1-4这章里即使导入老师的整个文件夹，运行后，当选择超链接，或单击按钮，都会报下面的错误。。。网上找了半天，说可能是web.xml有问题，但不知道到底是哪里错了

严重: Allocate exception for servlet HelloServlet

java.lang.ClassNotFoundException: servlet.HelloServlet

at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1285)

at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1119)

at org.apache.catalina.core.DefaultInstanceManager.loadClass(DefaultInstanceManager.java:520)

at org.apache.catalina.core.DefaultInstanceManager.loadClassMaybePrivileged(DefaultInstanceManager.java:501)

at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:118)

at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1050)

at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:779)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:133)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)

at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:474)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)

at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)

at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:783)

at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)

at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:789)

at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1437)

at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

at java.lang.Thread.run(Thread.java:745)