Master HTTP Caching: Using Last-Modified, ETag, Expires, and Cache-Control

Proper use of HTTP caching can dramatically lower server load and network bandwidth, making it essential to understand the HTTP caching protocol.

1. Use the Last-Modified header

On the first request, the server returns a Last-Modified header with a GMT timestamp, e.g., Wed, 22 Jul 2009 07:08:07 GMT. In a servlet you can add it with: response.addDateHeader("Last-Modified", date.getTime()); The corresponding request header is If-Modified-Since. The server compares this value with the resource's last modification time and returns a 304 Not Modified response if unchanged. To read the request header in a servlet:

long last = request.getDateHeader("If-Modified-Since");

2. Use the ETag header

When timestamps are insufficient, an ETag can be generated from the content (e.g., using CRC32 or MD5). The response includes the ETag header, and the client sends it back in If-None-Match. If they match, the server returns 304 without a body.

3. Use the Expires header

The Expires header tells the browser not to request the resource until the specified time, allowing the content to be served from the local cache. In a servlet you can set it with:

response.addDateHeader("Expires", date.getTime());

4. Use max‑age in Cache‑Control

The max-age directive specifies how many seconds the response is considered fresh. Example: response.addHeader("Cache-Control", "max-age=10"); If you need additional directives (e.g., private), combine them in a single header: response.addHeader("Cache-Control", "private, max-age=10"); Summary Last-Modified and ETag still require a request to the server, but only a 304 status is returned when the content is unchanged. Expires and max-age allow the browser to serve the resource directly from its cache without contacting the server. Note that a manual refresh (Ctrl+F5) bypasses all caches.

Useful HTTP monitoring tools

Firefox: httpfox, Live HTTP Header

Internet Explorer: HttpWatch, IEHttpHeader