JavaWeb应用中，很多的地方都和session有关。因此session相关的事件监听器，在日常工作中非常有用。

有时候我们需要统计当前在线的人数和访问人数总数，此时就可以使用监听器技术来很简单的实现这种功能。

注意：

session并不是浏览器关闭时销毁的，而是在session失效的时候销毁下列代码就是监测session创建、销毁。

销毁session，可以设置过期时间：

1

步骤说明

2、在第十五篇基础上继续实现功能——第十六篇JDBC操作数据库之监听器。

一.InitNumListener.java

对ServletContext对象进行监听的接口有ServletContextAttributeListener 和 ServletContext-Listener。package com.listener;

import javax.servlet.ServletContext;

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

import java.io.*;

/**

* Created by Ray on 2018/3/17 0017.

**/

public class InitNumListener implements ServletContextListener {

//初始化上下文

public void contextInitialized(ServletContextEvent sce) {

/* This method is called when the servlet context is

initialized(when the Web application is deployed).

You can initialize servlet context related data here.

*/

//创建接口

ServletContext servletContext = sce.getServletContext();

//访问总数的文件路径

String filename = servletContext.getRealPath("/WEB-INF/count.txt");

File file = new File(filename);

//初始化访问总数

Integer totalcount = 0;

if(file.exists()){

try{

//读取文件内容

FileReader fileReader = new FileReader(file);

BufferedReader bufferedReader = new BufferedReader(fileReader);

totalcount = Integer.valueOf(bufferedReader.readLine());

//关闭流

bufferedReader.close();

fileReader.close();

}catch (Exception e){

e.printStackTrace();

}

}

//设置属性name的值为value

servletContext.setAttribute("totalcount",totalcount); //访问总数

servletContext.setAttribute("accesscount",0); //在线用户数

}

//销毁上下文

public void contextDestroyed(ServletContextEvent sce) {

/* This method is invoked when the Servlet Context

(the Web application) is undeployed or

Application Server shuts down.

*/

//创建接口

ServletContext servletContext = sce.getServletContext();

//访问总数的文件路径

String filename = servletContext.getRealPath("/WEB-INF/count.txt");

File file = new File(filename);

try{

//将内容写入文件

FileWriter fileWriter = new FileWriter(file);

BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

bufferedWriter.write(servletContext.getAttribute("totalcount").toString());

//关闭流

bufferedWriter.close();

fileWriter.close();

}catch (Exception e){

e.printStackTrace();

}

}

}

二.UserEnterListener.java

实现HttpSessionListener接口的监听器，可以监听 HttpSession 对象本身的创建和销毁。package com.listener;

import javax.servlet.ServletContext;

import javax.servlet.http.HttpSessionEvent;

import javax.servlet.http.HttpSessionListener;

/**

* Created by Ray on 2018/3/17 0017.

**/

public class UserEnterListener implements HttpSessionListener {

public void sessionCreated(HttpSessionEvent se) {

/* Session is created. */

ServletContext servletContext = se.getSession().getServletContext();

//返回属性name的值

Integer totalcount = (Integer) servletContext.getAttribute("totalcount");

Integer accesscount = (Integer) servletContext.getAttribute("accesscount");

//使用指定的属性名字绑定一个对象并储存到session中

servletContext.setAttribute("totalcount",totalcount+1);

servletContext.setAttribute("accesscount",accesscount+1);

}

public void sessionDestroyed(HttpSessionEvent se) {

/* Session is destroyed. */

ServletContext servletContext = se.getSession().getServletContext();

//返回属性name的值

Integer accesscount = (Integer) servletContext.getAttribute("accesscount");

//使用指定的属性名字绑定一个对象并储存到session中

servletContext.setAttribute("accesscount",accesscount-1);

}

}

三.web.xml

1.监听器Listener

2.设置session过期时间

3.省略部分代码(懒得删)<?xml version="1.0" encoding="UTF-8"?>

xmlns="http://java.sun.com/xml/ns/javaee"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://java.sun.com/xml/ns/javaee

http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

com.listener.InitNumListener

com.listener.UserEnterListener

1

BookList

com.control.BookList

BookList

/BookList

BookAdd

com.control.BookAdd

BookAdd

/BookAdd

BookUpdate

com.control.BookUpdate

BookUpdate

/BookUpdate

BookDoUpdate

com.control.BookDoUpdate

BookDoUpdate

/BookDoUpdate

BookDelete

com.control.BookDelete

BookDelete

/BookDelete

BookDoDelete

com.control.BookDoDelete

BookDoDelete

/BookDoDelete

BookBatchAdd

com.control.BookBatchAdd

BookBatchAdd

/BookBatchAdd

BookBatchDelete

com.control.BookBatchDelete

BookBatchDelete

/BookBatchDelete

BookFind

com.control.BookFind

BookFind

/BookFind

Login

com.control.Login

Login

/Login

DrawImage

com.control.DrawImage

DrawImage

/DrawImage

Register

com.control.Register

Register

/Register

LoginUser

com.control.LoginUser

LoginUser

/LoginUser

ExitLogin

com.control.ExitLogin

ExitLogin

/ExitLogin

Upload

com.control.Upload

Upload

/Upload

ListFile

com.control.ListFile

ListFile

/ListFile

Download

com.control.Download

Download

/Download

CharSetFilter

com.Filter.CharSetFilter

charset

UTF-8

CharSetFilter

/*

404

/error/error404.jsp

500

/error/error500.jsp

ErrorFilter

com.Filter.ErrorFilter

ErrorFilter

/error.jsp

ERROR

四.listen.jsp

String path = request.getContextPath();

String basePath = request.getScheme() + "://" + request.getServerName() + ":" + request.getServerPort() + path + "/";

%>

Listen

访问人数：${totalcount}    在线人数：${accesscount}

五.count.txt(总访问人数19)

六.页面效果

另一个浏览器(模拟多个用户)

关闭浏览器(模拟在线人数退出)，等1-2分钟刷新

ok！