ReactiveUploadResource.java

@RestController
@RequestMapping(path = "/api/files")
public class ReactiveUploadResource {
    Logger LOGGER = LoggerFactory.getLogger(ReactiveUploadResource.class);

    /**
     * upload handler method, mapped to POST. Like any file upload handler it consumes MULTIPART_FORM_DATA.
     * Produces a JSON respomse
     *
     * @param parts a flux providing all part contained in the MULTIPART_FORM_DATA request
     * @return a flux of results - one element per uploaded file
     */
    @RequestMapping(method = RequestMethod.POST, consumes = MediaType.MULTIPART_FORM_DATA_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
    public Flux<String> uploadHandler(@RequestBody Flux<Part> parts) {
        return parts
                .filter(part -> part instanceof FilePart) // only retain file parts
                .ofType(FilePart.class) // convert the flux to FilePart
                .flatMap(this::saveFile); // save each file and flatmap it to a flux of results
    }
    
    ....
        
    private Mono<String> saveFile(FilePart filePart) {
        ....
    }
    
}