Tomcat 10报错javax.servlet.http.HttpServlet类未找到异常

实例化Servlet时触发500状态错误，查阅现有同类问题的解决方案均无法修复。
访问http://localhost:8080/SampServlet时返回错误：实例化Servlet类[com.example.SampServlet]失败，根因日志提示java.lang.ClassNotFoundException: javax.servlet.http.HttpServlet。

相关代码与配置 #

SampServlet类代码 #

package com.example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet(name = "SampServlet", value = "/SampServlet")
public class SampServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 向浏览器返回HTML内容
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        try {
            out.println("<h3>Testing Servlets</h3>");
        }
        finally {
            out.close();
        }
    }
}

web.xml配置 #

<?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>SampServlet</servlet-name>
        <servlet-class>com.example.SampServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>SampServlet</servlet-name>
        <url-pattern>/SampServlet</url-pattern>
    </servlet-mapping>
</web-app>

项目结构 #

项目结构截图

完整错误日志 #

Message Error instantiating servlet class [com.example.SampServlet]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

jakarta.servlet.ServletException: Error instantiating servlet class [com.example.SampServlet]
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
    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:356)
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399)
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:867)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1716)
    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$Worker.run(ThreadPoolExecutor.java:659)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.base/java.lang.Thread.run(Thread.java:833)
Root Cause

java.lang.NoClassDefFoundError: javax/servlet/http/HttpServlet
    java.base/java.lang.ClassLoader.defineClass1(Native Method)
    java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1013)
    java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
    org.apache.catalina.loader.WebappClassLoaderBase.findClassInternal(WebappClassLoaderBase.java:2527)
    org.apache.catalina.loader.WebappClassLoaderBase.findClass(WebappClassLoaderBase.java:877)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1413)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1257)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
    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:356)
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399)
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:867)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1716)
    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$Worker.run(ThreadPoolExecutor.java:659)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.base/java.lang.Thread.run(Thread.java:833)
Root Cause

java.lang.ClassNotFoundException: javax.servlet.http.HttpServlet
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1449)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1257)
    java.base/java.lang.ClassLoader.defineClass1(Native Method)
    java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1013)
    java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
    org.apache.catalina.loader.WebappClassLoaderBase.findClassInternal(WebappClassLoaderBase.java:2527)
    org.apache.catalina.loader.WebappClassLoaderBase.findClass(WebappClassLoaderBase.java:877)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1413)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1257)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
    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:356)
    org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399)
    org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:867)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1716)
    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$Worker.run(ThreadPoolExecutor.java:659)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.base/java.lang.Thread.run(Thread.java:833)
Note The full stack trace of the root cause is available in the server logs.

核心问题是Servlet API包命名空间和当前使用的Tomcat版本不匹配：
错误栈开头就出现了jakarta.servlet.ServletException，说明运行环境是Tomcat 10及以上版本。从Tomcat 10开始，Java EE正式迁移为Jakarta EE，所有Servlet相关类的包名从javax.servlet.*全部改为jakarta.servlet.*，服务器内置的Servlet API只有jakarta开头的类。
代码中导入的全是javax.servlet.*包下的类，服务器加载SampServlet时找不到旧命名空间下的HttpServlet类，直接抛出类找不到异常，导致Servlet实例化失败。
另外代码已经通过@WebServlet注解配置了Servlet映射，web.xml中又重复配置了同路径的映射，虽然不是触发本次500错误的原因，但存在映射冲突隐患。

二选一即可修复：

• 方案1（适配当前Tomcat版本，推荐）：将代码中所有javax.servlet开头的导入语句全部替换为jakarta.servlet，修改后的导入部分如下：

import jakarta.servlet.*;
import jakarta.servlet.http.*;
import jakarta.servlet.annotation.*;

修改后可以将web.xml的命名空间和版本替换为Jakarta EE对应版本，也可以直接删掉web.xml中重复的Servlet配置，仅保留注解配置即可。

• 方案2（保留现有代码写法）：将Tomcat降级到9.x及以下版本，Tomcat 9是最后一个使用javax.servlet命名空间的版本，和现有代码、web.xml配置完全兼容。

注意：如果使用Maven/Gradle管理项目依赖，要将Servlet API依赖的scope设置为provided，避免依赖包被打进项目部署包和Tomcat内置的Servlet包产生冲突。Tomcat 10及以上版本引入jakarta.servlet-api依赖，Tomcat 9及以下版本引入javax.servlet-api依赖，依赖版本和服务器版本对应即可。

内容的提问来源于stack exchange，提问作者user16482818