package SessionCount;
import javax.servlet.*;
import javax.servlet.http.*;

public class SessionCounter implements HttpSessionListener {

private static int activeSessions = 0;

public void sessionCreated(HttpSessionEvent se) {
activeSessions++;
}

public void sessionDestroyed(HttpSessionEvent se) {
if(activeSessions > 0)
activeSessions--;
}

public static int getActiveSessions() {
return activeSessions;
}
}

接着建立online.jsp文件用于显示在线人数
<％ page import="SessionCount.SessionCounter" ％>
在线：<％= SessionCounter.getActiveSessions() ％>

然后需要在你的网站的WEB-INF中建立web.xml
文件内容如下：
<!-- Web.xml -->
<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app
PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/j2ee/dtds/web-app_2.3.dtd";>

<web-app>

<!-- Listeners -->
<listener>
<listener-class>
SessionCount.SessionCounter
</listener-class>
</listener>

</web-app>