java.lang.reflect.InvocationTargetException，在线等待！！！

请问各位大侠，这个错误该怎么样改正呀！！在myEclips里，是缺少什么东西么？怎么会出现这个，很急，等待中……java.lang.reflect.InvocationTargetException
at sun.reflect.GeneratedMethodAccessor18.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.tomcat.util.IntrospectionUtils.callMethodN(IntrospectionUtils.java:953)
at org.apache.catalina.startup.CallMethodMultiRule.end(WebRuleSet.java:792)
at org.apache.tomcat.util.digester.Rule.end(Rule.java:229)
at org.apache.tomcat.util.digester.Digester.endElement(Digester.java:1140)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
at org.apache.tomcat.util.digester.Digester.parse(Digester.java:1644)
at org.apache.catalina.startup.ContextConfig.applicationWebConfig(ContextConfig.java:369)
at org.apache.catalina.startup.ContextConfig.start(ContextConfig.java:1062)
at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:261)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4252)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:791)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:771)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:525)
at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:924)
at org.apache.catalina.startup.HostConfig.deployDirectories(HostConfig.java:887)
at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:492)
at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1147)
at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:311)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1053)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:719)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1045)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:443)
at org.apache.catalina.core.StandardService.start(StandardService.java:516)
at org.apache.catalina.core.StandardServer.start(StandardServer.java:710)
at org.apache.catalina.startup.Catalina.start(Catalina.java:578)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)
Caused by: java.lang.IllegalArgumentException: Servlet mapping specifies an unknown servlet name /UserServlet
at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:2393)
at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:2373)
... 43 more