监听器--监听session创建后计数（servletContext）

这个servlet用于预置参数

package web;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;//初始化全局变量public class InitServlet extends HttpServlet {@Overridepublic void init() throws ServletException {ServletContext context = getServletContext();context.setAttribute("count", 0);}}

这个servlet用于创建session

package web;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class TestServlet extends HttpServlet {@Overrideprotected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {//发送请求创建自动request,启动request监听器HttpSession session = req.getSession();//创建session}}

这个servlet用于销毁session

package web;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class LogoutServlet extends HttpServlet {@Overrideprotected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {//销毁sessionHttpSession session = req.getSession();session.invalidate();System.out.println("销毁了session ID为:"+session.getId());}}

监听器如下：监听session

package web;import javax.servlet.ServletContext;import javax.servlet.http.HttpSession;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener;public class CountListener implements HttpSessionListener{private int count;public void sessionCreated(HttpSessionEvent se) {System.out.println("监听创建session");HttpSession session = se.getSession();ServletContext sct = session.getServletContext();count = (Integer)(sct.getAttribute("count"));sct.setAttribute("count", ++count);System.out.println("当前人数:"+count);}public void sessionDestroyed(HttpSessionEvent se) {System.out.println("监听销毁session");HttpSession session = se.getSession();ServletContext sct = session.getServletContext();count = (Integer)sct.getAttribute("count");if(count > 0){sct.setAttribute("count", --count);}System.out.println("当前人数:"+count);}}

配置文件：

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5">  <display-name>jsp6</display-name>  <welcome-file-list>    <welcome-file>index.html</welcome-file>    <welcome-file>index.htm</welcome-file>    <welcome-file>index.jsp</welcome-file>    <welcome-file>default.html</welcome-file>    <welcome-file>default.htm</welcome-file>    <welcome-file>default.jsp</welcome-file>  </welcome-file-list>    <!-- 监听器 -->   <listener>    <listener-class>web.CountListener</listener-class>  </listener>    <!-- 将全局参数在InitServlet中初始化,方便其他调用和修改 -->  <servlet>    <servlet-name>init</servlet-name>    <servlet-class>web.InitServlet</servlet-class>    <load-on-startup>1</load-on-startup>  </servlet>    <servlet>    <servlet-name>test</servlet-name>    <servlet-class>web.TestServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>test</servlet-name>    <url-pattern>/test</url-pattern>  </servlet-mapping>    <servlet>    <servlet-name>logout</servlet-name>    <servlet-class>web.LogoutServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>logout</servlet-name>    <url-pattern>/logout</url-pattern>  </servlet-mapping></web-app>

测试结果：（l两个不同的浏览器访问--创建2个session和销毁2个session）

请求路径：http://localhost:8080/jsp6/test      http://localhost:8080/jsp6/test     http://localhost:8080/jsp6/logout    http://localhost:8080/jsp6/logout

输出：

监听创建session
当前人数:1
监听创建session
当前人数:2
监听销毁session
当前人数:1
销毁了session ID为:703E789A938E1029007DCE9C4C3BB67E
监听销毁session
当前人数:0
销毁了session ID为:AB27D24FB29EE71825E4E44A430708A1

注意事项：servletContext可以用于读取常量（web.xml）  getInitParameter() 方法

若要读取变变量,则声明一个InitServlet,在init()方法中初始化servletContext中的参数

public void init() throws ServletException {
ServletContext context = getServletContext();
context.setAttribute("count", 0);
}

此时其他servlet可以使用setAttribute()和getAttribute()对该变量进行读取可修改读取变量和常量的方式不能混用