手把手教你学javaweb（七）项目中添加监听器

javaweb项目添加监听器

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监听器，主要是用来监听特定对象的创建、销毁或者属性的新增，覆盖，移除的。

监听器是观察者模式的经典实现！

javaweb中提供了很多监听器接口，主要有以下几个

/** 对象监听器 **/
javax.servlet.ServletContextListener //  监听 servletContext的创建和销毁
javax.servlet.http.HttpSessionListener//监听 session的创建和销毁
javax.servlet.ServletRequestListener    //监听request的创建和销毁
 /** 属性监听器 **/
javax.servlet.ServletRequestAttributeListener //监听request属性的新增、覆盖、移除
javax.servlet.ServletContextAttributeListener//监听servletContext属性的新增、覆盖、移除
javax.servlet.http.HttpSessionAttributeListener//监听session属性的新增、覆盖、移除

我在这里就不讨论具体在哪个业务场景使用了，直接上代码了

这些功能大概用到如下几个常用场景：

统计在线人数，防止重复登录，黑名单，功能增强，数据统计，日志打印…

package com.cgy.demos.web.listener;

import javax.servlet.ServletRequestAttributeEvent;
import javax.servlet.ServletRequestAttributeListener;
import javax.servlet.annotation.WebListener;


@WebListener
public class MyRequestAttrListener implements ServletRequestAttributeListener {

    @Override
    public void attributeAdded(ServletRequestAttributeEvent srae) {

        System.out.println("request属性添加了");
    }

    @Override
    public void attributeRemoved(ServletRequestAttributeEvent srae) {

        System.out.println("request属性移除了");
    }

    @Override
    public void attributeReplaced(ServletRequestAttributeEvent srae) {
        System.out.println("request属性移覆盖了");

    }
}

package com.cgy.demos.web.listener;

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

@WebListener
public class MyServletContextAttrListener implements ServletContextAttributeListener {
    @Override
    public void attributeAdded(ServletContextAttributeEvent event) {
        System.out.println("ServletContext 属性新增了");
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent event) {
        System.out.println("ServletContext 属性移除了");
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent event) {
        System.out.println("ServletContext 属性替换了");
    }
}

package com.cgy.demos.web.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class MyServletContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("Servlet Context 创建了");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("Servlet Context 销毁了");
    }
}

package com.cgy.demos.web.listener;

import com.alibaba.fastjson.JSON;

import javax.servlet.ServletContext;
import javax.servlet.ServletRequest;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpServletRequest;
import java.util.Enumeration;

/**
 * 任何请求的创建和销毁都会回调这个listener 下面是官方注释
 * Interface for receiving notification events about requests coming
 *  into and going out of scope of a web application.
 *
 */
@WebListener
public class MyServletRequestListener implements ServletRequestListener {
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {


        System.out.println("request destroyed 事件回调了");


    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("request  初始化 事件回调了");
    }
}

package com.cgy.demos.web.listener;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
@WebListener
public class MySessionAttrListener implements HttpSessionAttributeListener {
    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        System.out.println("session 属性新增了");
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent event) {
        System.out.println("session 属性移除了");
    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent event) {
        System.out.println("session 属性覆盖了");
    }
}

package com.cgy.demos.web.listener;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

@WebListener
public class MySessionListener implements HttpSessionListener {
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        System.out.println("session创建了,sessionId:" + se.getSession().getId());
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        System.out.println("session销毁了,sessionId:" + se.getSession().getId());
    }
}

具体要实现什么功能，请自行研究

至此，javaweb项目也开发的差不多了。后面会继续探讨 spring的相关内容