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  • Servlet 学习（九）

    Listener

    1、功能

    • Servlet 2.3 中新增加的另一个功能
    • 作用是监听Java Web 程序中的事件
    • 对应设计模式中的Listener 模式，当事件发生的时候会自动触发该事件对应的Listener
    • 主要用于对Request 、Session 、Context 等进行监控

    2、listener模型

     

    常见的监听接口

    3、【跟生命周期有关的监听器】

    • javax.servlet.ServletContextListener :

    　　　　public void contextInitialized( ServletContextEvent event )

    　　　　　　»当 容器 加载 ( deploy ) 一个 应用 ( "web application" ) 时 呼叫 contextInitialized

    　　　　　　»传入的事件对象是 ServletContextEvent

    　　　　public void contextDestroyed( ServletContextEvent event )

    　　　　　　»当 容器 卸载 ( undeploy ) 一个 应用 ( "web application" ) 时 呼叫 contextDestroyed

    　　　　　　»传入的事件对象是 ServletContextEvent

    测试案例：

    package ecut.listener.listener;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class ApplicationLifecycleListener implements ServletContextListener {

    public ApplicationLifecycleListener() {
        //System.out.println( "WebApplicationLifecycleListener" );
    }
    
    public void contextInitialized( ServletContextEvent sce )  { 
        ServletContext application = sce.getServletContext();
        String name = application.getServletContextName();
        String contextPath = application.getContextPath();
        System.out.println( "正在加载: " + name + " , 它相应的路径为: " + contextPath );
    }

    public void contextDestroyed( ServletContextEvent sce )  { 
        ServletContext application = sce.getServletContext();
        String name = application.getServletContextName();
        System.out.println( "正在卸载: " + name  + " 。");
    }
    
}
        <!--display-name（不是listener的子标记）标记设置ServletContextName -->
    <display-name>listener</display-name>

    <listener>
        <listener-class>ecut.listener.listener.ApplicationLifecycleListener</listener-class>
    </listener>

    运行结果如下：

    正在加载: listener , 它相应的路径为: /s
    ......................
    正在卸载: listener 。
    • javax.servlet.http.HttpSessionListener :

    　　　　public void sessionCreated( HttpSessionEvent event )

    　　　　　　»当 创建 会话 对象时，呼叫 sessionCreated

    　　　　public void sessionDestroyed( HttpSessionEvent event )

    　　　　　　»当 销毁 会话 对象时，呼叫 sessionDestroyed

    　　　　　　»销毁 会话对象的时机 : session 的 有效期 到了 、调用 session.invalidate()

    　　　　　　»可以在 web.xml 中配置 session 的有效期:
    　　　　　　   <session-config>
    　　　　　　　　<session-timeout>30</session-timeout>
    　　　　　　   </session-config>
    　　　　　　   注意，这里的有效期的时间的单位是 分钟

    测试案例：

    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

@WebServlet( "/session/show" )
public class ShowSessionIdServlet extends HttpServlet {

    private static final long serialVersionUID = 3683961647674453725L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        HttpSession session = request.getSession();
        
        // 通过代码来设置 会话的有效期 ( 时间单位是 秒 )
        session.setMaxInactiveInterval( 30 ); // 最长发呆时间 ( 从客户端最后一次访问 session 开始持续30秒 )
        
        response.setContentType( "text/html" );
        
        response.getWriter().println( "<h1>" + session.getId() + "</h1>" );
        
    }
    
    

}
    package ecut.listener.listener;

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

public class SessionLifecycleListener implements HttpSessionListener {

    @Override
    public void sessionCreated( HttpSessionEvent event ) {
        HttpSession session = event.getSession();
        System.out.println( "会话[ " + session.getId() + " ]创建成功" );
    }

    @Override
    public void sessionDestroyed( HttpSessionEvent event ) {
        HttpSession session = event.getSession();
        System.out.println( "准备销毁[ " + session.getId() + " ]会话" );
    }

}
        <listener>
        <listener-class>ecut.listener.listener.SessionLifecycleListener</listener-class>
    </listener>

    运行结果如下：

    会话[ D2920D969A73184E4A173615758A314C ]创建成功
准备销毁[ D2920D969A73184E4A173615758A314C ]会话
    • javax.servlet.ServletRequestListener :

    　　　　public void requestInitialized(ServletRequestEvent event)

    　　　　　　»当 客户端 发起请求时，即创建 请求对象 ，此时 呼叫 requestInitialized

    　　　　public void requestDestroyed(ServletRequestEvent event)

    　　　　　　»当请求 结束时 ( 响应数据已经全部返回到客户端 ) ，呼叫 requestDestroyed

    测试案例：

    package ecut.listener.listener;

import javax.servlet.DispatcherType;
import javax.servlet.ServletRequest;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;

public class RequestLifecycleListener implements ServletRequestListener {

    @Override
    public void requestDestroyed(ServletRequestEvent event) {
        ServletRequest req = event.getServletRequest();
        HttpServletRequest request = (HttpServletRequest) req ;
        String uri = request.getRequestURI();
        DispatcherType type = request.getDispatcherType();
        String name = req.getLocalName();
        System.out.println(  "来自"+name+"请求类型为"+type + " ,请求路径是 " + uri +"的请求结束");
    }

    @Override
    public void requestInitialized(ServletRequestEvent event) {
        ServletRequest req = event.getServletRequest();
        HttpServletRequest request = (HttpServletRequest) req ;
        String uri = request.getRequestURI();
        DispatcherType type = request.getDispatcherType();
        String name = req.getLocalName();
        System.out.println( "接收到请求：来自" +name+"的请求，请求类型为"+type + " ,请求路径是 " + uri );
        
    }

}
    <listener>
        <listener-class>ecut.listener.listener.RequestLifecycleListener</listener-class>
    </listener>

    运行结果如下：

    接收到请求：来自0:0:0:0:0:0:0:1的请求，请求类型为REQUEST ,请求路径是 /s/pages/listener/index.jsp
来自0:0:0:0:0:0:0:1请求类型为REQUEST ,请求路径是 /s/pages/listener/index.jsp的请求结束

     4、【跟 属性 操作 有关的监听器】

    • javax.servlet.ServletContextAttributeListener :

    　　　　void attributeAdded( ServletContextAttributeEvent event )

    　　　　　　»向 ServletContext 中添加一个不存在的 属性时 呼叫 attributeAdded
    　　　　　　ServletContext application = request.getServletContext();
    　　　　　　application.setAttribute( "counter" , 0 ) ; // 假设 application 中 根本就没有 counter ，这是首次添加 counter

    　　　　void attributeReplaced( ServletContextAttributeEvent event )

    　　　　　　»当 ServletContext 中已经包含 某个名称的属性时，又通过 setAttribute 来设置 设个属性值，则构成 替换操作，此时呼叫 attributeReplaced

    　　　　　　ServletContext application = request.getServletContext();
    　　　　　　application.setAttribute( "counter" , 1 ) ; // 替换某个属性的值时，呼叫 attributeReplaced
    　　　　

    　　　　void attributeRemoved( ServletContextAttributeEvent event )


    　　　　　　»当从 ServletContext 移除 某个名称的 属性时 呼叫 attributeRemoved

    测试案例：

    <%@ page language="java"   pageEncoding="UTF-8" %>
<%@ page contentType="text/html; charset=UTF-8" %>
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Listener</title>
        <style type="text/css">
            ol li {
                list-style-type: lower-roman ;
            }
            
            .lifecycle , .application , .session {
                border: 1px solid #dedede ;
                width: 90% ;
                margin: 20px auto ;
                box-shadow: 0px 0px 5px 4px #dfdfdf ;
                padding: 10px 10px ;
            }
        </style>
    </head>
    <body>
    
        <div class="lifecycle">
            <a href="<%= request.getContextPath() %>/session/show" >显示 Session Id </a>
        </div>
        
        <div class="application">
            Application
            <form action="<%= request.getContextPath() %>/application/add" method="post" >
                添加或替换属性:<input type="text" name="name" placeholder="请输入属性名">
                <input type="text" name="value" placeholder="请输入属性值">
                <input type="submit" value="添加" >
            </form>
            
            <hr>
            
            <form action="<%= request.getContextPath() %>/application/remove" method="post" >
                删除属性:<input type="text" name="name" placeholder="请输入属性名">
                <input type="submit" value="删除" >
            </form>
        
        </div>
        
        <div class="session">
        
            Session:
            <form action="<%= request.getContextPath() %>/session/add" method="post" >
                添加或替换属性:<input type="text" name="name" placeholder="请输入属性名">
                <input type="text" name="value" placeholder="请输入属性值">
                <input type="submit" value="添加" >
            </form>
            
            <hr>
            
            <form action="<%= request.getContextPath() %>/session/remove" method="post" >
                删除属性:<input type="text" name="name" placeholder="请输入属性名">
                <input type="submit" value="删除" >
            </form>
        
        </div>
        
        <div class="session">
        
            <form action="<%= request.getContextPath() %>/customer/login" method="post" >
                <input type="text" name="username" placeholder="请输入用户名">
                <input type="password" name="password" placeholder="请输入密码">
                <input type="submit" value="登录" >
            </form>
        
        </div>
        
        
    
    </body>
</html>
    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet( "/application/add" )
public class ApplicationAddServlet extends HttpServlet {

    private static final long serialVersionUID = -1218970600318038715L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        String name = request.getParameter( "name" );
        String value = request.getParameter( "value" );
        
        ServletContext application = this.getServletContext();
        
        application.setAttribute( name , value ); // ----> ApplicationAttributeListener # attributeAdded
        
        response.sendRedirect( request.getContextPath() + "/pages/listener/index.jsp" );
        
    }
    
    

}
    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet( "/application/remove" )
public class ApplicationRemoveServlet extends HttpServlet {

    private static final long serialVersionUID = -6802388429975749293L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        String name = request.getParameter( "name" );
        
        ServletContext application = this.getServletContext();
        
        application.removeAttribute( name ); // ----> ApplicationAttributeListener # attributeRemoved
        
        response.sendRedirect( request.getContextPath() + "/pages/listener/index.jsp" );
        
    }
    
    

}
    package ecut.listener.listener;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;

public class ApplicationAttributeListener implements ServletContextAttributeListener {

    @Override
    public void attributeAdded( ServletContextAttributeEvent event ) {
        ServletContext application = event.getServletContext();
        String name = event.getName() ; 
        Object value = event.getValue() ; 
        System.out.print( "向 " + application.getContextPath() + " 的 ServletContext 中添加属性: " );
        System.out.println(  name + " ，取值: " + value );
    }

    @Override
    public void attributeReplaced( ServletContextAttributeEvent event ) {
        
        ServletContext application = event.getServletContext();
        String name = event.getName() ; 
        Object value = event.getValue() ; // 原来的值
        
        System.out.print( application.getContextPath() + " 的 ServletContext 中的属性: " );
        System.out.print( name );
        System.out.print( " 的取值从 [ ");
        System.out.print( value );
        System.out.println( " ] 替换成 [ " + application.getAttribute( name )  +" ]" );

    }
    
    @Override
    public void attributeRemoved( ServletContextAttributeEvent event ) {
        ServletContext application = event.getServletContext();
        String name = event.getName() ; 
        Object value = event.getValue() ; 
        System.out.print( "从 " + application.getContextPath() + " 的 ServletContext 中移除属性: " );
        System.out.println(  name + " ，取值: " + value );
    }

}

    运行结果如下：

    向 /s 的 ServletContext 中添加属性: 女朋友 ，取值: 罗玉凤
    /s 的 ServletContext 中的属性: 女朋友 的取值从 [ 罗玉凤 ] 替换成 [ 芙蓉 ]
    从 /s 的 ServletContext 中移除属性: 女朋友 ，取值: 芙蓉
    • javax.servlet.http.HttpSessionAttributeListener : 监听 session 对象中的 属性变化的监听器


    　　　　void attributeAdded( HttpSessionBindingEvent event )

    　　　　void attributeReplaced( HttpSessionBindingEvent event )

    　　　　void attributeRemoved( HttpSessionBindingEvent event )

    测试案例：

    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

@WebServlet( "/session/add" )
public class SessionAddServlet extends HttpServlet {
    
    private static final long serialVersionUID = 7547888494941329407L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        String name = request.getParameter( "name" );
        String value = request.getParameter( "value" );
        
        HttpSession session = request.getSession();
        
        
        session.setAttribute( name , value ); // ----> SesssionAttributeListener # attributeAdded
        
        response.sendRedirect( request.getContextPath() + "/pages/listener/index.jsp" );
        
    }
    
    

}
    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

@WebServlet( "/session/remove" )
public class SessionRemoveServlet extends HttpServlet {

    private static final long serialVersionUID = -6802388429975749293L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        String name = request.getParameter( "name" );
        
        HttpSession session = request.getSession();
        
        session.removeAttribute( name ); // ----> SesssionAttributeListener# attributeRemoved
        
        response.sendRedirect( request.getContextPath() + "/pages/listener/index.jsp" );
        
    }
    
    

}
    package ecut.listener.listener;

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

public class SessionAttributeListener implements HttpSessionAttributeListener {

    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        HttpSession session = event.getSession();
        String name = event.getName() ; 
        Object value = event.getValue() ; 
        System.out.print( "向 会话[" + session.getId() + " ]中添加属性: " );
        System.out.println(  name + " ，取值: " + value );
        
    }

    @Override
    public void attributeRemoved(HttpSessionBindingEvent event) {
        HttpSession session = event.getSession();
        String name = event.getName() ; 
        Object value = event.getValue() ; 
        System.out.print( "从会话[ " + session.getId() + "] 中移除属性: " );
        System.out.println(  name + " ，取值: " + value );

    }

    @Override
    public void attributeReplaced(HttpSessionBindingEvent event) {
        

        HttpSession session = event.getSession();
        String name = event.getName() ; 
        Object value = event.getValue() ; // 原来的值
        
        System.out.print("会话[" + session.getId() + "] 中的属性: " );
        System.out.print( name );
        System.out.print( " 的取值从 [ ");
        System.out.print( value );
        System.out.println( " ] 替换成 [ " + session.getAttribute( name )  +" ]" );

    }

}

    运行结果如下：

    向 会话[E962FCF5380CE7E04CF599D20438F25 ]中添加属性: 男朋友 ，取值: 吴彦祖
会话[E962FC9F5380CE7E04CF599D20438F25] 中的属性: 男朋友 的取值从 [ 吴彦祖 ] 替换成 [ 彭于晏 ]
从会话[ E962FC9F5380CE7E04CF599D20438F25] 中移除属性: 男朋友 ，取值: 彭于晏
    • javax.servlet.ServletRequestAttributeListener : 监听 request 对象中的 属性变化的监听器

    　　　　void attributeAdded( ServletRequestAttributeEvent srae )

    　　　　void attributeReplaced( ServletRequestAttributeEvent srae )

    　　　　void attributeRemoved(ServletRequestAttributeEvent srae)


    5、【 跟 Session 钝化 和 锐化 有关的监听器 】

    • javax.servlet.http.HttpSessionActivationListener :


    　　　　void sessionWillPassivate( HttpSessionEvent event )

    　　　　　　»Passivate ( 钝化 )

    　　　　　　当 有客户端 关联的 Session 存在，并且 Session 中有数据时，如果此时 重新加载 当前Web应用，

    　　　　　　将导致 Session 中的数据被丢失，为了保存这部分数据，容器 可能会将 Session 对象中的数据 保存到 磁盘 、网络 等存储设备中，这种行为 称作 Session 的 钝化

    　　　　void sessionDidActivate( HttpSessionEvent event )

    　　　　　　»Activate ( 锐化 )

    　　　　　　从 硬盘或其它存储设备上的文件中重新读回 Session 数据，重新加载到 JVM 所管理的 内存中 的动作

    6、【跟 属性值 绑定到 Session 有关的监听器】

    • javax.servlet.http.HttpSessionBindingListener


    　　　　void valueBound( HttpSessionBindingEvent event )

    　　　　　　»如果该实例加入session 对象的属性，则呼叫valueBound()

    　　　　void valueUnbound( HttpSessionBindingEvent event )

    　　　　　　»假设 将要 添加到 session 中的 属性值 的类型是 Customer 类型

    　　　　　　»如果该实例从session 对象种移除，则呼叫valueUnbound()

    　　　　　　Customer c = new Customer();

    　　　　　　session.setAttribute( "customer" , c ) ; // 将 Customer 类型的 对象 绑定到 session 的属性值中

    　　　　　　session.removeAttribute( "customer" ) ; // 解除 Customer 类型的 对象 跟 session 的绑定

    　　　　　　为了监听 Customer 对象 跟 session 是否 发生关系，则需要 将 Customer 本身 做成是 监听器

     　　　　　　这个监听器无需在xml中或者是在类中使用注解注册Listener

    统计用户在线人数测试案例：

    package ecut.listener.entity;

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

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

public class Customer implements Serializable , HttpSessionBindingListener {

    private static final long serialVersionUID = 7499870485797565944L;
    
    private Integer id ;
    private String username ;
    private String password ;
    private String nickname ;
    private char gender ;
    private Date birthdate ;

    @Override
    public void valueBound( HttpSessionBindingEvent event ) {
        System.out.println( "绑定到 [ " + event.getSession().getId() + " ]" );
        ServletContext application = event.getSession().getServletContext();
        // 1、先尝试从 ServletContext 获取名称是 loginCounter 的属性
        Integer loginCounter = (Integer)application.getAttribute( "loginCounter" );
        // 2、判断 loginCounter 是否不存在
        if( loginCounter == null ){
            loginCounter = Integer.valueOf( 0 ) ;
        }
        // 3、将 loginCounter 增加 1  以后 再 放入到 ServletContext 中
        application.setAttribute( "loginCounter" , ++loginCounter );
    }

    @Override
    public void valueUnbound( HttpSessionBindingEvent event ) {
        System.out.println( "从 [ " + event.getSession().getId() + " ] 解除绑定" );
        ServletContext application = event.getSession().getServletContext();
        // 1、先尝试从 ServletContext 获取名称是 loginCounter 的属性
        Integer loginCounter = (Integer)application.getAttribute( "loginCounter" );
        // 2、判断 loginCounter 是否不存在
        if( loginCounter == null ){
            application.setAttribute( "loginCounter" , 0 );
        } else {
            // 3、将 loginCounter 减去 1  以后 再 放入到 ServletContext 中
            --loginCounter;
            loginCounter = loginCounter < 0 ? 0 : loginCounter ;
            application.setAttribute( "loginCounter" , loginCounter );
        }
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getNickname() {
        return nickname;
    }

    public void setNickname(String nickname) {
        this.nickname = nickname;
    }

    public char getGender() {
        return gender;
    }

    public void setGender(char gender) {
        this.gender = gender;
    }

    public Date getBirthdate() {
        return birthdate;
    }

    public void setBirthdate(Date birthdate) {
        this.birthdate = birthdate;
    }

}
    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import ecut.listener.entity.Customer;

@WebServlet( "/customer/login" )
public class CustomerLoginServlet  extends HttpServlet {

    private static final long serialVersionUID = 2694859066336538343L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        String username = request.getParameter( "username" );
        String password = request.getParameter( "password" );
        
        Customer c = new Customer();
        c.setUsername( username );
        c.setPassword( password );
        
        HttpSession session = request.getSession() ;
        
        session.setAttribute( "customer" ,  c ); // ---> Customer # valueBound
        
        response.sendRedirect( request.getContextPath() + "/customer/main" );
        return ;
    }

}
    package ecut.listener.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

@WebServlet( "/customer/logout" )
public class CustomerLogoutServlet extends HttpServlet {

    private static final long serialVersionUID = 1376263177132417940L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        HttpSession session = request.getSession();
        
        session.removeAttribute( "customer" ); // ---> Customer # valueUnbound
        
        response.sendRedirect( request.getContextPath() + "/pages/listener/index.jsp" );
    
    }

}
    package ecut.listener.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import ecut.listener.entity.Customer;

@WebServlet( "/customer/main" )
public class CustomerMainServlet  extends HttpServlet {

    private static final long serialVersionUID = 154588398950897364L;

    @Override
    protected void service( HttpServletRequest request , HttpServletResponse response ) 
            throws ServletException, IOException {
        
        response.setContentType( "text/html" );
        
        PrintWriter w = response.getWriter();
        
        ServletContext application = this.getServletContext() ;
        HttpSession session = request.getSession();
        
        Object o = session.getAttribute( "customer" );
        
        if( o instanceof Customer ){
            Customer c = (Customer) o ;
            w.println( "<h1>欢迎" + c.getUsername() + " 登录</h1>" );
            w.println( "<h1>当前登陆人数:" + application.getAttribute( "loginCounter" ) + " </h1>" );
            w.println( "<a href='" + request.getContextPath() + "/customer/logout'>注销</a>");
        } else {
            
            response.sendRedirect( request.getContextPath() + "/pages/listener/index.jsp" );
        }
    
    }

}

    运行结果如下：

    绑定到 [ E962FC9F5380CE7E04CF599D20438F25 ]
向 /s 的 ServletContext 中添加属性: loginCounter ，取值: 1
向 会话[E962FC9F5380CE7E04CF599D20438F25 ]中添加属性: customer ，取值: ecut.listener.entity.Customer@5be15f9f
    绑定到 [ 2E6EF661378654D7120122FCDEEA1F88 ]
    /s 的 ServletContext 中的属性: loginCounter 的取值从 [ 1 ] 替换成 [ 2 ]
    向 会话[2E6EF661378654D7120122FCDEEA1F88 ]中添加属性: customer ，取值: ecut.listener.entity.Customer@2098d9
    从 [ 2E6EF661378654D7120122FCDEEA1F88 ] 解除绑定
    /s 的 ServletContext 中的属性: loginCounter 的取值从 [ 2 ] 替换成 [ 1 ]
    从会话[ 2E6EF661378654D7120122FCDEEA1F88] 中移除属性: customer ，取值: ecut.listener.entity.Customer@2098d9

    7、注册监听器

    • 必须注册Listener ，容器才能为相应事件找到处理者
    • 除了HttpSessionBindingListener外所有的Listener 都必须注册
    • 只有注册过的Listener 容器才能进行调度
    • 注册Listener

    　　　　可以在web.xml 中添加相应的配置
    　　　　　　»配置文件中，一般把Listener 的配置放在servlet 和filter 之前
    　　　　Servlet 3.0 可以在监听器类中使用@WebListener 注解
    　　　　　　»这个注解用于标注这个类是一个监听器，并注册给容器
    　　　　　　»@WebListener 只有一个属性value，它是String 类型，指定当前Listener的描述

    转载请于明显处标明出处

    http://www.cnblogs.com/AmyZheng/p/9032454.html
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  • 原文地址：https://www.cnblogs.com/AmyZheng/p/9032454.html
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