如何知道一个访客在一个网站上停留多长时间？

package org.javaer.test;import java.util.Date;import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;public class TestSession extends HttpServlet implements HttpSessionListener {        private static final long serialVersionUID = 1L;        public void sessionCreated(HttpSessionEvent arg0) {
                // TODO Auto-generated method stub                System.out.print("访问时间--------");
                System.out.println(new Date().toString());
        }        public void sessionDestroyed(HttpSessionEvent arg0) {
                // TODO Auto-generated method stub                System.out.print("离开时间--------");
                System.out.println(new Date().toString());
        }}-------------------------------------------------------------------------------
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.4"
        xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee   http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">        <filter>
                <filter-name>EncodingFilter</filter-name>
                <filter-class>org.javaer.servlet.EncodingFilter</filter-class>
                <init-param>
                        <param-name>encoding</param-name>
                        <param-value>UTF-8</param-value>
                </init-param>
        </filter>
        <filter-mapping>
                <filter-name>EncodingFilter</filter-name>
                <url-pattern>/*</url-pattern>
        </filter-mapping>        <listener>
                <listener-class>org.javaer.test.TestSession</listener-class>
        </listener>        <servlet>
                <servlet-name>action</servlet-name>
                <servlet-class>
                        org.apache.struts.action.ActionServlet
                </servlet-class>
                <init-param>
                        <param-name>config</param-name>
                        <param-value>/WEB-INF/struts-config.xml</param-value>
                </init-param>                <load-on-startup>2</load-on-startup>
        </servlet>        <servlet-mapping>
                <servlet-name>action</servlet-name>
                <url-pattern>*.do</url-pattern>
        </servlet-mapping></web-app>
--------------------------------------------------------------------------上面两个分别是我的SERVLET 和web.xml文件。现在出现的问题是用户退出的时候。sessionDestroyed  方法不能被执行