• 《Head First Servlets & JSP》-6-会话管理-listener etc. demo

    工程结构

    上下文参数示例

    示例程序展示了如何从上下文读取参数，并在上下文监听器中生成属性对象和在上下文中设置属性。

    • 建立一个简单的JavaBean对象作为属性：Dog.java
    1. package com.example.entities;
    3. /**
    4.  * Created by LTN on 2016/11/2.
    5.  */
    6. public class Dog {
    7.     private String breed;
    9.     public Dog(String breed) {
    10.         this.breed = breed;
    11.     }
    13.     public String getBreed() {
    14.         return breed;
    15.     }
    17. }
    • 建立一个servlet上下文监听器DogServletContextListener.java
    1. package com.example.listener;
    3. import com.example.entities.Dog;
    5. import javax.servlet.ServletContext;
    6. import javax.servlet.ServletContextEvent;
    8. /**
    9.  * Created by LTN on 2016/11/2.
    10.  */
    11. public class DogServletContextListener implements javax.servlet.ServletContextListener {
    12.     public void contextInitialized(ServletContextEvent event) {
    13.         ServletContext sc = event.getServletContext();
    14.         String dogBreed = sc.getInitParameter("breed");
    15.         Dog d = new Dog(dogBreed);
    16.         sc.setAttribute("dog", d);
    17.     }
    19.     public void contextDestroyed(ServletContextEvent event) {
    20.         //do nothing.
    21.     }
    22. }
    • 建立Servlet进行测试ContextListenerTester.java
    1. package com.example.servlet;
    3. import com.example.entities.Dog;
    5. import javax.servlet.ServletException;
    6. import javax.servlet.http.HttpServlet;
    7. import javax.servlet.http.HttpServletRequest;
    8. import javax.servlet.http.HttpServletResponse;
    9. import java.io.IOException;
    10. import java.io.PrintWriter;
    12. /**
    13.  * Created by LTN on 2016/11/2.
    14.  */
    15. public class ContextListenerTester extends HttpServlet {
    16.     public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    17.         response.setContentType("text/html");
    18.         PrintWriter out = response.getWriter();
    19.         out.println("test context attributes set by listener<br>");
    20.         out.println("<br>");
    22.         Dog dog = (Dog) getServletContext().getAttribute("dog");
    23.         out.println("Dog's breed is: " + dog.getBreed());
    24.     }
    25. }
    • 在DD（web.xml）中配置上下文参数、监听器和servlet映射关系
    1. <web-app>
    3.   <display-name>Archetype Created Web Application</display-name>
    5.   <context-param>
    6.     <param-name>breed</param-name>
    7.     <param-value>Great Dane</param-value>
    8.   </context-param>
    10.   <servlet>
    11.     <servlet-name>ListenerTester</servlet-name>
    12.     <servlet-class>com.example.servlet.ContextListenerTester</servlet-class>
    13.   </servlet>
    14.   <servlet-mapping>
    15.     <servlet-name>ListenerTester</servlet-name>
    16.     <url-pattern>/ListenTest.do</url-pattern>
    17.   </servlet-mapping>
    19.   <listener>
    20.     <listener-class>com.example.listener.DogServletContextListener</listener-class>
    21.   </listener>
    23. </web-app>
    1. test context attributes set by listener
    3. Dog's breed is: Great Dane

    Session属性监听器

    使用Session属性监听器对Session中属性进行监听，当Session属性发生增加、删除或改变时，会被该监听器捕捉。

    • 创建一个Session属性监听类DogSessionAttributeListener.java
    1. package com.example.listener;
    3. import javax.servlet.http.HttpSessionAttributeListener;
    4. import javax.servlet.http.HttpSessionBindingEvent;
    6. /**
    7.  * Created by LTN on 2016/11/2.
    8.  */
    9. public class DogSessionAttributeListener implements HttpSessionAttributeListener {
    10.     public void attributeAdded(HttpSessionBindingEvent event) {
    11.         String name = event.getName();
    12.         Object value = event.getValue();
    14.         System.out.println("Attribute added: " + name + ": " + value);
    15.     }
    17.     public void attributeRemoved(HttpSessionBindingEvent event) {
    18.         String name = event.getName();
    19.         Object value = event.getValue();
    21.         System.out.println("Attribute removed: " + name + ": " + value);
    22.     }
    24.     public void attributeReplaced(HttpSessionBindingEvent event) {
    25.         String name = event.getName();
    26.         Object value = event.getValue();
    28.         System.out.println("Attribute replaced: " + name + ": " + value);
    29.     }
    30. }
    • 建立一个Servlet测试类AttributeListenerTester.java
    1. package com.example.servlet;
    3. import javax.servlet.ServletException;
    4. import javax.servlet.http.HttpServlet;
    5. import javax.servlet.http.HttpServletRequest;
    6. import javax.servlet.http.HttpServletResponse;
    7. import javax.servlet.http.HttpSession;
    8. import java.io.IOException;
    10. /**
    11.  * Created by LTN on 2016/11/2.
    12.  */
    13. public class AttributeListenerTester extends HttpServlet {
    14.     private static int labelCounter = 0;
    16.     public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    17.         HttpSession session = request.getSession();
    18.         session.setAttribute("label", "label" + labelCounter++);
    19.     }
    20. }
    • 在DD中配置servlet和listener
    1. <servlet>
    2.   <servlet-name>AttributeListenTester</servlet-name>
    3.   <servlet-class>com.example.servlet.AttributeListenerTester</servlet-class>
    4. </servlet>
    5. <servlet-mapping>
    6.   <servlet-name>AttributeListenTester</servlet-name>
    7.   <url-pattern>/AttributeListenerTest.do</url-pattern>
    8. </servlet-mapping>
    10. <listener>
    11.   <listener-class>com.example.listener.DogSessionAttributeListener</listener-class>
    12. </listener>
    1. Attribute added: label: label0

    然后连续三次刷新，后台依次打印：

    1. Attribute replaced: label: label0
    2. Attribute replaced: label: label1
    3. Attribute replaced: label: label2

    注意，event.getName()和event.getValue()都是获取该事件之前的session属性的值。

    统计全局活动的对话

    使用HttpSessionListener进行全局活动会话的统计，类似统计在线人数功能。

    • 建立全局活动会话监听器
    1. package com.example.listener;
    3. import javax.servlet.http.HttpSessionEvent;
    4. import javax.servlet.http.HttpSessionListener;
    6. /**
    7.  * Created by LTN on 2016/11/2.
    8.  */
    9. public class GlobalCounterListener implements HttpSessionListener {
    10.     static private int activeSessions;
    12.     public static int getActiveSessions() {
    13.         return activeSessions;
    14.     }
    16.     public void sessionCreated(HttpSessionEvent event) {
    17.         activeSessions++;
    18.     }
    20.     public void sessionDestroyed(HttpSessionEvent event) {
    21.         activeSessions--;
    22.     }
    23. }
    • 建立servlet测试类GlobalCounterTester.java
    1. package com.example.servlet;
    3. import com.example.listener.GlobalCounterListener;
    5. import javax.servlet.ServletException;
    6. import javax.servlet.http.HttpServlet;
    7. import javax.servlet.http.HttpServletRequest;
    8. import javax.servlet.http.HttpServletResponse;
    9. import javax.servlet.http.HttpSession;
    10. import java.io.IOException;
    11. import java.io.PrintWriter;
    13. /**
    14.  * Created by LTN on 2016/11/2.
    15.  */
    16. public class GlobalCounterTester extends HttpServlet {
    17.     public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    18.         PrintWriter out = response.getWriter();
    20.         HttpSession session = request.getSession();
    21.         session.setAttribute("name","counter");
    22.         String name=(String)session.getAttribute("name");
    24.         int activeSessions= GlobalCounterListener.getActiveSessions();
    26.         out.println("name: "+name);
    27.         out.println("activeSessions: "+activeSessions);
    28.     }
    29. }
    • 在DD中配置servlet和listener
    1. <servlet>
    2.   <servlet-name>SessionCounterTester</servlet-name>
    3.   <servlet-class>com.example.servlet.GlobalCounterTester</servlet-class>
    4. </servlet>
    5. <servlet-mapping>
    6.   <servlet-name>SessionCounterTester</servlet-name>
    7.   <url-pattern>/CounterTest.do</url-pattern>
    8. </servlet-mapping>
    10. <listener>
    11.   <listener-class>com.example.listener.GlobalCounterListener</listener-class>
    12. </listener>
    1. name: counter
    2. activeSessions: 1

    换一个浏览器，输入地址，会显示activeSessions数值递增。

    注意，同一个浏览器开新标签不能使得session个数增加；关闭当前浏览器再接着启动，也不能。

    属性绑定监听器

    当属性对象被加入session或从session删除使，对象本身可以知道。

    • 建立Session binding监听器Dog.java，该类继承了HttpSessionBindingListener，并实现valueBound和valueUnbound方法。
    1. package com.example.entities;
    3. import javax.servlet.http.HttpSessionBindingEvent;
    4. import javax.servlet.http.HttpSessionBindingListener;
    6. /**
    7.  * Created by LTN on 2016/11/2.
    8.  */
    9. public class Dog implements HttpSessionBindingListener {
    10.     private String breed;
    12.     public Dog(String breed) {
    13.         this.breed = breed;
    14.     }
    16.     public String getBreed() {
    17.         return breed;
    18.     }
    20.     public void valueBound(HttpSessionBindingEvent event) {
    21.         System.out.println("I am a dog, i know i am in a conversation now.");
    22.     }
    24.     public void valueUnbound(HttpSessionBindingEvent event) {
    25.         System.out.println("I am a dog, i know i am leaving a conversation now.");
    26.     }
    27. }
    • 建立servlet测试类EntityBindingTester.java
    1. package com.example.servlet;
    3. import com.example.entities.Dog;
    5. import javax.servlet.ServletException;
    6. import javax.servlet.http.HttpServlet;
    7. import javax.servlet.http.HttpServletRequest;
    8. import javax.servlet.http.HttpServletResponse;
    9. import javax.servlet.http.HttpSession;
    10. import java.io.IOException;
    12. /**
    13.  * Created by LTN on 2016/11/2.
    14.  */
    15. public class EntityBindingTester extends HttpServlet {
    16.     public void doGet(HttpServletRequest request, HttpServletResponse response)throws IOException,ServletException {
    18.         Dog dog = (Dog) getServletContext().getAttribute("dog");
    20.         HttpSession session = request.getSession();
    21.         session.setAttribute("dog-binding",dog);
    22.         session.invalidate();
    23.     }
    25. }
    • 在DD中配置servlet，注意，属性Dog不用/能在DD中配置
    1. <servlet>
    2.   <servlet-name>ObjectBindingTester</servlet-name>
    3.   <servlet-class>com.example.servlet.EntityBindingTester</servlet-class>
    4. </servlet>
    5. <servlet-mapping>
    6.   <servlet-name>ObjectBindingTester</servlet-name>
    7.   <url-pattern>/ObjectBindingTest.do</url-pattern>
    8. </servlet-mapping>
    1. I am a dog, i know i am in a conversation now.
    2. Attribute added: dog-binding: com.example.entities.Dog@6dbba86e
    3. I am a dog, i know i am leaving a conversation now.
    4. Attribute removed: dog-binding: com.example.entities.Dog@6dbba86e

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    • 原文地址：https://www.cnblogs.com/myitroad/p/6192526.html
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