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Listener in JSP

2022-06-12 20:28:00 【Fairy-KunKun】

Servlet The monitor in

demand ：mood The system counts the number of online people

The listener belongs to a third party , It should not affect the main program , Pluggable .

package com.njwbhz.mood.util;
public class OnlineCounter {
   private static int cnt = 0;
   public static synchronized void in () {
      cnt++;
   }
   public static synchronized void out () {
      cnt--;
   }

   public static int getOnline () {
      return cnt;
   }
}

Define a listener , Special monitoring session Data changes .

package com.njwbhz.mood.listener;
import com.njwbhz.mood.util.OnlineCounter;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
public class OnlineListener implements HttpSessionAttributeListener {
   @Override
   public void attributeAdded ( HttpSessionBindingEvent httpSessionBindingEvent ) {
      System.out.println (
            "a user is coming"
      );
      OnlineCounter.in ( );
   }
   @Override
   public void attributeRemoved ( HttpSessionBindingEvent httpSessionBindingEvent ) {
      System.out.println (
            "a user is outing"
      );
      OnlineCounter.out ( );
   }
   @Override
   public void attributeReplaced ( HttpSessionBindingEvent httpSessionBindingEvent ) {

   }
}

To configure

<listener >
<listener-class >com.njwbhz.mood.listener.OnlineListener</listener-class>
</listener>

Start the test

<%@ page import="com.njwbhz.mood.util.OnlineCounter" %>
<%
   String path = request.getContextPath ( );
   String basePath = request.getScheme ( ) + "://" + request.getServerName ( ) + ":" + request.getServerPort ( ) + path + "/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
   <base href="<%=basePath%>">
   <title> Number of people online </title>
   <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
   <meta http-equiv="description" content="this is my page">
   <meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
Number of people online ：<%=OnlineCounter.getOnline ( )%>
</body>
</html>

explain ：

The monitor is web Application services , Independent listener , Used to listen for state changes of objects .

Servlet There are many kinds of listeners in ,

law ： listening object +Listener, Implement the corresponding interface

Use annotations for configuration

Look at another listener

ServletContext When the object is created ？

The server is created at startup .

Servlet Configured with load-on-startupàtomcat Startup time , Some preparatory work can be done （ Initialization work ） hand servlet complete .

package com.njwbhz.mood.servlet;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
@WebServlet ( value = "/startup", loadOnStartup = 1 )
public class StartServlet extends HttpServlet {
   @Override
   public void init ( ) throws ServletException {
      System.out.println ( "StartServlet execute init" );
   }
}

package com.njwbhz.mood.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyServletContextListener implements ServletContextListener {
   @Override
   public void contextInitialized ( ServletContextEvent servletContextEvent ) {
      System.out.println (
            "MyServletContextListener execute init"
      );
   }
   @Override
   public void contextDestroyed ( ServletContextEvent servletContextEvent ) {
   }
}