在写一个struts2的小例子时出错了，大家帮忙看下
2011-12-31 10:49:20 org.apache.catalina.core.AprLifecycleListener init
信息: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:\zxb\Common\binary\com.sun.java.jdk.win32.x86_1.6.0.013\bin;D:\zxb\apache-tomcat-6.0.29\bin
2011-12-31 10:49:21 org.apache.coyote.http11.Http11Protocol init
信息: Initializing Coyote HTTP/1.1 on http-8080
2011-12-31 10:49:21 org.apache.catalina.startup.Catalina load
信息: Initialization processed in 1072 ms
2011-12-31 10:49:21 org.apache.catalina.core.StandardService start
信息: Starting service Catalina
2011-12-31 10:49:21 org.apache.catalina.core.StandardEngine start
信息: Starting Servlet Engine: Apache Tomcat/6.0.29
2011-12-31 10:49:22 org.apache.catalina.startup.HostConfig deployDescriptor
信息: Deploying configuration descriptor host-manager.xml
2011-12-31 10:49:22 org.apache.catalina.startup.HostConfig deployDescriptor
信息: Deploying configuration descriptor manager.xml
2011-12-31 10:49:22 org.apache.catalina.startup.HostConfig deployDirectory
信息: Deploying web application directory docs
2011-12-31 10:49:22 org.apache.catalina.startup.HostConfig deployDirectory
信息: Deploying web application directory examples
2011-12-31 10:49:23 org.apache.catalina.core.ApplicationContext log
信息: ContextListener: contextInitialized()
2011-12-31 10:49:23 org.apache.catalina.core.ApplicationContext log
信息: SessionListener: contextInitialized()
2011-12-31 10:49:23 org.apache.catalina.startup.HostConfig deployDirectory
信息: Deploying web application directory headerServlet
2011-12-31 10:49:23 org.apache.catalina.startup.HostConfig deployDirectory
信息: Deploying web application directory ROOT
2011-12-31 10:49:23 org.apache.catalina.startup.HostConfig deployDirectory
信息: Deploying web application directory strutsdemo
2011-12-31 10:49:24 com.opensymphony.xwork2.util.logging.commons.CommonsLogger info
信息: Parsing configuration file [struts-default.xml]
2011-12-31 10:49:24 org.apache.catalina.core.StandardContext filterStart
严重: Exception starting filter struts2
java.lang.NoClassDefFoundError: org/apache/commons/lang/StringUtils
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.register(XmlConfigurationProvider.java:201)
at org.apache.struts2.config.StrutsXmlConfigurationProvider.register(StrutsXmlConfigurationProvider.java:102)
at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reloadContainer(DefaultConfiguration.java:180)
at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:66)
at org.apache.struts2.dispatcher.Dispatcher.init_PreloadConfiguration(Dispatcher.java:390)
at org.apache.struts2.dispatcher.Dispatcher.init(Dispatcher.java:436)
at org.apache.struts2.dispatcher.FilterDispatcher.init(FilterDispatcher.java:195)
at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:295)
at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:422)
at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:115)
at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4001)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4651)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:791)
at org.apache.catalina.core.ContainerBase.addChil