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

package com.example.entities;
/**
 * Created by LTN on 2016/11/2.
 */
public class Dog {
    private String breed;
    public Dog(String breed) {
        this.breed = breed;
    }
    public String getBreed() {
        return breed;
    }
}

• 建立一个servlet上下文监听器DogServletContextListener.java

package com.example.listener;
import com.example.entities.Dog;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
/**
 * Created by LTN on 2016/11/2.
 */
public class DogServletContextListener implements javax.servlet.ServletContextListener {
    public void contextInitialized(ServletContextEvent event) {
        ServletContext sc = event.getServletContext();
        String dogBreed = sc.getInitParameter("breed");
        Dog d = new Dog(dogBreed);
        sc.setAttribute("dog", d);
    }
    public void contextDestroyed(ServletContextEvent event) {
        //do nothing.
    }
}

• 建立Servlet进行测试ContextListenerTester.java

package com.example.servlet;
import com.example.entities.Dog;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
/**
 * Created by LTN on 2016/11/2.
 */
public class ContextListenerTester extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("test context attributes set by listener<br>");
        out.println("<br>");
        Dog dog = (Dog) getServletContext().getAttribute("dog");
        out.println("Dog‘s breed is: " + dog.getBreed());
    }
}

• 在DD（web.xml）中配置上下文参数、监听器和servlet映射关系

<web-app>
  <display-name>Archetype Created Web Application</display-name>
  <context-param>
    <param-name>breed</param-name>
    <param-value>Great Dane</param-value>
  </context-param>
  <servlet>
    <servlet-name>ListenerTester</servlet-name>
    <servlet-class>com.example.servlet.ContextListenerTester</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ListenerTester</servlet-name>
    <url-pattern>/ListenTest.do</url-pattern>
  </servlet-mapping>
  <listener>
    <listener-class>com.example.listener.DogServletContextListener</listener-class>
  </listener>
</web-app>

• 效果
  启动项目，在浏览器输入http://localhost:8080/servlet-listener/ListenTest.do
  显示：

test context attributes set by listener
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

package com.example.listener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
/**
 * Created by LTN on 2016/11/2.
 */
public class DogSessionAttributeListener implements HttpSessionAttributeListener {
    public void attributeAdded(HttpSessionBindingEvent event) {
        String name = event.getName();
        Object value = event.getValue();
        System.out.println("Attribute added: " + name + ": " + value);
    }
    public void attributeRemoved(HttpSessionBindingEvent event) {
        String name = event.getName();
        Object value = event.getValue();
        System.out.println("Attribute removed: " + name + ": " + value);
    }
    public void attributeReplaced(HttpSessionBindingEvent event) {
        String name = event.getName();
        Object value = event.getValue();
        System.out.println("Attribute replaced: " + name + ": " + value);
    }
}

• 建立一个Servlet测试类AttributeListenerTester.java

package com.example.servlet;
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 java.io.IOException;
/**
 * Created by LTN on 2016/11/2.
 */
public class AttributeListenerTester extends HttpServlet {
    private static int labelCounter = 0;
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        HttpSession session = request.getSession();
        session.setAttribute("label", "label" + labelCounter++);
    }
}

• 在DD中配置servlet和listener

<servlet>
  <servlet-name>AttributeListenTester</servlet-name>
  <servlet-class>com.example.servlet.AttributeListenerTester</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>AttributeListenTester</servlet-name>
  <url-pattern>/AttributeListenerTest.do</url-pattern>
</servlet-mapping>
<listener>
  <listener-class>com.example.listener.DogSessionAttributeListener</listener-class>
</listener>

• 测试效果
  启动tomcat，在浏览器中输入URL：http://localhost:8080/servlet-listener/AttributeListenerTest.do
  第一次回车后，后台打印

Attribute added: label: label0

然后连续三次刷新，后台依次打印：

Attribute replaced: label: label0
Attribute replaced: label: label1
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

package com.example.listener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
/**
 * Created by LTN on 2016/11/2.
 */
public class GlobalCounterListener implements HttpSessionListener {
    static private int activeSessions;
    public static int getActiveSessions() {
        return activeSessions;
    }
    public void sessionCreated(HttpSessionEvent event) {
        activeSessions++;
    }
    public void sessionDestroyed(HttpSessionEvent event) {
        activeSessions--;
    }
}

• 建立servlet测试类GlobalCounterTester.java

package com.example.servlet;
import com.example.listener.GlobalCounterListener;
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 java.io.IOException;
import java.io.PrintWriter;
/**
 * Created by LTN on 2016/11/2.
 */
public class GlobalCounterTester extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        PrintWriter out = response.getWriter();
        HttpSession session = request.getSession();
        session.setAttribute("name","counter");
        String name=(String)session.getAttribute("name");
        int activeSessions= GlobalCounterListener.getActiveSessions();
        out.println("name: "+name);
        out.println("activeSessions: "+activeSessions);
    }
}

• 在DD中配置servlet和listener

<servlet>
  <servlet-name>SessionCounterTester</servlet-name>
  <servlet-class>com.example.servlet.GlobalCounterTester</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>SessionCounterTester</servlet-name>
  <url-pattern>/CounterTest.do</url-pattern>
</servlet-mapping>
<listener>
  <listener-class>com.example.listener.GlobalCounterListener</listener-class>
</listener>

• 测试效果
  在浏览器输入URL：http://localhost:8080/servlet-listener/CounterTest.do
  页面显示：

name: counter
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。

package com.example.entities;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
/**
 * Created by LTN on 2016/11/2.
 */
public class Dog implements HttpSessionBindingListener {
    private String breed;
    public Dog(String breed) {
        this.breed = breed;
    }
    public String getBreed() {
        return breed;
    }
    public void valueBound(HttpSessionBindingEvent event) {
        System.out.println("I am a dog, i know i am in a conversation now.");
    }
    public void valueUnbound(HttpSessionBindingEvent event) {
        System.out.println("I am a dog, i know i am leaving a conversation now.");
    }
}

• 建立servlet测试类EntityBindingTester.java

package com.example.servlet;
import com.example.entities.Dog;
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 java.io.IOException;
/**
 * Created by LTN on 2016/11/2.
 */
public class EntityBindingTester extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws IOException,ServletException {
        Dog dog = (Dog) getServletContext().getAttribute("dog");
        HttpSession session = request.getSession();
        session.setAttribute("dog-binding",dog);
        session.invalidate();
    }
}

• 在DD中配置servlet，注意，属性Dog不用/能在DD中配置

<servlet>
  <servlet-name>ObjectBindingTester</servlet-name>
  <servlet-class>com.example.servlet.EntityBindingTester</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>ObjectBindingTester</servlet-name>
  <url-pattern>/ObjectBindingTest.do</url-pattern>
</servlet-mapping>

• 效果测试
  启动tomcat，输入URL：http://localhost:8080/servlet-listener/ObjectBindingTest.do
  后台打印：

I am a dog, i know i am in a conversation now.
Attribute added: dog-binding: com.example.entities.Dog@6dbba86e
I am a dog, i know i am leaving a conversation now.
Attribute removed: dog-binding: com.example.entities.Dog@6dbba86e

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