为什么网页刷新时，都会执行一次 Request变更属性，再执行一次Request新增属性

为什么网页刷新时，都会执行一次 Request变更属性，再执行一次Request新增属性（控制台输出的第2行，第6行）

控制台输出如下：

HttpServletRequest8已被创建，URI:/listener-interface/HelloServlet

Request变更属性：org.apache.catalina.ASYNC_SUPPORTED:true           

ServletContext变更属性：sc-attr1:sc-attr1-value

ServletContext变更属性：sc-attr2:sc-attr2-value2

HttpSession变更属性：session-attr1:session-attr1-value

Request新增属性：request-attr1:request-attr-value

HttpServletRequest8已被销毁

代码如下：

package com.imooc.listener;

import java.io.IOException;

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 HelloServlet

 */

@WebServlet("/HelloServlet")

public class HelloServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

    /**

     * @see HttpServlet#HttpServlet()

     */

    public HelloServlet() {

        super();

        // TODO Auto-generated constructor stub

    }

/**

* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

*/

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

// TODO Auto-generated method stub

response.getWriter().println("hello world");

request.getServletContext().setAttribute("sc-attr1", "sc-attr1-value");

request.getServletContext().setAttribute("sc-attr2", "sc-attr2-value2");

request.getSession().setAttribute("session-attr1", "session-attr1-value");

request.setAttribute("request-attr1", "request-attr-value");

}

/**

* @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);

}

}

package com.imooc.listener;

import javax.servlet.ServletContextAttributeEvent;

import javax.servlet.ServletContextAttributeListener;

import javax.servlet.ServletRequestAttributeEvent;

import javax.servlet.ServletRequestAttributeListener;

import javax.servlet.http.HttpSessionAttributeListener;

import javax.servlet.http.HttpSessionBindingEvent;

public class WebAttributeListener

implements ServletContextAttributeListener, HttpSessionAttributeListener, ServletRequestAttributeListener {

@Override

public void attributeAdded(ServletRequestAttributeEvent arg0) {

// TODO Auto-generated method stub

System.out.println("Request新增属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeRemoved(ServletRequestAttributeEvent arg0) {

// TODO Auto-generated method stub

System.out.println("Request移除属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeReplaced(ServletRequestAttributeEvent arg0) {

// TODO Auto-generated method stub

System.out.println("Request变更属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeAdded(HttpSessionBindingEvent arg0) {

// TODO Auto-generated method stub

System.out.println("HttpSession新增属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeRemoved(HttpSessionBindingEvent arg0) {

// TODO Auto-generated method stub

System.out.println("HttpSession移除属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeReplaced(HttpSessionBindingEvent arg0) {

// TODO Auto-generated method stub

System.out.println("HttpSession变更属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeAdded(ServletContextAttributeEvent arg0) {

// TODO Auto-generated method stub

System.out.println("ServletContext新增属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeRemoved(ServletContextAttributeEvent arg0) {

// TODO Auto-generated method stub

System.out.println("ServletContext移除属性："+arg0.getName()+":"+arg0.getValue());

}

@Override

public void attributeReplaced(ServletContextAttributeEvent arg0) {

// TODO Auto-generated method stub

System.out.println("ServletContext变更属性："+arg0.getName()+":"+arg0.getValue());

}

}