java web 监听器的三个组件

 Listener

1、概念

注册监听：将事件、事件源、监听器绑定在一起。 当事件源上发生某个事件后，执行监听器代码

ServletContextListener 接口，它能够监听 ServletContext 对象的生命周期，实际上就是监听 Web 应用的生命周期。

2、代码实现

package com.beiyou.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class Listener1 implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        //ServletContext对象创建后会调用该方法
        System.out.println("Listener1 ->   contextInitialized");        //扩展内容明天讲        ServletContext sct=sce.getServletContext();
        sct.setAttribute("mysql", "127.0.0.1");

    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        //ServletContext对象被销毁之前会调用该方法
        System.out.println("Listener1 -> contextDestroyed");
    }
}

 <listener>
        <listener-class>com.beiyou.listener.Listener1</listener-class>
 </listener>
 <listener>
        <listener-class>com.beiyou.listener.Listener2</listener-class>
 </listener>

3. 使用注解

@WebListener

4.扩展

  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {

        ServletConfig servletConfig = getServletConfig();
        Object mysql = servletConfig.getServletContext().getAttribute("mysql");


        request.getRequestDispatcher("login.jsp").forward(request, response);


    }