监听器小例子（在线统计用户人数和用户信息）

package com.ndb.listener; import java.text.SimpleDateFormat; import java.util.ArrayList; import java.util.Date; import javax.servlet.ServletRequestEvent; import javax.servlet.ServletRequestListener; import javax.servlet.annotation.WebListener; import javax.servlet.http.HttpServletRequest; import com.ndb.entity.User; import com.ndb.util.sessionUtil; /** * Application Lifecycle Listener implementation class myServletRequestListener * */ @WebListener public class myServletRequestListener implements ServletRequestListener { private ArrayList<User> userList;//用户列表 /** * Default constructor. */ public myServletRequestListener() { // TODO Auto-generated constructor stub } /** * @see ServletRequestListener#requestDestroyed(ServletRequestEvent) */ public void requestDestroyed(ServletRequestEvent arg0) { // TODO Auto-generated method stub } /** * @see ServletRequestListener#requestInitialized(ServletRequestEvent) */ public void requestInitialized(ServletRequestEvent arg0) { // TODO Auto-generated method stub userList=(ArrayList<User>)arg0.getServletContext().getAttribute("userList"); if(userList==null){ userList=new ArrayList<>(); } HttpServletRequest request=(HttpServletRequest)arg0.getServletRequest(); String sessionId=request.getSession().getId(); sessionUtil su=new sessionUtil(); //如果当前没有该用户sessionId，则添加统计，有了，则不添加 if(su.getUserSessionId(userList,sessionId)==null){ User user=new User(); user.setSessionId(sessionId); user.setFirstTime(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date())); user.setIp(request.getRemoteAddr()); userList.add(user); } arg0.getServletContext().setAttribute("userList", userList); } }