Avoid Sending Unchanged Data

Clients send If-Modified-Since header, so, what is the Last-Modified of a document sent by a servlet?

You can set this with:

public long getLastModified(HttpServletRequest req) {
  return lastprimeModified.getTime() / 1000 * 1000;
}

Return a time in milliseconds since epoch, but round to nearest second to avoid problems in comparison.
You can expand on this idea to do server-side caching of your servlet responses. (how?)