Listener 监听Session内的对象

Listener用于监控Session内的对象，分别是HttpSessionBindingListener与HttpSessionActivationListener。它们的触发时机分别为：

HttpSessionBindingListener：当对象被放到Session里时执行valueBound(HttpSessionBindingEvent event)方法。当对象被从Session里移除时执行valueUnbound(HttpSessionBingdingEvent event)方法。对象必须实现该Listener接口。
HttpSessionActivationListener：服务器关闭时，会将Session里的内容保存到硬盘上，这个过程叫做钝化。服务器重新启动时，会将Session内容从硬盘上重新加载。当Session里的对象被钝化时会执行sessionWillPassivate(HttpSessionEvent event)方法。当对象被重新加载时执行sessionDidActivate(HttpSessionEvent event)。对象必须实现该Listener接口。

事例如下：

package linstener;

import java.io.Serializable;

import java.io.UnsupportedEncodingException;

import java.nio.charset.Charset;

import java.util.Date;

import java.util.Locale;

import javax.servlet.http.HttpSession;

import javax.servlet.http.HttpSessionActivationListener;

import javax.servlet.http.HttpSessionBindingEvent;

import javax.servlet.http.HttpSessionBindingListener;

import javax.servlet.http.HttpSessionEvent;

import org.apache.commons.logging.Log;

import org.apache.commons.logging.LogFactory;

public class PersonInfo implements HttpSessionActivationListener,HttpSessionBindingListener,Serializable {

    /**

     *

     */

    private static final long serialVersionUID = 1L;

    private Log log=LogFactory.getLog(this.getClass());

    private String name;

    private Date dateCreated;

    @Override

    public void sessionDidActivate(HttpSessionEvent arg0) {

    //从硬盘恢复后被调用

        HttpSession session =arg0.getSession();

        log.info(this+"已经成功硬盘中加载。sessionid：" +session.getId());

    }

    @Override

    public void sessionWillPassivate(HttpSessionEvent arg0) {

        //即将被钝化到硬盘时调用

        HttpSession session=arg0.getSession();

        log.info(this+"即将保存到硬盘。sessionid："+session.getId());

    }

    @Override

    public void valueBound(HttpSessionBindingEvent arg0) {

        //被放入session前调用

        HttpSession session=arg0.getSession();

        String name=arg0.getName();

        log.info(this+"被绑定到session\""+session.getId()+"\"的"+name+"属性上。");

        this.setDateCreated(new Date());

    }

    @Override

    public void valueUnbound(HttpSessionBindingEvent arg0) {

        //从session 中移除后调用

        HttpSession session=arg0.getSession();

        String name=arg0.getName();

        log.info(this+"被从session\""+session.getId()+"\"的"+name+"属性上移除。");

    }

    public Date getDateCreated() {

        return dateCreated;

    }

    public void setDateCreated(Date dateCreated) {

        this.dateCreated = dateCreated;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    @Override

    public String toString() {

        return "PersonInfo("+name+")";

    }

}

这两个Listener监听的是Session中的对象而非Session等，因此不需要在web.xml中声明。

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8"%>

<jsp:directive.page import="linstener.PersonInfo" />

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

    <%

        PersonInfo personInfo =(PersonInfo)session.getAttribute("personInfo");

        if(personInfo==null){

            personInfo=new PersonInfo();

            personInfo.setName("Alex Geng");

            session.setAttribute("personInfo", personInfo);

            out.println("PersonInfo 对象不存在。已经成功新信件。sessionId："+session.getId());

        }

        else{

            out.println("PersonInfo对象存在。无需新建。sessionId:"+session.getId());

        }

    %>

</body>

</html>

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Listener 监听Session内的对象

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