为什么点击登录报404错误。。。。

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>

   <groupId>org.example</groupId>
   <artifactId>springMVC</artifactId>
   <version>1.0-SNAPSHOT</version>


   <repositories>
       <repository>
           <id>aliyun</id>
           <name>aliyun</name>
           <url>https://maven.aliyun.com/repository/public</url>
       </repository>
   </repositories>
   <dependencies>
       <dependency>
           <groupId>org.springframework</groupId>
           <artifactId>spring-webmvc</artifactId>
           <version>5.1.9.RELEASE</version>
       </dependency>
   </dependencies>
</project>

<?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_3_1.xsd"
        version="3.1">

   <servlet>
       <servlet-name>springmvc</servlet-name>
       <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
       <init-param>
           <param-name>contextConfigLocation</param-name>
           <param-value>classpath:applicationContext.xml</param-value>
       </init-param>
       <load-on-startup>0</load-on-startup>
   </servlet>
   <servlet-mapping>
       <servlet-name>springmvc</servlet-name>
   <url-pattern>/</url-pattern>
   </servlet-mapping>

</web-app>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:mvc="http://www.springframework.org/schema/mvc"
      xmlns:context="http://www.springframework.org/schema/context"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:mv="http://www.springframework.org/schema/mvc"
      xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context.xsd
           http://www.springframework.org/schema/mvc
           http://www.springframework.org/schema/mvc/spring-mvc.xsd">
   <!--
   context:component-scan 标签作用
   在Spring IOC初始化过程中,自动创建并管理com.imooc.springmvc及子包中
   拥有以下注解的对象.
   @Repository
   @Service
   @Controller
   @Component
   -->
   <context:component-scan base-package="com.imooc.mvc"></context:component-scan>
   <!--启用Spring MVC的注解开发模式-->
   <mvc:annotation-driven conversion-service="conversionService">
       <mvc:message-converters>
           <bean class="org.springframework.http.converter.StringHttpMessageConverter">
               <property name="supportedMediaTypes">
                   <list>
                       <!-- response.setContentType("text/html;charset=utf-8") -->
                       <value>text/plain;charset=utf-8</value>
                       <value>text/html;charset=utf-8</value>
                   </list>
               </property>
           </bean>
       </mvc:message-converters>
   </mvc:annotation-driven>
   <!-- 将图片/JS/CSS等静态资源排除在外,可提高执行效率 -->
   <mvc:default-servlet-handler/>
</beans>

package com.imooc.mvc.controller;

import com.imooc.mvc.entity.User;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
@RequestMapping("/m")
public class URLMappingController {


   @PostMapping("/p")
   @ResponseBody
   public String login(User user){
       return "<fieldset><legend>登陆成功</legend>用户名：\" +user.getUsername()+\n" +
               "\n" +
               "                \"<br>密码：\"+user.getPassword()+\"</fieldset>";
   }

}

package com.imooc.mvc.entity;

public class User {
   private String username;
   private String password;

   public String getUsername() {
       return username;
   }

   public void setUsername(String username) {
       this.username = username;
   }

   public String getPassword() {
       return password;
   }

   public void setPassword(String password) {
       this.password = password;
   }
}

<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="UTF-8">
   <title>Title</title>
</head>
<body>
<form action="/m/p" method="post">
用户名：<input type="text" id="username" name="username">
密码：<input type="text" id="password" name="password">
<input type="submit" value="登录">
</form>
</body>
</html>