相关代码：

package com.example.web;

import javax.servlet.*;
import java.io.IOException;

public class DemoServlet implements Servlet {
    @Override
    public void init(ServletConfig servletConfig) throws ServletException {

    }
    @Override
    public ServletConfig getServletConfig() {
        return null;
    }
    // 在这里处理请求
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("处理请求！");
    }
    @Override
    public String getServletInfo() {
        return null;
    }
    @Override
    public void destroy() {

    }
}

<?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>DemoServlet</servlet-name>
        <servlet-class>com.example.web.DemoServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <!-- 这个时候访问地址为http://port/工程路径/hello -->
        <servlet-name>DemoServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
</web-app>

访问地址：http://localhost:8080/web_war/hello出现下面报错
错误信息：

jakarta.servlet.ServletException: 类com.example.web.DemoServlet不是Servlet
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:353)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
org.apache.coyote.AbstractProtocol C o n n e c t i o n H a n d l e r . p r o c e s s ( A b s t r a c t P r o t o c o l . j a v a : 872 ) o r g . a p a c h e . t o m c a t . u t i l . n e t . N i o E n d p o i n t ConnectionHandler.process(AbstractProtocol.java:872) org.apache.tomcat.util.net.NioEndpoint ConnectionHandler.process(AbstractProtocol.java:872)org.apache.tomcat.util.net.NioEndpointSocketProcessor.doRun(NioEndpoint.java:1705)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
org.apache.tomcat.util.threads.ThreadPoolExecutor W o r k e r . r u n ( T h r e a d P o o l E x e c u t o r . j a v a : 659 ) o r g . a p a c h e . t o m c a t . u t i l . t h r e a d s . T a s k T h r e a d Worker.run(ThreadPoolExecutor.java:659) org.apache.tomcat.util.threads.TaskThread Worker.run(ThreadPoolExecutor.java:659)org.apache.tomcat.util.threads.TaskThreadWrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Thread.java:748)
java.lang.ClassCastException: com.example.web.DemoServlet cannot be cast to jakarta.servlet.Servlet
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:353)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
org.apache.coyote.AbstractProtocol C o n n e c t i o n H a n d l e r . p r o c e s s ( A b s t r a c t P r o t o c o l . j a v a : 872 ) o r g . a p a c h e . t o m c a t . u t i l . n e t . N i o E n d p o i n t ConnectionHandler.process(AbstractProtocol.java:872) org.apache.tomcat.util.net.NioEndpoint ConnectionHandler.process(AbstractProtocol.java:872)org.apache.tomcat.util.net.NioEndpointSocketProcessor.doRun(NioEndpoint.java:1705)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
org.apache.tomcat.util.threads.ThreadPoolExecutor W o r k e r . r u n ( T h r e a d P o o l E x e c u t o r . j a v a : 659 ) o r g . a p a c h e . t o m c a t . u t i l . t h r e a d s . T a s k T h r e a d Worker.run(ThreadPoolExecutor.java:659) org.apache.tomcat.util.threads.TaskThread Worker.run(ThreadPoolExecutor.java:659)org.apache.tomcat.util.threads.TaskThreadWrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Thread.java:748)

报错原因
Spring (and Spring Boot) currently only support JavaEE and not JakartaEE. Which means that it will only run on/with JavaEE compatible servers. Tomcat10 is an implementation of the JakartaEE specification which currently isn’t supported.The only solution is to downgrade to Tomcat 9.
解决方案：
我用的是Tomcat10切换到版本9或以下, 我换成了Tomcat8就能成功了。