Extracting Request Parameters Dynamically for a multipart/form-data Request

While working on my Micropub endpoint I found that I needed to support multipart/form-data requests. Having never worked with them, there was a bit of a learning curve, especially around how to get the request parameters out of them (as I was not yet needing to support the media aspect of the request).

I had to dynamically parse these fields out because on any given request there could be anywhere between 4 and 10 parameters in a given request, and I'd not really ever know what that request would look like until receiving it.

Unfortunately most of the articles/answers on Stack Overflow I could find were not very helpful as they were based on the expectation that you had knowledge of which fields you were going to receive.

I did, however, manage to finally track down this StackOverflow answer which mentions you can use the MultipartHttpServletRequest.

By adding this parameter to your endpoint's method call, you then get access to the full key-value parameters for the request in a Map which allows you to perform more automated traversal through the request's parameters.

This can be hooked in as such:

import org.springframework.util.MultiValueMap;

@PostMapping(path = "/endpoint", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
public ResponseEntity<String> multipartForm(MultipartHttpServletRequest multipartRequest) {
  Map<String, String[]> multipartRequestParams = multipartRequest.getParameterMap();
  // iterate through the Map as you want, or get individual fields out
  String[] contentArr = multipartRequestParams.get("content");

  // or
  String[] contentArr = multiPartRequest.getParameter("content");
}

This has been tested with Spring Boot 2.1.4.RELEASE (which contains Spring 5.1.6.RELEASE).