Listener监听器

        listener主要完成的工作是对于三大作用域的监听

1、监听作用域的生命周期

        ServletContextListener

        HttpSessionListener

        ServletRequestListener

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import java.util.Map;


@WebListener
public class CustomServletContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("监听到服务器启动");
        //数据字典gender
        Map<Integer, String> gender = Map.of(1, "男", 2, "nv", 3, "保密");
        sce.getServletContext().setAttribute("gender",gender);
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("监听到服务器关闭");
    }
}

2、监听作用域的属性改变

        setAttribute( String ,object);

        removeAttribute( String ); 

        ServletContextAttributeListener

        HttpSessionAttriuteListener

        ServletRequestAttributeListener

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.annotation.WebListener;


@WebListener
public class CustomServletContextAttributeListener implements ServletContextAttributeListener {
    @Override
    public void attributeAdded(ServletContextAttributeEvent scae) {
        System.out.println("add");
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent scae) {
        System.out.println("replace");
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent scae) {
        System.out.println("remove");
    }
}