Downloading large files via Spring MVC

Question

I have a rest method for downloading files which works. But, it seems that the download doesn't start on the web client until the file is completely copied to the output stream, which can take a while for large files.

@GetMapping(value = "download-single-report")
public void downloadSingleReport(HttpServletResponse response) {

    File dlFile = new File("some_path");

    try {
        response.setContentType("application/pdf");
        response.setHeader("Content-disposition", "attachment; filename="+ dlFile.getName());
        InputStream inputStream = new FileInputStream(dlFile);
        IOUtils.copy(inputStream, response.getOutputStream());
        response.flushBuffer();
    } catch (FileNotFoundException e) {
        // error
    } catch (IOException e) {
        // error
    }
}

Is there a way to "stream" the file such that the download starts as soon as I begin writing to the output stream?

I also have a similar method that takes multiple files and puts them in a zip, adding each zip entry to the zip stream, and the download also only begins after the zip has been created:

        ZipEntry zipEntry = new ZipEntry(entryName);
        zipOutStream.putNextEntry(zipEntry);
        IOUtils.copy(fileStream, zipOutStream);

Answer 1

You can use InputStreamResource to return stream result. I tested and it is started copying to output immediately.

    @GetMapping(value = "download-single-report")
    public ResponseEntity<Resource> downloadSingleReport() {
        File dlFile = new File("some_path");
        if (!dlFile.exists()) {
            return ResponseEntity.notFound().build();
        }

        try {
            try (InputStream stream = new FileInputStream(dlFile)) {
                InputStreamResource streamResource = new InputStreamResource(stream);
                return ResponseEntity.ok()
                        .contentType(MediaType.APPLICATION_PDF)
                        .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + dlFile.getName() + "\"")
                        .body(streamResource);
            }

            /*
            // FileSystemResource alternative
            
            FileSystemResource fileSystemResource = new FileSystemResource(dlFile);
            return ResponseEntity.ok()
                    .contentType(MediaType.APPLICATION_PDF)
                    .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + dlFile.getName() + "\"")
                    .body(fileSystemResource);
           */ 
        } catch (IOException e) {
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).build();
        }
    }

The second alternative is a partial download method.

    @GetMapping(value = "download-single-report-partial")
    public void downloadSingleReportPartial(HttpServletRequest request, HttpServletResponse response) {
        File dlFile = new File("some_path");
        if (!dlFile.exists()) {
            response.setStatus(HttpStatus.NOT_FOUND.value());
            return;
        }
        try {
            writeRangeResource(request, response, dlFile);
        } catch (Exception ex) {
            response.setStatus(HttpStatus.INTERNAL_SERVER_ERROR.value());
        }
    }

    public static void writeRangeResource(HttpServletRequest request, HttpServletResponse response, File file) throws IOException {
        String range = request.getHeader("Range");
        if (StringUtils.hasLength(range)) {
            //http
            ResourceRegion region = getResourceRegion(file, range);
            long start = region.getPosition();
            long end = start + region.getCount() - 1;
            long resourceLength = region.getResource().contentLength();
            end = Math.min(end, resourceLength - 1);
            long rangeLength = end - start + 1;

            response.setStatus(206);
            response.addHeader("Accept-Ranges", "bytes");
            response.addHeader("Content-Range", String.format("bytes %s-%s/%s", start, end, resourceLength));
            response.setContentLengthLong(rangeLength);
            try (OutputStream outputStream = response.getOutputStream()) {
                try (InputStream inputStream = new BufferedInputStream(new FileInputStream(file))) {
                    StreamUtils.copyRange(inputStream, outputStream, start, end);
                }
            }
        } else {
            response.setStatus(200);
            response.addHeader("Accept-Ranges", "bytes");
            response.setContentLengthLong(file.length());
            try (OutputStream outputStream = response.getOutputStream()) {
                try (InputStream inputStream = new BufferedInputStream(new FileInputStream(file))) {
                    StreamUtils.copy(inputStream, outputStream);
                }
            }
        }
    }

    private static ResourceRegion getResourceRegion(File file, String range) {
        List<HttpRange> httpRanges = HttpRange.parseRanges(range);
        if (httpRanges.isEmpty()) {
            return new ResourceRegion(new FileSystemResource(file), 0, file.length());
        }
        return httpRanges.get(0).toResourceRegion(new FileSystemResource(file));
    }

Spring Framework Resource Response Process

Resource response managed by ResourceHttpMessageConverter class. In writeContent method, StreamUtils.copy is called.

package org.springframework.http.converter;

public class ResourceHttpMessageConverter extends AbstractHttpMessageConverter<Resource> {
..
    protected void writeContent(Resource resource, HttpOutputMessage outputMessage)
            throws IOException, HttpMessageNotWritableException {
        try {
            InputStream in = resource.getInputStream();
            try {
                StreamUtils.copy(in, outputMessage.getBody());
            }
            catch (NullPointerException ex) {
                // ignore, see SPR-13620
            }
            finally {
                try {
                    in.close();
                }
                catch (Throwable ex) {
                    // ignore, see SPR-12999
                }
            }
        }
        catch (FileNotFoundException ex) {
            // ignore, see SPR-12999
        }
    }
}

out.write(buffer, 0, bytesRead); sends data immediately to output (I have tested on my local machine). When whole data is transferred, out.flush(); is called.

package org.springframework.util;

public abstract class StreamUtils {
..
    public static int copy(InputStream in, OutputStream out) throws IOException {
        Assert.notNull(in, "No InputStream specified");
        Assert.notNull(out, "No OutputStream specified");
        int byteCount = 0;

        int bytesRead;
        for(byte[] buffer = new byte[4096]; (bytesRead = in.read(buffer)) != -1; byteCount += bytesRead) {
            out.write(buffer, 0, bytesRead);
        }

        out.flush();
        return byteCount;
    }
}