Listener

绝地灬酷狼 2022-05-17 00:21 176阅读 0赞

### 观察者模式 ###

观察者模式定义了一种一对多的关联关系, 让多个观察者对象同时监听某一个主题对象. 这个主题对象在状态上发生变化时, 会通知所有观察者对象, 使它们能够自动更新自己.  
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//观察者接口
    public interface IObserver {
        //处理目标对象发送来的消息
        void handleNotify(String message);
    }
    public class FirstObserver implements IObserver {
        @Override
        public void handleNotify(String message) {
            System.out.println("一号观察者接收到消息["+ message + "], 正在处理消息");
        }
    }
    public class SecondObserver implements IObserver {
        @Override
        public void handleNotify(String message) {
            System.out.println("二号观察者接收到消息["+ message + "], 正在处理消息");
        }
    }
    
    //目标对象接口
    public interface ISomeService {
        //添加观察者
        void addObserver(IObserver observer);
        //删除观察者
        void removeObserver(IObserver observer);
        //向观察者发送消息
        void notifyObserver(String message);
    }
    public class SomeServiceImpl implements ISomeService {
        //观察者列表
        private List<IObserver> observers;
    
        public SomeServiceImpl() {
            observers = new ArrayList<>();
        }
    
        @Override
        public void addObserver(IObserver observer) {
            observers.add(observer);
        }
    
        @Override
        public void removeObserver(IObserver observer) {
            observers.remove(observer);
        }
    
        @Override
        public void notifyObserver(String message) {
            for (IObserver o : observers){
                o.handleNotify(message);
            }
        }
    }
    
    public class Test {
        public static void main(String[] args) {
            //创建多个观察者
            IObserver first = new FirstObserver();
            IObserver second = new SecondObserver();
    
            SomeServiceImpl service = new SomeServiceImpl();
            //添加观察者
            service.addObserver(first);
            service.addObserver(second);
    
            //向观察者发送消息
            service.notifyObserver("猥琐发育, 别浪");
        }
    }
    一号观察者接收到消息[猥琐发育, 别浪], 正在处理消息
    二号观察者接收到消息[猥琐发育, 别浪], 正在处理消息

### 监听器 ###

Servlet规范中定义了八个监听器接口, 它们监听的对象分别是Request、Session、ServletContext, 触发监听器的事件是这三个对象的创建与销毁, 它们域属性的添加、删除、修改, 以及Session的钝化与活化操作.  
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**LifeCycleListeners**

ServletRequestListener、HttpSessionListener、ServletContextListener这三个监听器分别用来监听Request对象、Session对象以及ServletContext对象的创建和销毁.

public class MyRequestListener implements ServletRequestListener{
        @Override
        public void requestDestroyed(ServletRequestEvent sre) {
            System.out.println("请求对象被销毁");
        }
    
        @Override
        public void requestInitialized(ServletRequestEvent sre) {
            System.out.println("请求对象被创建");
        }
    }
    
    <listener>
    <listener-class>org.listeners.MyRequestListener</listener-class>
    </listener>
    
    请求对象被创建
    请求对象被销毁

**EventListeners**

(1)ServletRequestAttributeListener、HttpSessionAttributeListener、ServletContextAttributeListener这三个监听器分别用来监听Request对象、Session对象以及ServletContext对象的域属性的的添加、删除、修改操作.

public class MyRequestListener implements ServletRequestAttributeListener{
        @Override
        public void attributeAdded(ServletRequestAttributeEvent srae) {
            System.out.println("向Request域中添加了一个属性: "+ srae.getName() + "=" + srae.getValue());
        }
    
        @Override
        public void attributeRemoved(ServletRequestAttributeEvent srae) {
            System.out.println("从Request域中删除了一个属性: "+ srae.getName() + "=" + srae.getValue());
        }
    
        @Override
        public void attributeReplaced(ServletRequestAttributeEvent srae) {
            System.out.println("修改了Request域中的一个属性: "+ srae.getName() + "=" + srae.getValue());
        }
    }
    
    <listener>
    <listener-class>org.listeners.MyRequestListener</listener-class>
    </listener>
    
    <html>
    <body>
        <%
            request.setAttribute("username", "tyshawn");
            request.setAttribute("username", "lee");
            request.removeAttribute("username");
        %>
    </body>
    </html>
    
    向Request域中添加了一个属性: username=tyshawn
    修改了Request域中的一个属性: username=tyshawn
    从Request域中删除了一个属性: username=lee

(2)HttpSessionBindingListener监听器用来监听对象是否在Session域中. 该监听器不需要注册.

public class User implements HttpSessionBindingListener {
        private String username;
    
        public User(String username) {
            this.username = username;
        }
    
        public String getUsername() {
            return username;
        }
    
        public void setUsername(String username) {
            this.username = username;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "username='" + username + '\'' +
                    '}';
        }
    
        //当前对象放入Session域中触发该方法
        @Override
        public void valueBound(HttpSessionBindingEvent event) {
            System.out.println("User对象放入到了Session域中");
            System.out.println(event.getName() + "=" + event.getValue());
        }
    
        //当前对象从Session域中删除触发该方法
        @Override
        public void valueUnbound(HttpSessionBindingEvent event) {
            System.out.println("User对象从Session域中删除了");
            System.out.println(event.getName() + "=" + event.getValue());
        }
    }
    
    <html>
    <body>
        <%
            User user = new User("tyshawn");
            session.setAttribute("user", user);
            session.removeAttribute("user");
        %>
    </body>
    </html>
    
    User对象放入到了Session域中
    user=User{username='tyshawn'}
    User对象从Session域中删除了
    user=User{username='tyshawn'}

(3)HttpSessionActivationListener用来监听Session中对象的活化与钝化. 该监听器不需要注册.

public class User implements HttpSessionActivationListener, Serializable {
        private String username;
    
        public User(String username) {
            this.username = username;
        }
    
        public String getUsername() {
            return username;
        }
    
        public void setUsername(String username) {
            this.username = username;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "username='" + username + '\'' +
                    '}';
        }
    
        //内存中的User对象写入到硬盘中时触发该方法
        @Override
        public void sessionWillPassivate(HttpSessionEvent se) {
            System.out.println("user对象将要被钝化");
        }
    
        //硬盘中的User对象恢复到内存中时触发该方法
        @Override
        public void sessionDidActivate(HttpSessionEvent se) {
            System.out.println("user对象已经活化");
        }
    }
    
    <html>
    <body>
        <%
            User user = new User("tyshawn");
            session.setAttribute("user", user);
        %>
    </body>
    </html>
    
    //stop服务器
    user对象将要被钝化
    
    //start服务器
    user对象已经活化

### 监听器应用 ###

**在线客户端统计**

这里的一个客户端指的是一个IP, 而不是一个浏览器, 更不是一次请求. 注意一个客户端可能开启多个浏览器.

思路: 使用ServletContextListener监听器在ServletContext初始化时创建一个map, key为客户端ip, value为该ip发出的所有session会话的集合, map的大小为客户端的数量. 然后使用ServletRequestListener监听器在每一次请求初始化时, 把本次会话放入与请求ip对应的session集合中. 最后使用HttpSessionListener监听器在session会话销毁时, 从session集合中删除本次会话, 当session集合为空时, 从map中删除该ip.

/**
    * 在ServletContext初始化时触发该监听器, 创建一个map, key为ip, value该ip发出的所有session集合, map的size()就为客户端的个数.
    */
    public class MyServletContextListener implements ServletContextListener {
        @Override
        public void contextInitialized(ServletContextEvent sce) {
            //key为ip, value该ip发出的所有session
            Map<String, Set<HttpSession>> map = new HashMap<>();
            ServletContext sc = sce.getServletContext();
            sc.setAttribute("map", map);
        }
    
        @Override
        public void contextDestroyed(ServletContextEvent sce) {
        }
    }
    
    /**
    * 对于每一次客户端请求都判断是否是来自于新的客户端
    */
    public class MyRequestListener implements ServletRequestListener{
        @Override
        public void requestInitialized(ServletRequestEvent sre) {
            HttpServletRequest request = (HttpServletRequest) sre.getServletRequest();
            //获取当前发送请求的ip
            String clientIp = request.getRemoteAddr();
            //获取当前session, 用来存储当前ip, 当用户销毁会话时用来判断是哪个ip发起的会话
            HttpSession currentSession = request.getSession();
    
            //判断是否是新的客户端, 把ip和会话存入map中
            ServletContext sc = sre.getServletContext();
            Map<String, Set<HttpSession>>  map = (Map<String, Set<HttpSession>>) sc.getAttribute("map");
            Set<HttpSession> sessions = map.get(clientIp);
            if(null == sessions){
                sessions = new HashSet<>();
            }
            sessions.add(currentSession);
            map.put(clientIp, sessions);
            sc.setAttribute("map", map);
    
            //将当前ip放入到当前session中
            currentSession.setAttribute("clientIp", clientIp);
        }
    
        @Override
        public void requestDestroyed(ServletRequestEvent sre) {
        }
    }
    
    /**
     * 当会话关闭时, 删除map中与该ip对应的会话. 注意:浏览器关闭了会话并没有关闭.
     */
    public class MySessionListener implements HttpSessionListener {
        @Override
        public void sessionDestroyed(HttpSessionEvent se) {
            HttpSession currentSession = se.getSession();
            String clientIp = (String) currentSession.getAttribute("clientIp");
    
            //删除map中与该ip对应的会话
            ServletContext sc = currentSession.getServletContext();
            Map<String, Set<HttpSession>> map = (Map<String, Set<HttpSession>>) sc.getAttribute("map");
            Set<HttpSession> sessions = map.get(clientIp);
            sessions.remove(currentSession);
    
            //当sessions的大小等于0时, 删除该ip的map数据
            if(sessions.size() == 0){
                map.remove(clientIp);
            }else{
                map.put(clientIp, sessions);
            }
            //保存修改后的map
            sc.setAttribute("map", map);
        }
    
        @Override
        public void sessionCreated(HttpSessionEvent se) {
        }
    }

**管理员踢除用户**

用户登录后, 服务器将用户信息存储在session中, 踢除用户就是让该用户的session失效, 重新登录.

思路: 使用ServletContextListener监听器在ServletContext初始化时创建一个map, key为用户名, value为当前用户发起的session. 使用HttpSessionBindingListener监听器在服务器将用户信息存储在session中时, 将该session存储在全局map中. 当管理员踢除用户时, 通过用户名从全局map中获取当前session, 让该session失效, 并从map中删除该用户名.

public class MyServletContextListener implements ServletContextListener {
        @Override
        public void contextInitialized(ServletContextEvent sce) {
            //key为用户名, value为当前用户发起的session
            Map<String, HttpSession> map = new HashMap<>();
            ServletContext sc = sce.getServletContext();
            sc.setAttribute("map", map);
        }
    
        @Override
        public void contextDestroyed(ServletContextEvent sce) {
        }
    }
    
    public class User implements HttpSessionBindingListener{
        private String username;
    
        public User(String username) {
            this.username = username;
        }
        public String getUsername() {
            return username;
        }
        public void setUsername(String username) {
            this.username = username;
        }
    
        /**
         *  当user对象被放入session中时, 把username和session对象放入全局map中
         */
        @Override
        public void valueBound(HttpSessionBindingEvent event) {
            HttpSession session = event.getSession();
            ServletContext sc = session.getServletContext();
            Map<String, HttpSession> map = (Map<String, HttpSession>) sc.getAttribute("map");
            map.put(username, session);
            sc.setAttribute("map", map);
        }
    
        @Override
        public void valueUnbound(HttpSessionBindingEvent event) {
        }
    }
    
    public class LoginServlet extends HttpServlet {
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            //登录成功, 获取用户名
            String username = request.getParameter("username");
            User user = new User(username);
    
            //将登录用户信息保存在session中
            HttpSession session = request.getSession();
            session.setAttribute("user", user);
    
            //跳转到剔除用户界面
            request.getRequestDispatcher("/welcome.jsp").forward(request,response);
        }
    }
    
    public class KickServlet extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            //被踢除的用户名
            String username = request.getParameter("username");
    
            //踢除用户, 让该用户发起的session失效, 并从map中删除该username
            ServletContext sc = request.getSession().getServletContext();
            Map<String, HttpSession> map = (Map<String, HttpSession>) sc.getAttribute("map");
            HttpSession session = map.get(username);
            session.invalidate();
            map.remove(username);
            sc.setAttribute("map", map);
    
            //跳转到登录页面
            request.getRequestDispatcher("/index.jsp").forward(request,response);
        }
    }
    
    <html>
    <body>
        <form action="${pageContext.request.contextPath}/loginServlet" method="post">
            用户名: <input type="text" name="username"><br>
                    <input type="submit" value="登录">
        </form>
    </body>
    </html>
    
    <html>
    <body>
        <table>
            <caption>已登录用户</caption>
            <tr>
                <th>用户名</th>
                <th>Session</th>
                <th>踢除</th>
            </tr>
            <c:forEach items="${map}" var="entry">
                <tr>
                    <td>${entry.key}</td>
                    <td>${entry.value}</td>
                    <td><a href="${pageContext.request.contextPath}/kickServlet?username=${entry.key}" >踢除</a> </td>
                </tr>
            </c:forEach>
        </table>
    </body>
    </html>

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