利用监听器实现在线人数统计

登录Servlet
package web;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import bean.User;

public class LoginServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
//将值传进去;
User user=new User();
String id=req.getParameter("id");
user.setId(id);
user.setUsername(req.getParameter("username"));

HttpSession session=req.getSession();
session.setAttribute("user",user);
session.setAttribute(id,id);

System.out.println(((User)session.getAttribute("user")).getId());

}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
this.doGet(req, resp);
}
}

package listener;

import java.util.*;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

import bean.User;

public class OnLineUserListener implements ServletContextListener,
HttpSessionListener, HttpSessionAttributeListener {

//私有属性；
private ServletContext application;

public void contextDestroyed(ServletContextEvent arg0) {

}

/*web服务器启动开始监听
* @see javax.servlet.ServletContextListener#contextInitialized(javax.servlet.ServletContextEvent)
*/
public void contextInitialized(ServletContextEvent se) {
//应用启动时，创建一个set，保存所有在线用户;
Set<User>onLineUsers=new HashSet<User>();
//将set添加到application范围中去
application=se.getServletContext();
application.setAttribute("onLineUsers",onLineUsers);
}

public void sessionCreated(HttpSessionEvent arg0) {

}

public void sessionDestroyed(HttpSessionEvent se) {
//用户注销后，得到用户注销的信息;
HttpSession session=se.getSession();
//得到注销的用户信息;
User logoutUser=(User)session.getAttribute("user");

//从application中取出在线用户列表；
Set<User>onLineUsers=(Set)application.getAttribute("onLineUsers");
//把注销用户从列中删除;
onLineUsers.remove(logoutUser);
}

/*用户登录后
* @see javax.servlet.http.HttpSessionAttributeListener#attributeAdded(javax.servlet.http.HttpSessionBindingEvent)
*/
public void attributeAdded(HttpSessionBindingEvent se) {
//用户登录成功后
System.out.println("增加属性：");
HttpSession session=se.getSession();
//得到新登录用户信息
User newUser=(User)session.getAttribute("user");
System.out.println(newUser.getUsername());
//从application中取出在线用户列表；
Set<User>onLineUsers=(Set)application.getAttribute("onLineUsers");
//将新用户添加到列中去
System.out.println("o"+onLineUsers);
onLineUsers.add(newUser);
}

public void attributeRemoved(HttpSessionBindingEvent se) {
//用户注销后，得到注销后的用户信息;
User logoutUser=(User)se.getValue();
//从application中取出在线用户列表；
Set<User>onLineUsers=(Set)application.getAttribute("onLineUsers");
//把注销用户从列中删除;
onLineUsers.remove(logoutUser);
}

public void attributeReplaced(HttpSessionBindingEvent arg0) {

}

}

<c:forEach items="${onLineUsers}" var="user">
id：${user.id },name:${user.username }
</c:forEach>