建立和使用一个上下文监听者（Head first servlet+Jsp）（实验二）

1.创建一个监听者类

package com.example;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyServletContextListener implements ServletContextListener {

    @Override
public void contextDestroyed(ServletContextEvent event) {

    }

    @Override
public void contextInitialized(ServletContextEvent event) {
        ServletContext sc = event.getServletContext();//由事件得到ServletContext
        String dogBreed = sc.getInitParameter("breed");//用上下文得到初始化参数
        Dog d = new Dog(dogBreed);
        sc.setAttribute("dog", d);
//使用上下文来设置一个属性（名/对象对），这个属性都是Dog，现在应用的其他部分就能得到这个属性（Dog）的值了


    }

}

2.属性类

package com.example;

public class Dog {
private String breed;

public Dog(String breed) {
this.breed = breed;
    }

public String getBreed() {
return breed;
    }

//    public void setBreed(String breed) {
//        this.breed = breed;
//    }
    

}

3.Servlet

package com.example;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ListenerTest
*/
@WebServlet("/ListenerTest")
public class ListenerTest extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
     * @see HttpServlet#HttpServlet()
*/
public ListenerTest() {
super();
// TODO Auto-generated constructor stub
    }

/**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        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");
//现在从ServletContext得到Dog，如果监听者正常，第一次调用服务方法之前Dog就已经放在上下文之中了。
        out.println("dog's breed is:"+dog.getBreed());
    }

/**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
//    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//        // TODO Auto-generated method stub
//        doGet(request, response);
//    }

}

4.编写DD文件

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
<display-name>FirstListener</display-name>
<servlet>
<servlet-name>ListenerTest</servlet-name>
<servlet-class>com.example.ListenerTest</servlet-class>
</servlet>

<servlet-mapping>
<servlet-name>ListenerTest</servlet-name>
<url-pattern>/ListenTest.do</url-pattern>
</servlet-mapping>

<context-param>
<param-name>breed</param-name>
<param-value>Greet Dane</param-value>
</context-param>

<listener>
<listener-class>com.example.MyServletContextListener</listener-class>
</listener>
</web-app>

测试如下：