Возможность докачки файла

Используйте jetty, есть готовый проект по вашему случаю -> https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-websocket-jetty

А изобретать костыли с использованием WebSocket не комильфо. Или пробуйте стандартными средствами RMI - быстро/проверенно

Кому интересно, воспользовался этим методом. Всё работает, подкачка поддерживается.

https://gist.github.com/davinkevin/b97e39d7ce89198774b4

MultipartFileSender.fromPath(file.getLocalPath())
                    .with(request)
                    .with(response)
                    .serveResource();

public class MultipartFileSender {
    protected final Logger logger = LoggerFactory.getLogger(this.getClass());
    private static final int DEFAULT_BUFFER_SIZE = 20480; // ..bytes = 20KB.
    private static final long DEFAULT_EXPIRE_TIME = 604800000L; // ..ms = 1 week.
    private static final String MULTIPART_BOUNDARY = "MULTIPART_BYTERANGES";
    Path filepath;
    HttpServletRequest request;
    HttpServletResponse response;
    public MultipartFileSender() {
    }
    public static MultipartFileSender fromPath(Path path) {
        return new MultipartFileSender().setFilepath(path);
    }
    public static MultipartFileSender fromFile(File file) {
        return new MultipartFileSender().setFilepath(file.toPath());
    }
    public static MultipartFileSender fromURIString(String uri) {
        return new MultipartFileSender().setFilepath(Paths.get(uri));
    }
    //** internal setter **//
    private MultipartFileSender setFilepath(Path filepath) {
        this.filepath = filepath;
        return this;
    }
    public MultipartFileSender with(HttpServletRequest httpRequest) {
        request = httpRequest;
        return this;
    }
    public MultipartFileSender with(HttpServletResponse httpResponse) {
        response = httpResponse;
        return this;
    }
    public void serveResource() throws Exception {
        if (response == null || request == null) {
            return;
        }
        if (!Files.exists(filepath)) {
            logger.error("File doesn't exist at URI : {}", filepath.toAbsolutePath().toString());
            response.sendError(HttpServletResponse.SC_NOT_FOUND);
            return;
        }
        Long length = Files.size(filepath);
        String fileName = filepath.getFileName().toString();
        FileTime lastModifiedObj = Files.getLastModifiedTime(filepath);
        if (StringUtils.isEmpty(fileName) || lastModifiedObj == null) {
            response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
            return;
        }
        long lastModified = LocalDateTime.ofInstant(lastModifiedObj.toInstant(), ZoneId.of(ZoneOffset.systemDefault().getId())).toEpochSecond(ZoneOffset.UTC);
        String contentType = MimeTypeUtils.probeContentType(filepath);
        // Validate request headers for caching ---------------------------------------------------
        // If-None-Match header should contain "*" or ETag. If so, then return 304.
        String ifNoneMatch = request.getHeader("If-None-Match");
        if (ifNoneMatch != null && HttpUtils.matches(ifNoneMatch, fileName)) {
            response.setHeader("ETag", fileName); // Required in 304.
            response.sendError(HttpServletResponse.SC_NOT_MODIFIED);
            return;
        }
        // If-Modified-Since header should be greater than LastModified. If so, then return 304.
        // This header is ignored if any If-None-Match header is specified.
        long ifModifiedSince = request.getDateHeader("If-Modified-Since");
        if (ifNoneMatch == null && ifModifiedSince != -1 && ifModifiedSince + 1000 > lastModified) {
            response.setHeader("ETag", fileName); // Required in 304.
            response.sendError(HttpServletResponse.SC_NOT_MODIFIED);
            return;
        }
        // Validate request headers for resume ----------------------------------------------------
        // If-Match header should contain "*" or ETag. If not, then return 412.
        String ifMatch = request.getHeader("If-Match");
        if (ifMatch != null && !HttpUtils.matches(ifMatch, fileName)) {
            response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
            return;
        }
        // If-Unmodified-Since header should be greater than LastModified. If not, then return 412.
        long ifUnmodifiedSince = request.getDateHeader("If-Unmodified-Since");
        if (ifUnmodifiedSince != -1 && ifUnmodifiedSince + 1000 <= lastModified) {
            response.sendError(HttpServletResponse.SC_PRECONDITION_FAILED);
            return;
        }
        // Validate and process range -------------------------------------------------------------
        // Prepare some variables. The full Range represents the complete file.
        Range full = new Range(0, length - 1, length);
        List<Range> ranges = new ArrayList<>();
        // Validate and process Range and If-Range headers.
        String range = request.getHeader("Range");
        if (range != null) {
            // Range header should match format "bytes=n-n,n-n,n-n...". If not, then return 416.
            if (!range.matches("^bytes=\\d*-\\d*(,\\d*-\\d*)*$")) {
                response.setHeader("Content-Range", "bytes */" + length); // Required in 416.
                response.sendError(HttpServletResponse.SC_REQUESTED_RANGE_NOT_SATISFIABLE);
                return;
            }
            String ifRange = request.getHeader("If-Range");
            if (ifRange != null && !ifRange.equals(fileName)) {
                try {
                    long ifRangeTime = request.getDateHeader("If-Range"); // Throws IAE if invalid.
                    if (ifRangeTime != -1) {
                        ranges.add(full);
                    }
                } catch (IllegalArgumentException ignore) {
                    ranges.add(full);
                }
            }
            // If any valid If-Range header, then process each part of byte range.
            if (ranges.isEmpty()) {
                for (String part : range.substring(6).split(",")) {
                    // Assuming a file with length of 100, the following examples returns bytes at:
                    // 50-80 (50 to 80), 40- (40 to length=100), -20 (length-20=80 to length=100).
                    long start = Range.sublong(part, 0, part.indexOf("-"));
                    long end = Range.sublong(part, part.indexOf("-") + 1, part.length());
                    if (start == -1) {
                        start = length - end;
                        end = length - 1;
                    } else if (end == -1 || end > length - 1) {
                        end = length - 1;
                    }
                    // Check if Range is syntactically valid. If not, then return 416.
                    if (start > end) {
                        response.setHeader("Content-Range", "bytes */" + length); // Required in 416.
                        response.sendError(HttpServletResponse.SC_REQUESTED_RANGE_NOT_SATISFIABLE);
                        return;
                    }
                    // Add range.                    
                    ranges.add(new Range(start, end, length));
                }
            }
        }
        // Prepare and initialize response --------------------------------------------------------
        // Get content type by file name and set content disposition.
        String disposition = "inline";
        // If content type is unknown, then set the default value.
        // For all content types, see: http://www.w3schools.com/media/media_mimeref.asp
        // To add new content types, add new mime-mapping entry in web.xml.
        if (contentType == null) {
            contentType = "application/octet-stream";
        } else if (!contentType.startsWith("image")) {
            // Else, expect for images, determine content disposition. If content type is supported by
            // the browser, then set to inline, else attachment which will pop a 'save as' dialogue.
            String accept = request.getHeader("Accept");
            disposition = accept != null && HttpUtils.accepts(accept, contentType) ? "inline" : "attachment";
        }
        logger.debug("Content-Type : {}", contentType);
        // Initialize response.
        response.reset();
        response.setBufferSize(DEFAULT_BUFFER_SIZE);
        response.setHeader("Content-Type", contentType);
        response.setHeader("Content-Disposition", disposition + ";filename=\"" + fileName + "\"");
        logger.debug("Content-Disposition : {}", disposition);
        response.setHeader("Accept-Ranges", "bytes");
        response.setHeader("ETag", fileName);
        response.setDateHeader("Last-Modified", lastModified);
        response.setDateHeader("Expires", System.currentTimeMillis() + DEFAULT_EXPIRE_TIME);
        // Send requested file (part(s)) to client ------------------------------------------------
        // Prepare streams.
        try (InputStream input = new BufferedInputStream(Files.newInputStream(filepath));
             OutputStream output = response.getOutputStream()) {
            if (ranges.isEmpty() || ranges.get(0) == full) {
                // Return full file.
                logger.info("Return full file");
                response.setContentType(contentType);
                response.setHeader("Content-Range", "bytes " + full.start + "-" + full.end + "/" + full.total);
                response.setHeader("Content-Length", String.valueOf(full.length));
                Range.copy(input, output, length, full.start, full.length);
            } else if (ranges.size() == 1) {
                // Return single part of file.
                Range r = ranges.get(0);
                logger.info("Return 1 part of file : from ({}) to ({})", r.start, r.end);
                response.setContentType(contentType);
                response.setHeader("Content-Range", "bytes " + r.start + "-" + r.end + "/" + r.total);
                response.setHeader("Content-Length", String.valueOf(r.length));
                response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT); // 206.
                // Copy single part range.
                Range.copy(input, output, length, r.start, r.length);
            } else {
                // Return multiple parts of file.
                response.setContentType("multipart/byteranges; boundary=" + MULTIPART_BOUNDARY);
                response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT); // 206.
                // Cast back to ServletOutputStream to get the easy println methods.
                ServletOutputStream sos = (ServletOutputStream) output;
                // Copy multi part range.
                for (Range r : ranges) {
                    logger.info("Return multi part of file : from ({}) to ({})", r.start, r.end);
                    // Add multipart boundary and header fields for every range.
                    sos.println();
                    sos.println("--" + MULTIPART_BOUNDARY);
                    sos.println("Content-Type: " + contentType);
                    sos.println("Content-Range: bytes " + r.start + "-" + r.end + "/" + r.total);
                    // Copy single part range of multi part range.
                    Range.copy(input, output, length, r.start, r.length);
                }
                // End with multipart boundary.
                sos.println();
                sos.println("--" + MULTIPART_BOUNDARY + "--");
            }
        }
    }
    private static class Range {
        long start;
        long end;
        long length;
        long total;
        /**
         * Construct a byte range.
         * @param start Start of the byte range.
         * @param end End of the byte range.
         * @param total Total length of the byte source.
         */
        public Range(long start, long end, long total) {
            this.start = start;
            this.end = end;
            this.length = end - start + 1;
            this.total = total;
        }
        public static long sublong(String value, int beginIndex, int endIndex) {
            String substring = value.substring(beginIndex, endIndex);
            return (substring.length() > 0) ? Long.parseLong(substring) : -1;
        }
        private static void copy(InputStream input, OutputStream output, long inputSize, long start, long length) throws IOException {
            byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
            int read;
            if (inputSize == length) {
                // Write full range.
                while ((read = input.read(buffer)) > 0) {
                    output.write(buffer, 0, read);
                    output.flush();
                }
            } else {
                input.skip(start);
                long toRead = length;
                while ((read = input.read(buffer)) > 0) {
                    if ((toRead -= read) > 0) {
                        output.write(buffer, 0, read);
                        output.flush();
                    } else {
                        output.write(buffer, 0, (int) toRead + read);
                        output.flush();
                        break;
                    }
                }
            }
        }
    }
    private static class HttpUtils {
        /**
         * Returns true if the given accept header accepts the given value.
         * @param acceptHeader The accept header.
         * @param toAccept The value to be accepted.
         * @return True if the given accept header accepts the given value.
         */
        public static boolean accepts(String acceptHeader, String toAccept) {
            String[] acceptValues = acceptHeader.split("\\s*(,|;)\\s*");
            Arrays.sort(acceptValues);
            return Arrays.binarySearch(acceptValues, toAccept) > -1
                    || Arrays.binarySearch(acceptValues, toAccept.replaceAll("/.*$", "/*")) > -1
                    || Arrays.binarySearch(acceptValues, "*/*") > -1;
        }
        /**
         * Returns true if the given match header matches the given value.
         * @param matchHeader The match header.
         * @param toMatch The value to be matched.
         * @return True if the given match header matches the given value.
         */
        public static boolean matches(String matchHeader, String toMatch) {
            String[] matchValues = matchHeader.split("\\s*,\\s*");
            Arrays.sort(matchValues);
            return Arrays.binarySearch(matchValues, toMatch) > -1
                    || Arrays.binarySearch(matchValues, "*") > -1;
        }
    }
}