servlet无法访问到是为什么？

刚学JavaWeb使用的IDE是intell J IDEA2018

老是报错：The origin server did not find a current representation for the target resource or is not willing to disclose that one exists.

HelloServlet的代码：

package Servlets;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet(name = "Demo")
public class Demo extends HttpServlet {
    private String message;

    @Override
    public void init() throws ServletException {
        message = "Hello world, this message is from servlet!";
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //设置网页响应类型
        response.setContentType("text/html");
        //实现具体操作
        PrintWriter out = response.getWriter();
        out.println("This is a new servlet page");
    }
}

web.xml的代码：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
   <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>servlets.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/servlets/HelloServlet</url-pattern>
    </servlet-mapping>
</web-app>

可以运行

但是浏览器只显示index.jsp的内容如下：

一直找不到原因，脑阔疼配置文件了，配置部署如下

我试了用浏览器输入地址访问：结果

求大佬赐教。QAQ。。。。。。