JavaWeb三大组件之Listener------Listener详细讲解

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🌳Listener

🌲Listener介绍

概念：Listener 表示监听器，是 JavaWeb 三大组件(Servlet、Filter、Listener)之一。

• 监听器可以监听就是在application,session,request三个对象创建、销毁或者往其中添加修改删除属性时自动执行代码的功能组件

Listener分类：JavaWeb中提供了8个监听器

三个事件源(application,session,request)或者三个容器

第一类:监听容器本身的创建和销毁

• ServletContextListener
• HttpSessionListener
• ServletRequestListener

第二类:监听容器的内容变化

• ServletContextAttributeListener
• HttpSessionAttributeListener
• ServletRequestAttributeListener

第三类:专门针对Javabean的,让Javabean实现接口重写方法即可

• HttpSessionBindingListener
• HttpSessionActivationListener

🌾Listener使用

1. 定义类，实现ServletContextListener接口

2. 在类上添加@WebListener 注解

3.其他监听器的使用方法基本一致

package com.moming;
 
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
//使用频率不高
//一般在spring框架初始化会使用到
public class ContextLoaderListener implements ServletContextListener {
    //初始化对象方法
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        //加载资源,
        System.out.println("ContextLoaderListener...");
    }
 
    //销毁方法
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        //释放资源
    }
}

第三类:专门针对Javabean的使用方法

User实体类实现监听器

package com.moming.pojo;
 
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
 
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
 
@Data
@AllArgsConstructor
@NoArgsConstructor
public class User implements HttpSessionBindingListener {
    private int id;
    private String name;
 
 
    @Override
    public void valueBound(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("[绑定]id为:"+id+"的用户:"+name+"被添加到session中");
    }
 
    @Override
    public void valueUnbound(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("[解绑定]id为:"+id+"的用户:"+name+"被移除session");
    }
}

AServlet

package com.moming.servlet;
 
import com.moming.pojo.User;
 
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
 
@WebServlet("/a")
public class AServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //绑定Session
        User user = new User(110,"沫洺");
        request.getSession().setAttribute("u", user);
    }
 
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

BServlet

package com.moming.servlet;
 
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
 
@WebServlet("/b")
public class BServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //解除绑定
        request.getSession().removeAttribute("u");
    }
 
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

绑定

 解绑定