Kotlin + Spring Webflux File Download

Kotlin + Spring Webflux File Download - REST API Example

September 21, 2021

In this article, We will learn how to download files in a RESTful Kotlin + Spring WebFlux application.

More Spring WebFlux practice:

Technologies Used:

Kotlin
Spring Boot 2.5.4
Spring Webflux
Java 11
Gradle

Project Structure:

Project Dependency Management(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.GetMapping
import kotlin.Throws
import java.io.IOException
import org.springframework.web.bind.annotation.PathVariable
import reactor.core.publisher.Mono
import java.lang.Void
import org.springframework.http.ZeroCopyHttpOutputMessage
import org.springframework.core.io.ClassPathResource
import org.springframework.http.HttpHeaders
import org.springframework.http.MediaType
import org.springframework.http.server.reactive.ServerHttpResponse

@RestController
class DownloadFile {
    @GetMapping("/download/{fileName}")
    @Throws(IOException::class)
    fun downloadFile(
        @PathVariable fileName: String,
        response: ServerHttpResponse
    ): Mono<Void> {
        val zeroCopyResponse = response as ZeroCopyHttpOutputMessage
        response.getHeaders()[HttpHeaders.CONTENT_DISPOSITION] =
            "attachment; filename=$fileName"
        response.getHeaders().contentType = MediaType.APPLICATION_OCTET_STREAM
        val resource = ClassPathResource(fileName)
        val file = resource.file
        return zeroCopyResponse.writeWith(
            file, 0,
            file.length()
        )
    }
}

Main Class

package com.knf.dev.demo

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

@SpringBootApplication
class KotlinspringwebfluxdownloadfileApplication

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