Java基于servlet监听器实现在线人数监控功能的方法

package com.smalle.listener;import java.util.LinkedList;import java.util.List;import javax.servlet.ServletContext;import javax.servlet.ServletContextEvent;import javax.servlet.ServletContextListener;import javax.servlet.http.HttpSessionAttributeListener;import javax.servlet.http.HttpSessionBindingEvent;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener;public class OnlineListener implements  ServletContextListener, HttpSessionAttributeListener, HttpSessionListener {  private ServletContext application = null;  //应用上下文初始时会回调的方法  @Override  public void contextInitialized(ServletContextEvent e) {    //初始化一个application对象    application = e.getServletContext();    //设置一个列表属性，用于保存在线用户名    this.application.setAttribute("online", new LinkedList<String>());  }  //往会话中添加属性时的回调方法  @Override  public void attributeAdded(HttpSessionBindingEvent e) {    //取得用户名列表    List<String> onlines = (List<String>) this.application.getAttribute("online");    if("username".equals(e.getName())){      onlines.add((String) e.getValue());    }    //将添加后的列表重新设置列application属性中.    this.application.setAttribute("online", onlines);  }  //会话销毁时会回调的方法  @Override  public void sessionDestroyed(HttpSessionEvent e) {    //取得用户名列表    List<String> onlines = (List<String>) this.application.getAttribute("online");    //取得当前用户名    String username = (String) e.getSession().getAttribute("username");    //将此用户从列表中删除    onlines.remove(username);    //讲删除后的列表重新设置到application属性中.    this.application.setAttribute("online", onlines);  }  public void sessionCreated(HttpSessionEvent e) {}  public void attributeRemoved(HttpSessionBindingEvent e) {}  public void attributeReplaced(HttpSessionBindingEvent e) {}  public void contextDestroyed(ServletContextEvent e) {}}

package com.smalle.listener;import java.io.IOException;import java.io.PrintWriter;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LoginServlet extends HttpServlet {  private static final long serialVersionUID = 1L;  public void doGet(HttpServletRequest request, HttpServletResponse response)      throws ServletException, IOException {    this.doPost(request, response);  }  public void doPost(HttpServletRequest request, HttpServletResponse response)      throws ServletException, IOException {    request.setCharacterEncoding("utf-8");  //设置响应内容类型        String username= request.getParameter("username");  //获取请求参数中的用户名        //往session中添加属性,会触发HttpSessionAttributeListener中的attributeAdded方法        if(username != null && !username.equals("")) {          request.getSession().setAttribute("username",username);        }        //从应用上下文中获取在线用户名列表        List<String> online = (List<String>)getServletContext().getAttribute("online");System.out.println("LoginServlet" + online);        response.setContentType("text/html;charset=utf-8");        PrintWriter out = response.getWriter();        out.println("");        out.println(" <title>用户列表</title>");        out.println(" ");        out.println("当前用户是：" + username);        out.print("  <hr><h3>在线用户列表</h3>");        int size = online == null ? 0 : online.size();        for (int i = 0; i < size; i++) {          if(i > 0){            out.println("<br>");          }          out.println(i + 1 + "." + online.get(i));        }        //注意: 要对链接URL进行自动重写处理        out.println("<hr/><a href=/"" + response.encodeURL("logoutListener") + "/">注销</a>");        out.println(" ");        out.println("");        out.flush();        out.close();  }}

package com.smalle.listener;import java.io.IOException;import java.io.PrintWriter;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LogoutServlet extends HttpServlet{  public void doGet(HttpServletRequest request, HttpServletResponse response)      throws ServletException, IOException {    this.doPost(request, response);  }  public void doPost(HttpServletRequest request, HttpServletResponse response)      throws ServletException, IOException {    request.setCharacterEncoding("utf-8");  //设置响应内容类型    //销毁会话,会触发SessionLinstener中的sessionDestroyed方法    request.getSession().invalidate();    //从应用上下文中获取在线用户名列表    List<String> online = (List<String>)getServletContext().getAttribute("online");    response.setContentType("text/html;charset=utf-8");    PrintWriter out = response.getWriter();    out.println("");    out.println(" <title>用户列表</title>");    out.println(" ");    out.print("  <h3>在线用户列表</h3>");    int size = online == null ? 0 : online.size();    for (int i = 0; i < size; i++) {      if(i > 0){        out.println("<br>");      }      out.println(i + 1 + "." + online.get(i));    }    out.println("<hr><a href='/'index.html/''>主页</a>");    out.println(" ");    out.println("");    out.flush();    out.close();  }}

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0"> <display-name>testServlet</display-name>  <listener>    <listener-class>com.smalle.listener.OnlineListener</listener-class>  </listener>  <servlet>    <servlet-name>LoginServlet</servlet-name>    <servlet-class>com.smalle.listener.LoginServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>LoginServlet</servlet-name>    <url-pattern>/loginListener</url-pattern>  </servlet-mapping>  <servlet>    <servlet-name>LogoutServlet</servlet-name>    <servlet-class>com.smalle.listener.LogoutServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>LogoutServlet</servlet-name>    <url-pattern>/logoutListener</url-pattern>  </servlet-mapping> <welcome-file-list>  <welcome-file>index.jsp</welcome-file> </welcome-file-list></web-app>

<!DOCTYPE html><html> <head>  <title>index.html</title>  <meta name="content-type" content="text/html; charset=UTF-8"> </head> <body>  <form action="loginListener" method="post">    用户名：<input type="text" name="username">  <input type="submit" value="登录"><br><br>  </form> </body></html>