1. What is a servlet listener?

Serlet listeners are also called web listeners. Is a special class in servlet s. It can help developers monitor specific events in Web applications. For example, the creation and destruction of ServletContext, ServletSession, ServletRequest, variable creation and destruction, etc.

2 Common uses of monitors

Web listeners are usually used to do the following:

Statistics of online population, using HttpSession Lisener

Load initialization information: using ServletContextListener

Statistical website visits

Implementing access monitoring

3. Classification of listeners

According to the servlet object, listeners can be divided into three types: ServletContext, ServletSession and ServletRequest.

Application of 4 Monitors

4.1 ServletContex: Corresponding to the creation and destruction of built-in objects for monitoring application s.

When the web container is open, the contextInitialized method is executed; when the container is closed or restarted, the contextDestroyed method is executed.

Implementation: Direct implementation of ServletContextListener interface

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 monitoring: Creation and destruction of built-in objects corresponding to monitoring session s.

When a new page is opened, a session session session session is opened and the session Created method is executed; when the page closes the session expires, or when the container closes and destroys, the session Destroyed method is executed.

Implementation: Direct implementation of HttpSessionListener interface:

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("Newly build session，sessionId: "+session.getId());

    }

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

4.3 ServletRequest monitoring: Creation and destruction of built-in objects corresponding to monitoring request s.

When a page is accessed, a request request request is made to execute the request Initialized method; when the page is closed, the request Destroyed method is executed.

Implementing the ServletRequestListener interface directly:

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("Time consumed:"+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);

    }
}

Configuration of 5 listener web.xml

    <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>