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JSP中的监听器

2022-06-12 20:24:00 【Fairy-KunKun】

Servlet中的监听器

需求：mood系统中统计在线人数

监听器属于第三者，不应该影响主体程序，具有可插拔性。

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;
   }
}

定义一个监听器，专门监听session数据变化。

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 ) {

   }
}

配置

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

启动测试

<%@ 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>在线人数</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>
在线人数：<%=OnlineCounter.getOnline ( )%>
</body>
</html>

说明：

监听器为web应用服务的，独立的监听程序，用于监听对象的状态变化。

Servlet中的监听器有很多种，

规律：监听对象+Listener,实现相应的接口

使用注解做配置

再看一个监听器

ServletContext对象什么时候被创建？

服务器启动时被创建。

Servlet配置了load-on-startupàtomcat启动时，可以将一些准备工作（初始化工作）交给servlet完成。

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 ) {
   }
}