老问题!!

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class WebSessionListener implements HttpSessionListener {

private static final String ONLINE_USERS_COUNT = "ONLINE_USERS_COUNT";

public void sessionCreated(HttpSessionEvent arg0) {
//

ServletContext servletContext = arg0.getSession().getServletContext();
int count;
if (servletContext.getAttribute(ONLINE_USERS_COUNT)!=null) {
count = ((Integer) servletContext.getAttribute(ONLINE_USERS_COUNT)).intValue() + 1;
} else {
count = 1;
}
System.out.println("-------------session created online users count= " + count);
servletContext.setAttribute(ONLINE_USERS_COUNT, new Integer(count));
}

public void sessionDestroyed(HttpSessionEvent arg0) {
//
ServletContext servletContext = arg0.getSession().getServletContext();
int count;
Integer countInteger = (Integer) servletContext.getAttribute(ONLINE_USERS_COUNT);
if (countInteger!=null && (countInteger).intValue()>0) {
count = (countInteger).intValue() - 1;
} else {
count = 0;
}
System.out.println("-------------session Destroyed online users count= " + count);
servletContext.setAttribute(ONLINE_USERS_COUNT, new Integer(count));
}

}

web.xml
<!-- HttpSession listener 统计当前在线用户数 -->
<listener>
<listener-class>web.listener.WebSessionListener</listener-class>
</listener>

就这么简单!