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>