tomcat10 帶來的報錯：jakarta.servlet.ServletException: 類com.kang.servlet.HelloServlet不是Servlet

 類com.kuang.servlet.HelloServlet不是Servlet

描述 服務器遇到一個意外的情況，阻止它完成請求。

例外情況

jakarta.servlet.ServletException: 類com.kuang.servlet.HelloServlet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$ConnectionHandler.process(AbstractProtocol.java:872)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1695)
    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.lang.Thread.run(Thread.java:748)
根本原因。

java.lang.ClassCastException: com.kuang.servlet.HelloServlet 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$ConnectionHandler.process(AbstractProtocol.java:872)
    org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1695)
    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.lang.Thread.run(Thread.java:748)

這是因為使用了最新的Tomcat10，而最新的Tomcat10有個最大的變動就是Tomcat 10 m1 將 JavaEE 也換成了Jakarta® EE 

Jakarta® EE  較以前JavaEE有一個非常非常重要的更新：包名不再是“javax.servlet”.而是改成了“jakarta。servlet”

第一種解決方案：將Tomcat 10降級 改成Tomcat 9就可以了

第二種解決方案：繼續使用Tomcat 10

  導入正確依賴

<!-- https://mvnrepository.com/artifact/jakarta.servlet/jakarta.servlet-api -->
<dependency>
    <groupId>jakarta.servlet</groupId>
    <artifactId>jakarta.servlet-api</artifactId>
    <version>5.0.0</version>
    <scope>provided</scope>
</dependency>

<!-- https://mvnrepository.com/artifact/jakarta.servlet.jsp/jakarta.servlet.jsp-api -->
<dependency>
    <groupId>jakarta.servlet.jsp</groupId>
    <artifactId>jakarta.servlet.jsp-api</artifactId>
    <version>3.0.0</version>
    <scope>provided</scope>
</dependency>