Linstener:监听器

事件监听机制
事件:一个事情
事件源:时间发生的地方
监听器:一段对象
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,执行监听器事件

ServletContextListener:监听ServletContext 对象的创建和销毁
void contextDestroyed()
void contextInitialized()
步骤:

1. 定义一个类,实现ServletContextListener接口
2. 复写方法
3. 配置
4. web.xml

<!--监听器 -->
<listener>
    <listener-class>com.itcast.listener.ContextDemo1</listener-class>
</listener>

指定初始化参数<context-param>

2. 注解
   @WebListener

3. 监听并初始化资源加载资源

package com.itcast.listener;

import jakarta.servlet.ServletContext;
import jakarta.servlet.ServletContextEvent;
import jakarta.servlet.ServletContextListener;

import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class ContextDemo1 implements ServletContextListener {
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("ServletContextListener contextDestroyed");
    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        //加载资源文件
        //1.获取ServletContext对象
        ServletContext servletContext = sce.getServletContext();

        String contextConifg = servletContext.getInitParameter("contextConifg");
        //3.获取真实路径
        String realPath = servletContext.getRealPath(contextConifg);
        //4.加载进内存
        try {
            FileInputStream fileInputStream = new FileInputStream(realPath);
            System.out.println(fileInputStream);
        } catch (FileNotFoundException e) {
            throw new RuntimeException(e);
        }
        System.out.println("ServletContextListener contextInitialized");
    }
}