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Learn how to stream files using `Multipart/form-data` in Spring WebFlux without running into memory issues.
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Stream Files Efficiently with Multipart/Form-Data in Spring WebFlux

Handling file uploads can be a tricky part of web application development, especially when you're working with large files. When you receive Multipart/form-data from a client, it is imperative to handle this data efficiently, particularly in reactive applications using Spring WebFlux. In this guide, we will go through how to effectively stream file content received in Multipart/form-data format to another backend service without bloating our application's memory.

The Problem

When dealing with file uploads in Spring WebFlux, many developers follow a common approach where they read the entire file content into memory as a byte array. Here’s an example of such an approach:

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While this method works, it has significant downsides:

Memory Consumption: Reading whole files into memory can lead to high memory usage, especially with larger files.

Array Size Limitation: The file size is limited by Integer.MAX_VALUE, which can quickly become a bottleneck.

Extra Encoding: Encoding files as base64 strings increases the file size and uses additional memory.

Configuration Issues: The application’s codec memory limit must be greater than the file size, which can cause configuration headaches.

The Solution

To fix these problems, we can streamline the process by directly streaming the file content instead of converting it into a byte array. Here is how to properly set up your Spring WebFlux application to handle this seamlessly.

Step 1: Setting Up the External Controller

You'll need to create an external controller that accepts the multipart file and prepares it for streaming. Here’s a working example of how to do that:

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Step 2: Internal Service Controller

This is essential for handling the incoming streamed content in the service that you are forwarding to:

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Detailed Breakdown

MultipartBodyBuilder: We create a MultipartBodyBuilder object to construct the body of the request we'll send to another service.

Streaming: The asyncPart method streams the file directly, avoiding the creation of a byte array.

Filename: The filename is retained by specifying .filename(document.filename()), ensuring that the receiving service gets the appropriate file name.

WebClient: We utilize Spring's WebClient, a reactive HTTP client that is well-suited for making asynchronous requests.

Conclusion

By following the method described in this guide, you can stream files efficiently using Multipart/form-data with Spring WebFlux. This avoids memory issues and ensures that your application can handle file uploads effectively, even for larger file sizes. Remember to incorporate proper error handling and validation in your production applications to make your file upload process robust.

For more details, refer to the Spring WebFlux documentation and best practices to keep your reactive applications efficient and scalable.