private void setNotModified(HttpServletResponse response, String eTag, long expires) {
        response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
        response.setHeader("ETag", eTag); // Required in 304.
        response.setDateHeader("Expires", expires); // Postpone cache with 1 week.
        response.setHeader("Cache-Control", String.format("public, max-age=%d", DEFAULT_EXPIRE_TIME));
    }
Process the actual request.

@param request  The request to be processed.
@param response The response to be created.
@param content  Whether the request body should be written (GET) or not (HEAD).
@throws IOException If something fails at I/O level.
src/main/java/com/kodedu/controller/FileService.java