Building Reactive File Upload with Spring Boot WebFlux and Swagger

Tags: Spring Boot reactive programming swagger

Written by: Tushar Sharma

Project Overview

This solution implements a reactive REST API for handling multiple file uploads using:

• Spring Boot WebFlux (Reactive Web)
• SpringDoc OpenAPI 3 documentation
• Project Reactor
• Java 21

Project Setup (build.gradle)

Key Dependencies

• webflux: Enables reactive web programming

• springdoc-openapi: Generates OpenAPI 3 documentation

• reactor-test: Provides testing utilities for reactive streams

Main Application Class

Key Annotations:

• @OpenAPIDefinition: Enables Swagger/OpenAPI documentation

• Customizes API metadata through @Info

Reactive File Upload Controller

Why Flux instead of MultipartFile?

• Reactive Paradigm:
  • Flux handles streams of data asynchronously
  • Processes files as they arrive without buffering entire request
• Memory Efficiency:
  • Handles large files without memory overload
  • Backpressure-aware (controls data flow rate)
• Non-Blocking I/O:
  • Works with WebFlux's event-loop model
  • Maintains high concurrency with minimal threads

Testing with Swagger UI

Access the API documentation at:

http://localhost:8080/swagger-ui/index.html

Sample Request

POST /fileUploader/upload
Content-Type: multipart/form-data

Request Body

• Form field name: files
• Attach multiple files

Sample Response

"Received files: 1.pdf, 2.pdf"

Understanding Documentation Components

OpenAPI Specification (openapi.json)

• Machine-readable API contract

• Generated at runtime via /v3/api-docs endpoint

• Contains endpoints, schemas, and examples

Swagger UI (index.html)

• Human-friendly documentation interface

• Auto-generated from openapi.json

• Accessible via /swagger-ui.html

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