在第一次写Servlet程序时，在Eclipse中启动Tomcat后，通过浏览器访问页面报如下错误：

HTTP Status 500 - Error instantiating servlet class HelloWorld

type Exception report

messageError instantiating servlet class HelloWorld

descriptionThe server encountered an internal error that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Error instantiating servlet class HelloWorldorg.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526)org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655)org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1566)org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1523)java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)java.lang.Thread.run(Thread.java:745)

root cause

java.lang.ClassNotFoundException: HelloWorldorg.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1324)org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1177)org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:526)org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:655)org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1566)org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1523)java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)java.lang.Thread.run(Thread.java:745)

noteThe full stack trace of the root cause is available in the Apache Tomcat/8.0.9 logs.

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Apache Tomcat/8.0.9