servlet监听器（三）

1.什么是servlet监听器？

　　servlet监听器也叫web监听器。是servlet中一种特殊类。能够帮助开发者监听web应用中的特定事件。比如ServletContext，ServletSession，ServletRequest的创建和销毁，变量的创建和销毁等。

2 监听器常用的用途

　　通常使用Web监听器做以下的内容：

　　统计在线人数，利用HttpSessionLisener

　　加载初始化信息：利用ServletContextListener

　　统计网站访问量

　　实现访问监控

3.监听器的分类

　　根据servlet对象可以将监听器划分为三种：ServletContext，ServletSession,ServletRequest。

4监听器的应用

　　4.1 ServletContex :对应监控application内置对象的创建和销毁。

　　当web容器开启时，执行contextInitialized方法；当容器关闭或重启时，执行contextDestroyed方法。

　　实现方式：直接实现ServletContextListener接口

package com.learn;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import java.io.*;

/**
 * Created by Administrator on 2017/09/23.
 */
public class MyServletContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {

        System.out.println("Webapp initital.......");
        ServletContext servletContext = sce.getServletContext();
       InputStreamReader inputStreamReader = new InputStreamReader(servletContext.getResourceAsStream("/count/count.txt"));
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
        try {
            int icount = Integer.parseInt(bufferedReader.readLine());
            icount++;
            servletContext.setAttribute("count",icount);
            System.out.println("Webapp initital success");
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {

        System.out.println("webApp destroyed ........");
        ServletContext servletContext = sce.getServletContext();

        Integer count = (Integer) servletContext.getAttribute("count");
        if(count != null){
            count ++;
            String filePath = servletContext.getRealPath("/count");
            filePath = filePath+"/count.txt";
            try {
                PrintWriter printWriter = new PrintWriter(filePath);
                printWriter.write(count);
                printWriter.close();
                System.out.println("webApp desdroyed success ");
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }
    }
}

　　4.2HttpSession监控：对应监控session内置对象的创建和销毁。

　　当打开一个新的页面时，开启一个session会话，执行sessionCreated方法；当页面关闭session过期时，或者容器关闭销毁时，执行sessionDestroyed方法。

　　实现方式：直接实现HttpSessionListener接口：

package com.learn;

import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

/**
 * Created by Administrator on 2017/09/23.
 */
public class MySessionListener implements HttpSessionListener{
    @Override
    public void sessionCreated(HttpSessionEvent se) {

        HttpSession session = se.getSession();
        System.out.println("新建session，sessionId："+session.getId());

    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        HttpSession session = se.getSession();
        System.out.println("销毁session，sessionId："+session.getId());
    }
}

　　4.3ServletRequest监控：对应监控request内置对象的创建和销毁。

　　当访问某个页面时，出发一个request请求，执行requestInitialized方法；当页面关闭时，执行requestDestroyed方法。

　　实现方式，直接实现ServletRequestListener接口：

　　

package com.learn;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;

/**
 * Created by Administrator on 2017/09/23.
 */
public class MyRequestListener implements ServletRequestListener {
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {


        HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
        Long date =   System.currentTimeMillis()-(Long) (request.getAttribute("dateCreated"));
        System.out.println("消耗时间："+date);
    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
        String uri = request.getRequestURI();
        uri = request.getQueryString() == null ? uri : (uri + "?" + request.getQueryString());

        request.setAttribute("dateCreated", System.currentTimeMillis());
        System.out.println("IP:"+request.getRemoteAddr()+"URI:"+uri);

    }
}

5监听器web.xml的配置

    <listener>
     <listener-class>com.learn.MyServletContextListener</listener-class>
    </listener>

    <listener>
        <listener-class>com.learn.MySessionListener</listener-class>
    </listener>

    <listener>
        <listener-class>com.learn.MyRequestListener</listener-class>
    </listener>