What this guide builds

Welcome to the not boring world of file upload APIs. We will build a Spring Boot REST API that accepts multipart uploads using MultipartFile, validates input so jerky clients do not wreck your server, and stores files in a predictable place. Expect clear error responses and no surprises in logs.

Project setup and dependencies

Create a Maven or Gradle project and include Spring Web. Optionally add Apache Commons IO to make file handling less painful. You do not need a circus of libraries, just sensible dependencies so file streams and content type checks behave.

Recommended dependencies

• spring-boot-starter-web for the REST API and Spring MVC support
• commons-io for handy file utilities and safe copying
• spring-boot-starter-test for unit tests if you want to sleep better at night

Controller responsibilities

The upload controller should be small and unopinionated. Expose a POST endpoint, accept a MultipartFile parameter, and call a storage service. Let the service do the heavy lifting so your controller is not dramatic.

Key controller checks

• Accept MultipartFile file as a parameter
• Reject empty files early with a clear 400 style response
• Return a simple payload with file name and status so clients can parse it without therapy

Storage service and validation

The storage service does one job and does it well. Save files to disk or hand them off to cloud storage. Validate content type and size before you touch the disk. Check the content type string and the file length and throw a custom exception for violations so the API stays consistent.

Validation checklist

• Accept only expected content types such as image slash png or image slash jpeg when you expect images
• Enforce a maximum size and reject anything larger than your limits
• Sanitize file names to avoid path traversal or weird characters

Configure multipart settings

Tune multipart settings in application properties so the server does not swallow huge files by accident. For example set spring.servlet.multipart.max-file-size to a reasonable value and spring.servlet.multipart.max-request-size to match. These values keep resource usage predictable.

Error handling and responses

Add exception handlers for multipart exceptions and validation failures. Return JSON error responses with status codes and human friendly messages. That way a client can understand what went wrong without calling you at 2 AM.

Testing tips

Test with curl or Postman. Use form data with the file field name matching your controller parameter. For unit tests mock MultipartFile and assert that your service validates and stores as expected. Integration tests can use a temp directory for storage so you do not litter your dev machine with mystery files.

Best practices and tips

• Store file metadata in a database and files on a dedicated storage layer for scalability
• Log uploads with request id so you can trace problems without guessing
• Return small, consistent JSON responses so clients do not have to parse a novel

Summary

Follow the steps above and you get a robust Spring Boot file upload REST API that validates inputs, stores files where developers expect, and returns sensible errors when clients try to be creative. You will spend less time debugging file mishaps and more time pretending your code was perfect all along.