Spring Webclient multipart file upload

Upasana | September 12, 2020 | 2 min read | 16 views

In this article we will learn how to use Spring 5 WebClient for multipart file upload to a remote file upload server.

Prerequisites

Spring Boot 2.3.3
IDE - IntelliJ or Eclipse
Java 8/11
File upload server refer to this post

Upload server setup

Make sure you have a file upload server up and running. Follow instructions from this article.

Gradle setup

We will create a Spring Boot project using https://start.spring.io/ and edit the build.gradle with the following changes:

build.gradle

plugins {
    id 'org.springframework.boot' version '2.3.3.RELEASE'
    id 'io.spring.dependency-management' version '1.0.10.RELEASE'
    id 'java'
}

group = 'com.carvia'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '11'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.boot:spring-boot-starter-webflux'
    testImplementation('org.springframework.boot:spring-boot-starter-test') {
        exclude group: 'org.junit.vintage', module: 'junit-vintage-engine'
    }
    testImplementation 'io.projectreactor:reactor-test'
}

test {
    useJUnitPlatform()
}

Spring 5 Webclient

Now we will create a WebClient based client that will take a file and upload it to remote service.

ReactiveFileUploadClient.java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.io.FileSystemResource;
import org.springframework.http.HttpEntity;
import org.springframework.http.MediaType;
import org.springframework.http.client.MultipartBodyBuilder;
import org.springframework.util.MultiValueMap;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.reactive.function.BodyInserters;
import org.springframework.web.reactive.function.client.WebClient;

import java.io.File;

@RestController
public class ReactiveFileUploadClient {

    @Autowired
    private WebClient.Builder webClientBuilder; (1)

    @GetMapping(value = "/client")
    public void upload() {
        final WebClient webClient = webClientBuilder.build();
        webClient.post()
                .uri("http://localhost:8080/upload")
                .contentType(MediaType.MULTIPART_FORM_DATA)
                .body(BodyInserters.fromMultipartData(fromFile(new File("HELP.md"))))
                .retrieve()
                .bodyToMono(String.class)   (2)
                .block();   (3)
    }

    public MultiValueMap<String, HttpEntity<?>> fromFile(File file) {
        MultipartBodyBuilder builder = new MultipartBodyBuilder();
        builder.part("file", new FileSystemResource(file));
        return builder.build();
    }
}

1	Spring Boot auto configures a WebClientBuilder instance for us.
2	Our upload server returns a string response `success`
3	We are blocking the current thread till we receive the response from server. We can choose not to do so and move forward.