读书笔记--监听器Listener

其他 2019-10-11 23:44:00 阅读次数: 0

对应设计模式的Listener模式，事件触发时会自动触发该事件对应的Listener。主要监听session、request、context等进行监听。

Listener的概述

使用Listener和Event来完成相应事件的处理。

listener的使用

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import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class SessionListenerTest implements HttpSessionListener {

	public void sessionCreated(HttpSessionEvent se) {
		HttpSession session = se.getSession();
		System.out.println("新创建了一个session: " + session);
	}

	public void sessionDestroyed(HttpSessionEvent se) {
		HttpSession session = se.getSession();
		System.out.println("销毁了一个session: " + session);
	}

}
 
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<listener>
	<listener-class>
		com.helloweenvsfei.listener.SessionListenerTest
	</listener-class>
</listener>

Listener的分类

8中listener，主要监听session、request、context的创建和销毁。6中Event

监听对象的创建和销毁

HttpSessionListener、ServletContextListener、ServletRequestListener分别用于监听Session、content、request的创建和销毁。

  • HttpSessionListener：创建Session时：sessionCreated(HttpSessionEvent se)；超时或者执行session.invalidate()时： sessionDestroyed(HttpSessionEvent se)。用于收集在线信息。

  • ServletContextListener：服务启动或者热部署时：contextInitialized(ServletContextEvent event)；服务器关闭或者只关闭web时：contextDestroyed(ServletContextEvent event)。该listener可用获取web.xml里面的配置的初始化信息。

  • ServletRequestListener：用户请求时：requestInitialized(ServletRequestEvent event)；请求完成自动销毁时：requestDestroyed(ServletRequestEvent event)。如果html页面有多个图片会多次触发listener事件。

实例：监听session、request、context

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import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

public class ListenerTest implements HttpSessionListener,
		ServletContextListener, ServletRequestListener {

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

	// 创建 session
	public void sessionCreated(HttpSessionEvent se) {
		HttpSession session = se.getSession();
		log.info("新创建一个session, ID为: " + session.getId());
	}

	// 销毁 session
	public void sessionDestroyed(HttpSessionEvent se) {
		HttpSession session = se.getSession();
		log.info("销毁一个session, ID为: " + session.getId());
	}

	// 加载 context
	public void contextInitialized(ServletContextEvent sce) {
		ServletContext servletContext = sce.getServletContext();
		log.info("即将启动" + servletContext.getContextPath());
	}

	// 卸载 context
	public void contextDestroyed(ServletContextEvent sce) {
		ServletContext servletContext = sce.getServletContext();
		log.info("即将关闭" + servletContext.getContextPath());
	}

	// 创建 request
	public void requestInitialized(ServletRequestEvent sre) {

		HttpServletRequest request = (HttpServletRequest) sre
				.getServletRequest();

		String uri = request.getRequestURI();
		uri = request.getQueryString() == null ? uri : (uri + "?" + request
				.getQueryString());

		request.setAttribute("dateCreated", System.currentTimeMillis());

		log.info("IP " + request.getRemoteAddr() + " 请求 " + uri);
	}

	// 销毁 request
	public void requestDestroyed(ServletRequestEvent sre) {

		HttpServletRequest request = (HttpServletRequest) sre
				.getServletRequest();

		long time = System.currentTimeMillis()
				- (Long) request.getAttribute("dateCreated");

		log.info(request.getRemoteAddr() + "请求处理结束, 用时" + time + "毫秒. ");
	}

}

监听对象的属性变化

另一个Listener用于监听Session、context、request的属性变化，借口名格式为：xxxAttributeListener，包括HttpSessionAttributeListener、ServletContextAttributeListener、ServletRequestAttributeListener。当当向被监听对象添加、更新、移除属性时，会分别执行xxxAdded()/xxxReplaced()/xxxRemoved()方法，xxx分别代表Session、context、request。

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import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

public class SessionAttributeListenerTest implements
		HttpSessionAttributeListener {

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

	// 添加属性
	public void attributeAdded(HttpSessionBindingEvent se) {
		HttpSession session = se.getSession();
		String name = se.getName();
		log.info("新建session属性：" + name + ", 值为：" + se.getValue());
	}

	// 删除属性
	public void attributeRemoved(HttpSessionBindingEvent se) {
		HttpSession session = se.getSession();
		String name = se.getName();
		log.info("删除session属性：" + name + ", 值为：" + se.getValue());
	}

	// 修改属性
	public void attributeReplaced(HttpSessionBindingEvent se) {
		HttpSession session = se.getSession();
		String name = se.getName();
		Object oldValue = se.getValue();
		log.info("修改session属性：" + name + ", 原值：" + oldValue + ", 新值："
				+ session.getAttribute(name));
	}
}

监听Session内的对象

HttpSessionBindingListener：当对象放到Session里面执行：valueBound(HttpSessionBindingEvent event)；当对象从Session中移除时执行：valueUnbound(HttpSessionBindingEvent event)。对象必须实现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里的内容被钝化时执行：sessionWillPassivate(HttpSessionEvent se)；当对象重新加载时：sessionDidActivte(HttpSessionEvent se)。对象必须实现Listener接口。

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import java.io.Serializable;
import java.util.Date;

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 = -4780592776386225973L;

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

	private String name;

	private Date dateCreated;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Date getDateCreated() {
		return dateCreated;
	}

	public void setDateCreated(Date dateCreated) {
		this.dateCreated = dateCreated;
	}

	// 从硬盘加载后
	public void sessionDidActivate(HttpSessionEvent se) {
		HttpSession session = se.getSession();
		log.info(this + "已经成功从硬盘中加载。sessionId: " + session.getId());
	}

	// 即将被钝化到硬盘时
	public void sessionWillPassivate(HttpSessionEvent se) {
		HttpSession session = se.getSession();
		log.info(this + "即将保存到硬盘。sessionId: " + session.getId());
	}

	// 被放进session前
	public void valueBound(HttpSessionBindingEvent event) {
		HttpSession session = event.getSession();
		String name = event.getName();
		log.info(this + "被绑定到session "" + session.getId() + ""的" + name
				+ "属性上");

		// 记录放到session中的时间
		this.setDateCreated(new Date());
	}

	// 从session中移除后
	public void valueUnbound(HttpSessionBindingEvent event) {
		HttpSession session = event.getSession();
		String name = event.getName();
		log.info(this + "被从session "" + session.getId() + ""的" + name
				+ "属性上移除");
	}

	@Override
	public String toString() {
		return "PersonInfo(" + name + ")";
	}

}

Listener使用案例

单点登录

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<%@ page language="java" contentType="text/html; charset=UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
<jsp:directive.page import="com.helloweenvsfei.singleton.PersonInfo" />
<%
	String action = request.getParameter("action");
	String account = request.getParameter("account");

	if("login".equals(action) && account.trim().length() > 0){
	
		// 登录，将personInfo放入session
		PersonInfo personInfo = new PersonInfo();
		personInfo.setAccount(account.trim().toLowerCase());
		personInfo.setIp(request.getRemoteAddr());
		personInfo.setLoginDate(new java.util.Date());
		
		session.setAttribute("personInfo", personInfo);
		
		response.sendRedirect(response.encodeRedirectURL(request.getRequestURI()));
		return;
	}
	else if("logout".equals(action)){
	
		// 注销，将personInfo从session中移除
		session.removeAttribute("personInfo");
		
		response.sendRedirect(response.encodeRedirectURL(request.getRequestURI()));
		return;
	}
%>
<!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>
		<style type="text/css">
		body {
			font-size:12px; 
		}
		</style>
	</head>
	<body>
		
		<c:choose>
		
			<c:when test="${ personInfo != null }">
				<!-- 已经登录，将显示帐号信息 -->
				欢迎您，${ personInfo.account }。<br/> 
				您的登录IP为${ personInfo.ip }，<br/>
				登录时间为<fmt:formatDate value="${ personInfo.loginDate }" pattern="yyyy-MM-dd HH:mm"/>。
				<a href="${ pageContext.request.requestURI }?action=logout">退出</a>
				
				<!-- 每5秒钟刷新一次页面 -->
				<script>setTimeout("location=location; ", 5000); </script>
			</c:when>
			
			<c:otherwise>
				<!-- 没有登录，将显示登录页面 -->
				${ msg } 
				<c:remove var="msg" scope="session" />
				<form action="${ pageContext.request.requestURI }?action=login" method="post">
					帐号：
					<input name="account" />
					<input type="submit" value="登录">
				</form>
			</c:otherwise>
		
		</c:choose>

	</body>
</html>

原文:大专栏  读书笔记--监听器Listener


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