Perhaps there would be some people wondering, why should i use expiry date on my static files? Well for a first-time visitor to your page may have to make several HTTP requests to load all your web page’s content, but by using the Expires header you make those components cacheable. This avoids unnecessary HTTP requests on subsequent page views. Expires headers are most often used with images, but they should be used on all components including scripts, stylesheets, and Flash components.

But dont forget, using a far future Expires header affects page views only after a user has already visited your site. It has no effect on the number of HTTP requests when a user visits your site for the first time and the browser’s cache is empty. Therefore the impact of this performance improvement depends on how often users hit your pages with a primed cache.

Okay, so basically i use a simple ServletFilter to add Expiry header on each static contents, on this example would be .css, .png and .gif files. So here is my code,

package com.edw.fw.server.filter;

import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;

public class ExpiryFilter implements Filter {

	// add a five years expiry
	private Integer years = 5;

	@Override
	public void destroy() {
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		if (years > -1) {
			Calendar c = Calendar.getInstance();
			c.setTime(new Date());
			c.add(Calendar.YEAR, years);

			// HTTP header date format: Thu, 01 Dec 1994 16:00:00 GMT
			String o = new SimpleDateFormat("EEE, d MMM yyyy HH:mm:ss zzz")
					.format(c.getTime());
			((HttpServletResponse) response).setHeader("Expires", o);
		}

		chain.doFilter(request, response);
	}

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {		
	}
}

And dont forget to register your Filter to your web.xml file

		<!--  
			expiration date filter
		-->
	  	<filter>
			<description>Set cache expiry for static content</description>
			<filter-name>ExpiryFilter</filter-name>
			<filter-class>com.edw.fw.server.filter.ExpiryFilter</filter-class>
		</filter>
		<filter-mapping>
			<filter-name>ExpiryFilter</filter-name>
			<url-pattern>*.css</url-pattern>
			<dispatcher>REQUEST</dispatcher>
		</filter-mapping>
		<filter-mapping>
			<filter-name>ExpiryFilter</filter-name>
			<url-pattern>*.png</url-pattern>
			<dispatcher>REQUEST</dispatcher>
		</filter-mapping>
		<filter-mapping>
			<filter-name>ExpiryFilter</filter-name>
			<url-pattern>*.gif</url-pattern>
			<dispatcher>REQUEST</dispatcher>
		</filter-mapping>

This is what it looks like on my browser’s http log,

as you can see, my http request get 304 not modified header, due to accessing my browser’s cache instead of the targetted web page.

Okay, i hope this helped other. Have Fun