web程序统计在线人数的实现方式！

//统计当前在线用户个数
public class UserCountListener implements HttpSessionListener {

public void sessionCreated(HttpSessionEvent se) {

ServletContext context = se.getSession().getServletContext();
Integer num = (Integer) context.getAttribute("num");
if(num==null){
context.setAttribute("num", 1);
}else{
num++;
context.setAttribute("num", num);
}

}

public void sessionDestroyed(HttpSessionEvent se) {

ServletContext context = se.getSession().getServletContext();
Integer num = (Integer) context.getAttribute("num");
if(num==null){
context.setAttribute("num", 1);
}else{
num--;
context.setAttribute("num", num);
}
}

}