How to modify Spring controller to prevent downloading a file that hasn't changed

Question

I have an implementation similar to this one that allows the caller to ask for a specific file from the filesystem. The file is one that doesn't change frequently and is being downloaded by a mobile application. Is there a way I can change this to use the 304 - Not Modified HTTP status code so the client wouldn't have to download the file when it hasn't changed? If so, would the interface to this method have to change in some way to accept a date? Is there some standard way to implement this?

@RequestMapping(value = "/files/datafile", method = RequestMethod.GET)
public void getDataFile(HttpServletResponse response) {
    try {
      // get your file as InputStream
      InputStream is = ...;
      // copy it to response's OutputStream
      org.apache.commons.io.IOUtils.copy(is, response.getOutputStream());
      response.flushBuffer();
    } catch (IOException ex) {
      log.info("Error writing file to output stream. Filename was '{}'", fileName, ex);
      throw new RuntimeException("IOError writing file to output stream");
    }

}

Ralph · Answer 1 · 2015-12-07T20:56:17.277

0

Use Springs ETag Header support.

You just need to add this filter to your web.xml

<filter>
    <filter-name>etagFilter</filter-name>
    <filter-class>org.springframework.web.filter.ShallowEtagHeaderFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>etagFilter</filter-name>
    <servlet-name>petclinic</servlet-name>
</filter-mapping>

Or you do it more sophisticated in your controller:

Using Spring support, but then you need to rewrite it so that it return a ResponseEntity instead of using HttpResponse directly, like described in Spring Reference Chapter 21.14.3 Support for the Cache-Control, ETag and Last-Modified response headers in Controllers.
Of course you could also set the etag header directly to your HttpResponse: RFC 2616 Chapter 14.19 ETag

edited Dec 07 '15 at 20:56

answered Dec 07 '15 at 20:49

Ralph

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Wouldn't the client code need to change as well? How would the server know that the value hasn't changed if the client doesn't provide a hash of the old value? – Chris Williams Dec 08 '15 at 19:17
If your client is an not to old webbrowser, then it should work. The key is that the client send the etag header to the browser back if it request the same url, than the server can decide if it must send back the content or just the statuscode that nothing has changed. – Ralph Dec 08 '15 at 19:28
The client is a mobile app. – Chris Williams Dec 08 '15 at 19:29
Then maybe, the client needs to modified too. to send the etag header back to the server. – Ralph Dec 08 '15 at 19:31

How to modify Spring controller to prevent downloading a file that hasn't changed

1 Answers1