Downloading a large file in spring MVC through Web client

Question

I'm trying to download a large file with with Spring web client without loading an entire file to the memory. I have such service method:

@Override
public StreamingResponseBody download() {
    Flux<DataBuffer> dataBufferFlux = webClient.get()
            .uri("/download")
            .retrieve()
            .bodyToFlux(DataBuffer.class);

    StreamingResponseBody streamingResponseBody = outputStream -> {
        DataBufferUtils.write(dataBufferFlux, outputStream)
                .doOnNext(DataBufferUtils::release)
                .blockLast();
    };
    return streamingResponseBody;
}

Also we use Spring MVC framework so I can't use Flux in my endpoints. The endpoint code is:

@GetMapping(value = "/download", produces = "application/octet-stream")
public ResponseEntity<StreamingResponseBody> down() {
    var download = service.download();
    return ResponseEntity.ok().contentType(MediaType.APPLICATION_OCTET_STREAM).body(download);
}

But this code does not work. When I call my endpoint I get an empty body with 0 content type. I can't load all bytes[] to memory because I'll get OOM.

Also we use Spring MVC framework so I can't use Flux in my endpoints. Sure you can, you will end-up with only the async part of Webflux but there is nothing preventing you from returning a Flux. — M. Deinum
– M. Deinum, Commented Nov 17 at 13:28

life888888 · Accepted Answer · 2025-11-18 09:52:57Z

Project Tree

demo-download-file-spring-mvc-webclient
├── pom.xml
├── Dockerfile
└── src
    └── main
        ├── java
        │   └── com
        │       └── example
        │           ├── controller
        │           │   └── DownloadController.java
        │           └── DemoDownloadFileSpringMVCApplication.java
        └── resources
            └── application.properties

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.5.7</version>
        <relativePath/>
    </parent>
    <groupId>com.example</groupId>
    <artifactId>demo-download-file-spring-mvc-webclient</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo-download-file-spring-mvc-webclient</name>
    <description>Demo project for Spring Boot</description>

    <properties>
            <maven.compiler.source>17</maven.compiler.source>
                <maven.compiler.target>17</maven.compiler.target>
                <maven.compiler.encoding>UTF-8</maven.compiler.encoding>
                <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
                <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>

        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.15.1</version>
        </dependency>

    </dependencies>

    <build>
    <finalName>app</finalName>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

application.properties

config remote download url

spring.application.name=demo-download-file
remote-download-base-url=http://demo.example.com/download

DemoDownloadFileSpringMVCApplication.java

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoDownloadFileSpringMVCApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoDownloadFileSpringMVCApplication.class, args);
    }
}

DownloadController.java

package com.example.controller;

import org.apache.commons.io.IOUtils;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.servlet.mvc.method.annotation.StreamingResponseBody;
import org.springframework.web.reactive.function.client.WebClient;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferUtils;
import reactor.core.publisher.Flux;

import jakarta.servlet.http.HttpServletResponse;
import java.io.InputStream;
import java.io.OutputStream;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Controller
public class DownloadController {
    static final Logger log= LoggerFactory.getLogger(DownloadController.class);
    
    private final WebClient webClient;

    @Value("${remote-download-base-url}")
    private String baseUrl; // This can be overridden via application.properties + environment.

    public DownloadController(WebClient.Builder builder) {
        this.webClient = builder.build();
    }

    @GetMapping("/download-remote")
    public ResponseEntity<StreamingResponseBody> downloadRemote(
            @RequestParam("param1") String param1,
            @RequestParam("param2") String param2) {

        log.info("param1 = {}, param2 = {}", param1, param2);
        
        String remoteUrl = baseUrl + "?param1=" + param1 +"&param2="+ param2;
        log.info("remoteUrl = {}", remoteUrl);

        StreamingResponseBody body = outputStream -> {

            // Obtain Flux<DataBuffer> from remote stream
            Flux<DataBuffer> dataBufferFlux = webClient.get()
                    .uri(remoteUrl)
                    .retrieve()
                    .bodyToFlux(DataBuffer.class);
           log.info("dataBufferFlux ok");
           
            // Writing Flux to OutputStream (blocking MVC environment)
            DataBufferUtils.write(dataBufferFlux, outputStream)
                    .doOnNext(DataBufferUtils::release)
                    .blockLast();  // MVC needs to block until the streaming is complete.
        };
           log.info("DataBufferUtils.write ok");
           
           
        return ResponseEntity.ok()
                .header(HttpHeaders.CONTENT_DISPOSITION,
                        "attachment; filename=" + fileName)
                .contentType(MediaType.APPLICATION_OCTET_STREAM)
                .body(body);
    }
}

Dockerfile

optional - only for verify.

FROM eclipse-temurin:17-jre-alpine

WORKDIR /app-data

COPY target/app.jar app.jar

EXPOSE 8080

ENTRYPOINT ["java", "-jar", "app.jar"]

Build

mvn clean package

Run

export REMOTE_DOWNLOAD_BASE_URL="http://demo.server.local:58080/download"

java -jar target/app.jar

Test

Open Browser

http://localhost:8080/download-remote?param1=TEMP&param2=TEST

NOTE:

Spring Boot Controller will download file from http://demo.server.local:58080/download?param1=TEMP&param2=TEST

String remoteUrl = baseUrl + "?param1=" + param1 +"&param2="+ param2;

To verify that the program does not download the entire file to memory, we create a Docker image and limit the Docker container's memory to 1GB during execution. We then test downloading a remote file up to 4GB (exceeding the 1GB allocated to the Docker container) to verify that the file is not downloaded entirely to memory.

Build Docker Image

docker build -t spring-download-remote .

Run Docker

docker run \
  -it \
  --rm \
  --name download-remote \
  -p 8080:8080 \
  --memory="1g" \
  --memory-swap="1g" \
  -e REMOTE_DOWNLOAD_BASE_URL="http://demo.server.local:58080/download" \
  spring-download-remote

Test

Open Browser:

http://localhost:8080/download-remote?param1=TEMP&param2=TEST

In fact, the program will connect to this URL (http://demo.server.local:58080/download?param1=TEMP&param2=TEST) to download the file.

Code:

MVC Controller Download remoteUrl file:


    @Value("${remote-download-base-url}")
    private String baseUrl; // This can be overridden via application.properties + environment.

    @GetMapping("/download-remote")
    public ResponseEntity<StreamingResponseBody> downloadRemote(
            @RequestParam("param1") String param1,
            @RequestParam("param2") String param2) {

    ...
           String remoteUrl = baseUrl + "?param1=" + param1 +"&param2="+ param2;
    ...

+--------+                     +-------------------------+                           +--------+ 
| Broser +-- Download File --> + Spring Boot Application +                           +        +
|        |                     + (MVC Controller) +------------+                     + Remote +
+--------+                     +------------------+ Web Client +-- Download File --> + Server +
                                                  +------------+                     +--------+

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Downloading a large file in spring MVC through Web client

1 Answer 1

Project Tree

pom.xml

application.properties

DemoDownloadFileSpringMVCApplication.java

DownloadController.java

Dockerfile

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Test

Build Docker Image

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1 Answer 1

Project Tree

pom.xml

application.properties

DemoDownloadFileSpringMVCApplication.java

DownloadController.java

Dockerfile

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Test

Build Docker Image

Run Docker

Test

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