Web笔记-Listener

作者: PengFly | 来源:发表于2017-08-08 15:07 被阅读19次

事件源、事件、监听器

事件源：发生事件的对象
事件：事件封装了事件源，方便监听器的某个方法获取到事件源对象
监听器：监听事件源的行为和动作
注：监听器一般都是接口，谁用谁实现。

Servlet规范中提供的8个监听器

监听ServletContext、HttpSession、ServletRequest对象的初始化和销毁的监听器

ServletContextListener

void contextInitialized(ServletContextEvent sce) ：ServletContext初始化
void contextDestroyed(ServletContextEvent sce) ：ServletContext销毁
使用场景：
应用启动时执行，可将整个应用的初始化代码写到该监听器里

HttpSessionListener

void sessionCreated(HttpSessionEvent sce)：第一次执行request.getSession()执行
void sessionDestroyed(HttpSessionEvent sce)：HttpSession销毁时调用
注：关闭浏览器不会导致HttpSession销毁；HttpSession销毁时机（超时或者手动调用invalidate()方法）
使用场景：
统计网站在线用户量

ServletRequestListener

void requestInitialized(ServletRequestEvent arg0)：用户请求时执行
void requestDestroyed(ServletRequestEvent arg0)：请求结束后执行
使用场景：根据用户访问的url资源id进行页面访问量统计

监听ServletContext、HttpSession、ServletRequest域对象数据变化（增删改）的监听器

ServletContextAttributeListener

void attributeAdded(ServletContextAttributeEvent arg0)：当向应用范围内添加属性时执行
void attributeRemoved(ServletContextAttributeEvent arg0)：当从应用范围内移除属性时执行
void attributeReplaced(ServletContextAttributeEvent arg0)：当应用范围内当属性值替换时执行

HttpSessionAttributeListener

void attributeAdded(HttpSessionBindingEvent arg0)：当向会话范围内添加属性时执行
void attributeRemoved(HttpSessionBindingEvent arg0)：当从会话范围内移除属性时执行
void attributeReplaced(HttpSessionBindingEvent arg0)：当会话范围内当属性值替换时执行
使用场景：统计在线用户，踢人

ServletRequestAttributeListener

void attributeAdded(ServletRequestAttributeEvent arg0)：当向请求范围内添加属性时执行
void attributeRemoved(ServletRequestAttributeEvent arg0)：当从请求范围内移除属性时执行
void attributeReplaced(ServletRequestAttributeEvent arg0)：当请求范围内当属性值替换时执行

感知型监听器（无需注册监听器）

HttpSessionBindingListener：监听自身是否被添加到量session中去

void valueBound(HttpSessionBindingEvent arg0)：绑定时执行
void valueUnbound(HttpSessionBindingEvent arg0)：解绑时执行

HttpSessionActivationListener：监听自身合适被钝化和激活

void sessionWillPassivate(HttpSessionEvent arg0)：钝化时执行
void sessionDidActivate(HttpSessionEvent arg0)：被激活时执行

HttpSession的钝化和激活

当用户第一次调用request.getSession()时，则会创建session对象，session对象会存储在缓存当中。

session销毁：只有在session超时和手动调用invalidata()方法时，session对象才会销毁。
session钝化：将session对象持久化到本地硬盘上（tomcat/work/catalina/localhost/应用名/下的*.ser文件）
1、服务器内存告急
2、HttpSession对象长时间没有使用
3、服务器正常停止或应用被卸载
注：在编写javabean时，最好实现java.io.Serializable接口，若没有实现该接口，则会导致session的激活会失败

在线用户统计及踢人案例

流程分析：
1. 创建HttpSessionAttributeListener
2. 当用户登录成功后，则在session中设置当前用户信息
3. 在HttpSessionAttributeListener的attributeAdded方法中判断并创建Map<String,HttpSession>是否有当前用户信息，没有则添加，并将该map集合设置到全局属性中
4. 展示页面中通过获取ServletContext中的attribute的map集合进行遍历展示在线用户信息
5. 踢下线处理则将目标用户的username作为参数传递给Servlet，在踢下线Servlet中获取全局map，删除对应的session中的目标用户信息，再从map中移除目标元素

示例代码：
index.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!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>
    <c:if test="${sessionScope.user == null }">
        <c:redirect url="/login.jsp"></c:redirect>
    </c:if>
    欢迎您，${sessionScope.user.username}
    <c:if test="${sessionScope.user.username eq 'admin' }">
        <a href="${ pageContext.request.contextPath }/online.jsp">查看在线用户</a>
    </c:if>
</body>
</html>

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!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>登录</title>
</head>
<body>
    <form action="${pageContext.request.contextPath }/servlet/LoginServlet"
        method="post">
        <table align="center" border="1" cellpadding="4px">
            <tr>
                <td align="right">用户名</td>
                <td><input type="text" name="username" /></td>
            </tr>
            <tr>
                <td align="right">密码</td>
                <td><input type="password" name="password" /></td>
            </tr>
            <tr>
            <tr>
                <td colspan="2" align="center"><input type="submit" value="登录" /></td>
            </tr>
        </table>
    </form>
</body>
</html>

LoginServlet.java

package com.gaoshiyi.web.servlet;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import org.apache.commons.beanutils.BeanUtils;
import com.gaoshiyi.web.domain.User;
import com.gaoshiyi.web.service.BusinessSevice;
import com.gaoshiyi.web.service.impl.BusinessServiceImpl;
public class LoginServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    public LoginServlet() {
        super();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;chartset=utf-8");
        Map<String, String[]> map = request.getParameterMap();
        User user = new User();
        try {
            BeanUtils.populate(user, map);
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        BusinessSevice sevice = new BusinessServiceImpl();
        User exitUser = sevice.login(user);
        if (exitUser == null) {
            
            response.getWriter().write("用户或密码错误，请重新登录！");
            response.setHeader("Refresh", "1;url=" + request.getContextPath() + "/login.jsp");
            return;
        }
        
        //进行session的存储
        HttpSession session = request.getSession();
        session.setAttribute("user", user);
        response.sendRedirect(request.getContextPath());
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

LoginInfoListener.java

package com.gaoshiyi.web.listener;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import com.gaoshiyi.web.domain.User;
/**
 * 用户登录信息监听器
 *
 */
@WebListener
public class LoginInfoListener implements HttpSessionAttributeListener {
    public LoginInfoListener() {
    }
    public void attributeAdded(HttpSessionBindingEvent arg0)  { 
    
        HttpSession session = arg0.getSession();
        ServletContext servletContext = session.getServletContext();
        Object obj = session.getAttribute("user");
        if (obj != null && obj instanceof User) {
            
            //建立Map集合存储用户信息
            Map<String, HttpSession> usersMap = (Map<String, HttpSession>) servletContext.getAttribute("users");
            if (usersMap == null) {
                //同步处理否则会创建多个Map
                usersMap = Collections.synchronizedMap(new HashMap<>());
                servletContext.setAttribute("users", usersMap);
                
            }
            User user = (User) obj;
            if (!usersMap.containsKey(user.getUsername())) {
                
                //若集合中没有该用户则添加
                usersMap.put(user.getUsername(), session);
                System.out.println("新用户：" + user.getUsername() + "登录...");
            }
            System.out.println(usersMap);
        }
    }
    public void attributeRemoved(HttpSessionBindingEvent arg0)  { 
    }
    public void attributeReplaced(HttpSessionBindingEvent arg0)  { 
    }
}

online.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!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>在线用户</title>
</head>
<body>
    <c:if test="${sessionScope.user.username != 'admin' }">
        <c:redirect url="/index.jsp"></c:redirect>
    </c:if>
    <h3 align="center">当前在线用户列表</h3>
    <table align="center" border="1" width="50%" height="auto">
        <tr>
            <th>用户名</th>
            <th>操作</th>
        </tr>
        <c:forEach items="${applicationScope.users }" var="um">
            <tr align="center">
            <c:url var="url" value="servlet/KickServlet">
                <c:param name="username" value="${ um.key}"></c:param>
            </c:url>
                <td>${ um.key }</td>
                <td><a href="${ url }">踢下线</a></td>
            </tr>
        </c:forEach>
    </table>
</body>
</html>

KickServlet.java

package com.gaoshiyi.web.servlet;
import java.io.IOException;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
/**
 * 踢下线处理
 * @author gaopengfei
 *
 */
public class KickServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    public KickServlet() {
        super();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");
        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=utf-8");
        String username = request.getParameter("username");
//      username = new String(username.getBytes("ISO-8859-1"), "UTF-8");
        System.out.println("被踢用户：" + username);
        //获取全局用户登录信息集合
        Map<String, HttpSession> usersMap = (Map<String, HttpSession>) getServletContext().getAttribute("users");
        if (usersMap.containsKey(username)) {
            
            //先从session中移除
            usersMap.get(username).removeAttribute("user");
            //再从map中移除元素
            usersMap.remove(username);
            System.out.println("用户：" + username + "已被踢下线...");
        }
        response.getWriter().write("操作成功^_^");
        response.setHeader("Refresh", "1;url=" + request.getContextPath()+"/online.jsp");
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

演示：

在线踢下线

Web笔记-Listener

事件源、事件、监听器

Servlet规范中提供的8个监听器

HttpSession的钝化和激活

在线用户统计及踢人案例

相关文章

网友评论

延伸阅读

深度阅读

栏目导航

热点阅读