[FIXED] Return a file from a Spring Boot Service without using disk

Issue

I'd like to know if there's any idiomatic way to return data from a Spring Boot Service, via a REST controller, without having to resort to writing any File() objects to the disk of the service itself.

Below is an example, where I've managed to get a Rest Endpoint to return a ByteArray of the data, without resorting to disk use. Unfortunately, with this implementation I seem to lose any control over the final file extension and name.

(I'm guessing this is down to how whatever client, making the call, handles the response body of bytes)

@RestController
@RequestMapping("/dataComparison")
@Profile("!prod")
class ComparisonReportController(
    private val reportService: ReportMappingService,
) {
    /*
    If you get errors parsing your csv file, make sure your file is saved in utf-8 encoding
    */

    @PostMapping(
        "/requestReport",
        consumes = [MULTIPART_FORM_DATA_VALUE],
        produces = [APPLICATION_OCTET_STREAM_VALUE]
    )
    @ResponseStatus(HttpStatus.OK)
    fun getReport(
        @RequestParam("file", required = true) file: MultipartFile,
        @RequestParam(
            "fromDate",
            required = true
        ) @DateTimeFormat(iso = DateTimeFormat.ISO.DATE) fromDate: LocalDate,
        @RequestParam("numDays", required = true) numDays: Long,
        @RequestParam("reportType", required = true) reportType: ReportType,
    ): ByteArray {
        withLoggingContext(getItemControllerTags()) {
            log.info { "Received csv file of size [${file.size}b]. Beginning to parse the file..." }

            if (numDays <= 0) {
                throw IllegalArgumentException("The report cannot be run for $numDays days")
            }
            if (file.isEmpty) {
                throw IllegalArgumentException("Cannot run a data comparison with an an empty file: ${file.name} ")
            }

            val config = createReportConfig(file, fromDate, numDays, reportType)
            val report = reportService.getReport(config)
            val lines = report.render()
            val bytes = ReserveGroupReport.getCsvBytes(lines)

            return bytes
        }
    }
}

Solution

You should set some headers to your response, to let the client know, that you return not just bytes, but a file. For example, you can do it directly by returning a ResponseEntity from your controller with HttpHeaders inside.

I'm sorry, but I don't know Kotlin and will show this in java, for a situation when you need to send a pdf file:

@PostMapping(value = "/requestReport", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
public ResponseEntity<byte[]> getReport(MultipartFile file) {
    byte[] bytes = ReportService.createReport();
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_PDF);
    headers.setContentDisposition(ContentDisposition.attachment().build());
    headers.setContentDispositionFormData("report.pdf", "report.pdf");
    return new ResponseEntity<>(bytes, headers, HttpStatus.OK);
}

You can choose any disposition and file name here.

Answered By - AndrewThomas
Answer Checked By - Marie Seifert (JavaFixing Admin)