eclipse开发web中遇到的有关问题

求助 eclipse开发web中遇到的问题
大家知道这个错误的原因吗?谢啦严重:   End   event   threw   exception
java.lang.reflect.InvocationTargetException
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.commons.beanutils.MethodUtils.invokeMethod(MethodUtils.java:252)
at   org.apache.commons.digester.SetNextRule.end(SetNextRule.java:256)
at   org.apache.commons.digester.Rule.end(Rule.java:276)
at   org.apache.commons.digester.Digester.endElement(Digester.java:1058)
at   org.apache.catalina.util.CatalinaDigester.endElement(CatalinaDigester.java:123)
at   org.apache.xerces.parsers.AbstractSAXParser.endElement(Unknown   Source)
at   org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown   Source)
at   org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown   Source)
at   org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown   Source)
at   org.apache.xerces.parsers.XML11Configuration.parse(Unknown   Source)
at   org.apache.xerces.parsers.DTDConfiguration.parse(Unknown   Source)
at   org.apache.xerces.parsers.XMLParser.parse(Unknown   Source)
at   org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown   Source)
at   org.apache.commons.digester.Digester.parse(Digester.java:1567)
at   org.apache.catalina.core.StandardHostDeployer.install(StandardHostDeployer.java:512)
at   org.apache.catalina.core.StandardHost.install(StandardHost.java:890)
at   org.apache.catalina.startup.HostConfig.deployDescriptors(HostConfig.java:523)
at   org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:468)
at   org.apache.catalina.startup.HostConfig.start(HostConfig.java:1002)
at   org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:393)
at   org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:166)
at   org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1133)
at   org.apache.catalina.core.StandardHost.start(StandardHost.java:816)
at   org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1125)
at   org.apache.catalina.core.StandardEngine.start(StandardEngine.java:518)
at   org.apache.catalina.core.StandardService.start(StandardService.java:519)
at   org.apache.catalina.core.StandardServer.start(StandardServer.java:2343)
at   org.apache.catalina.startup.Catalina.start(Catalina.java:581)
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:297)
at   org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:398)
Caused   by:   java.lang.NoClassDefFoundError:   javax/servlet/http/HttpServlet
at   java.lang.ClassLoader.defineClass1(Native   Method)
at   java.lang.ClassLoader.defineClass(Unknown   Source)
at   java.security.SecureClassLoader.defineClass(Unknown   Source)
at   java.net.URLClassLoader.defineClass(Unknown   Source)
at   java.net.URLClassLoader.access$100(Unknown   Source)
at   java.net.URLClassLoader$1.run(Unknown   Source)