public class OnlineListener implementss HttpSessionListener
private int onlineCount;
public OnlineListener()
{onlineCount=0;
}
public void sessionCreated(HttpSessionEvent sessionEvent)
{
onlineCount++;
sessionEvent.getSession().getServletContext().setAttribute("online",new Integer(onlineCount));
}
public void sessionDestroyed(HttpSessionEvent sessionEvent)
{
onlineCount--;
sessionEvent.getSession().getServletContext().setAtrribute("online",new Integer(onlineCount));
}
}

------解决方案--------------------
我们刚学ＪＳＰ，学了个application的内置对象……可以用它实现在线用户统计，好像，你可以看看，我也是刚学的，一起讨论讨论
index.jsp页面如下：
<%
Integer acount;
acount = (Interger)application.getAttrubute("acounter");
if(acount==null){
 acount = new Integer(1);
}else{
 acount = new Integer(requset.intValue()+1);
}
application.setAttribute("acounter","acount");
%>
<table>
 <tr>
 <th>在线用户为</th>
 <Td><%=application.getAttribute("acounter")%></Td>
 </tr>
</table>