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：