Listener:監(jiān)聽(tīng)器的原理和實(shí)例

八大監(jiān)聽(tīng)器：
????? ServletRequest對(duì)象：
???????? 生命周期監(jiān)聽(tīng)：ServletRequestListener
???????? 屬性監(jiān)聽(tīng)：ServletRequestAttributeListener
????? HttpSession對(duì)象：
???????? 生命周期監(jiān)聽(tīng)：HttpSessionListener
???????? 屬性監(jiān)聽(tīng)：HttpSessionAttributeListener
???????? 對(duì)象綁定監(jiān)聽(tīng)1：HttpSessionBindingListener
???????? 對(duì)象鈍化活化監(jiān)聽(tīng)2：HttpSessionActivationListener
????? ServletContext對(duì)象：
???????? 生命周期監(jiān)聽(tīng)：ServletContextListener
???????? 屬性監(jiān)聽(tīng)：ServletContextAttributeListener


一、監(jiān)聽(tīng)對(duì)象，主要監(jiān)聽(tīng)servlet的三大域?qū)ο髍equest，session，application（ServletRequest,HttpSession,ServletContext）

??? 監(jiān)聽(tīng)內(nèi)容：
??????? A：監(jiān)聽(tīng)域?qū)ο蟮膭?chuàng)建和銷毀，也就是生命周期的監(jiān)聽(tīng)
??????? B：監(jiān)聽(tīng)域?qū)ο蟮膶傩蕴砑?#xff0c;移除和更改
??????? C：監(jiān)聽(tīng)被加入域?qū)ο笾械膶?duì)象

二、三大域?qū)ο蟮纳芷?br />??? ServletRequest 什么時(shí)候被創(chuàng)建：
??????? A: 請(qǐng)求一個(gè)jsp頁(yè)面時(shí)，tomcat 翻譯jsp頁(yè)面為一個(gè)servlet類，執(zhí)行servlet 類中的service()方法是，tomcat幫我們創(chuàng)建了9大內(nèi)置對(duì)象，其中就有request。
??????? B: 請(qǐng)求一個(gè)servlet時(shí)，doGet() 或者是doPost()方法，tomcat 自動(dòng)也會(huì)幫我們創(chuàng)建request，
?? ServletRequest 什么時(shí)候被銷毀：
??????? 執(zhí)行完service()方法或者 doGet()方法，doPost()方法即銷毀

??? HttSesssion 什么時(shí)候被創(chuàng)建：
??????? Request.getSession():? 先判斷是否存在session對(duì)象，如果存在就直接返回，如果不存在就幫我們創(chuàng)建一個(gè)session，然后返回。
??? HttSesssion 什么時(shí)候被銷毀：
?????? 是根據(jù)cookie的生命周期來(lái)判斷的，如果cookie是瀏覽器生命周期的話，那么瀏覽器關(guān)閉后，session即銷毀；如果cookie是保存到文件中的，那么就具體情況，具體對(duì)待。
?
??? ServletContext 什么時(shí)候被創(chuàng)建：
?????? Tomcat啟動(dòng)項(xiàng)目，即創(chuàng)建了application對(duì)象。
??? ServletContext 什么時(shí)候被銷毀：
?????? Tomcat關(guān)閉項(xiàng)目，即銷毀了application對(duì)象。

三、【實(shí)例】
?? 創(chuàng)建一個(gè)監(jiān)聽(tīng)器
? A：選擇一個(gè)接口，根據(jù)監(jiān)聽(tīng)對(duì)象，監(jiān)聽(tīng)內(nèi)容的不同選擇不同的接口
? B：在web.xml中添加監(jiān)聽(tīng)器的配置信息
????? <listener>
????????? <listener-class>監(jiān)聽(tīng)器文件的相對(duì)路徑</listener-class>
????? </listener>


監(jiān)聽(tīng)器： HttpSessionAttributeListener
??? 【原理】
??? HttpSessionAttributeListener是對(duì)SessionAttribute的監(jiān)聽(tīng)，當(dāng)在會(huì)話對(duì)象中加入屬性，移除屬性和替換屬性時(shí)就會(huì)觸發(fā)HttpSessionAttributeListener監(jiān)

聽(tīng)器。
??? 接口HttpSessionAttributeListener有3個(gè)方法：
??? 1、public void attributeAdded(HttpSessionBindingEvent sbe){}:該方法在session添加對(duì)象是觸發(fā)
??? 2、public void attributeRemoved(HttpSessionBindingEvent sbe){}:該方法是在session移除對(duì)象時(shí)觸發(fā)的
??? 3、public voidattributeReplaced(HttpSessionBindingEvent se)：在Session屬性被重新設(shè)置時(shí)

?? 【實(shí)例】統(tǒng)計(jì)一下系統(tǒng)的登錄人數(shù)
package com.aaa.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSessionBindingEvent;

@WebListener()
public class Listener2 implements ServletContextListener,
??????? HttpSessionListener, HttpSessionAttributeListener {

????? public int count=0;//統(tǒng)計(jì)登錄的人數(shù)

??? // Public constructor is required by servlet spec
??? public Listener2() {
??? }

??? // -------------------------------------------------------
??? // ServletContextListener implementation
??? // -------------------------------------------------------
??? public void contextInitialized(ServletContextEvent sce) {
????? /* This method is called when the servlet context is
???????? initialized(when the Web application is deployed).
???????? You can initialize servlet context related data here.
????? */
??? }

??? public void contextDestroyed(ServletContextEvent sce) {
????? /* This method is invoked when the Servlet Context
???????? (the Web application) is undeployed or
???????? Application Server shuts down.
????? */
??? }

??? // -------------------------------------------------------
??? // HttpSessionListener implementation
??? // -------------------------------------------------------
??? public void sessionCreated(HttpSessionEvent se) {
??????? /* Session is created. */
??? }

??? public void sessionDestroyed(HttpSessionEvent se) {
??????? /* Session is destroyed. */
??? }

??? // -------------------------------------------------------
??? // HttpSessionAttributeListener implementation
??? // -------------------------------------------------------

??? public void attributeAdded(HttpSessionBindingEvent sbe) {
????? /* This method is called when an attribute
???????? is added to a session.
????? */
???? //session對(duì)象中加入屬性了觸發(fā)此方法
???? //sbe.getName()得到session里的屬性名
????? if(sbe.getName().equals("user")){
???????? count++;
???????? //將count放在全局域?qū)ο骃ervletContext里
???????? sbe.getSession().getServletContext().setAttribute("count",count);
???????? System.out.println("現(xiàn)在系統(tǒng)登錄的用戶人數(shù)"+count);
?????? }
????? ?
??? }

??? public void attributeRemoved(HttpSessionBindingEvent sbe) {
????? /* This method is called when an attribute
???????? is removed from a session.
????? */
????? //session對(duì)象中刪除屬性時(shí)觸發(fā)此方法
????? if(sbe.getName().equals("user")){
?????????? count--;
?????????? sbe.getSession().getServletContext().setAttribute("count",count);
?????????? System.out.println("現(xiàn)在系統(tǒng)登錄的用戶人數(shù)"+count);
?????? }
??? }

??? public void attributeReplaced(HttpSessionBindingEvent sbe) {
????? /* This method is invoked when an attibute
???????? is replaced in a session.
????? */
??? }
}

轉(zhuǎn)載于:https://www.cnblogs.com/fbbg/p/11099169.html

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