatibility = JavaVersion.VERSION_11
repositories {
   mavenCentral()
}
dependencies {
   implementation("org.springframework.boot:spring-boot-starter-webflux")
   implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
   implementation("io.projectreactor.kotlin:reactor-kotlin-extensions")
   implementation("org.jetbrains.kotlin:kotlin-reflect")
   implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
   implementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor")
   testImplementation("org.springframework.boot:spring-boot-starter-test")
   testImplementation("io.projectreactor:reactor-test")
}
tasks.withType<KotlinCompile> {
   kotlinOptions {
      freeCompilerArgs = listOf("-Xjsr305=strict")
      jvmTarget = "11"
   }
}
tasks.withType<Test> {
   useJUnitPlatform()
}

Rest Controller

package com.knf.dev.demo.controller
import org.springframework.web.bind.annotation.RestController
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.PostMapping
import org.springframework.web.bind.annotation.RequestPart
import reactor.core.publisher.Mono
import org.springframework.http.codec.multipart.FilePart
import java.lang.Void
import reactor.core.publisher.Flux
import java.nio.file.Paths
@RestController
@RequestMapping("api/v1")
class UploadDownloadController {
    private val basePath = Paths.
    get("./src/main/resources/files/")
    //single file upload
    @PostMapping("single/upload")
    fun uploadFile(@RequestPart("file") filePartMono:
                   Mono<FilePart>): Mono<Void> {
        return filePartMono
            .doOnNext { fp: FilePart -> 
                println("Received File : " + fp.filename()) }
            .flatMap { fp: FilePart -> fp.
            transferTo(basePath.resolve(fp.filename())) }
            .then()
    }
    //multiple file upload
    @PostMapping("multi/upload")
    fun uploadMultipleFiles(@RequestPart("files") partFlux: 
                            Flux<FilePart>): Mono<Void> {
        return partFlux
            .doOnNext { fp: FilePart -> 
                println(fp.filename()) }
            .flatMap { fp: FilePart -> fp.
            transferTo(basePath.resolve(fp.filename())) }
            .then()
    }
}

Main Class

package com.knf.dev.demo
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
@SpringBootApplication
class KotlinspringwebfluxfileuploaddownloadApplication
fun main(args: Array<String>) {
   runApplication<KotlinspringwebfluxfileuploaddownloadApplication>(*args)
}

Run

Start Spring Boot with the default embedded Tomcat gradle bootRun.

API testing using postman:

Single file upload:

Multiple file upload:

Download: click here

More Spring WebFlux practice:

• Kotlin + Spring Webflux File Download - REST API Example
• Spring Webflux File Upload (Single/Multiple)- REST API Example
• Spring Webflux File Download - REST API Example
• Reactive Rest CRUD APIs using Spring Boot, WebFlux, and Reactive Mongo