Expires with dynamic cache in nginx

Question

I'm trying to set some Expires in a lot of JavaScript that I got here, but they are not static on filesystem, it is dynamic generated by a Java application.

If I do this:

  location /bla {
    root /tmp;
    expires 30m;
  }

And put some .js files inside /tmp/bla everything works fine.

But when I try to do this:

  location /script {
    expires 30m;
    proxy_cache cache;
    proxy_pass http://localhost:8080;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
  }

Does not work. The reply from the server is always 200, never 304 as it should be.

What can I do to fix this?

I also tried to use nginx 1.3.3 with Etag support, but also only works with static files :(

Thanks!!