完整的Exception：
Exception starting filter struts2
javax.xml.transform.TransformerFactoryConfigurationError: Provider org.apache.xalan.processor.TransformerFactoryImpl not found
at javax.xml.transform.TransformerFactory.newInstance(Unknown Source)
at com.opensymphony.xwork2.util.DomHelper$DOMBuilder.<clinit>(DomHelper.java:167)
at com.opensymphony.xwork2.util.DomHelper.parse(DomHelper.java:115)
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.loadConfigurationFiles(XmlConfigurationProvider.java:786)
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.loadDocuments(XmlConfigurationProvider.java:132)
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.init(XmlConfigurationProvider.java:100)
at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reload(DefaultConfiguration.java:130)
at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:52)
at org.apache.struts2.dispatcher.Dispatcher.init_PreloadConfiguration(Dispatcher.java:398)
at org.apache.struts2.dispatcher.Dispatcher.init(Dispatcher.java:455)
at org.apache.struts2.dispatcher.FilterDispatcher.init(FilterDispatcher.java:201)
at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:225)
at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:308)
at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:79)
at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:3698)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4349)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:823)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:807)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:595)
at org.apache.catalina.core.StandardHostDeployer.install(StandardHostDeployer.java:277)
at org.apache.catalina.core.StandardHost.install(StandardHost.java:832)
at org.apache.catalina.startup.HostConfig.deployDirectories(HostConfig.java:701)
at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:432)
at org.apache.catalina.startup.HostConfig.start(HostConfig.java:983)
at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:349)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1091)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:789)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1083)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:478)
at org.apache.catalina.core.StandardService.start(StandardService.java:480)
at org.apache.catalina.core.StandardServer.start(StandardServer.java:2313)
at org.apache.catalina.startup.Catalina.start(Catalina.java:556)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:287)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:425)
错误原因：
认为是由于jdk1.5 与 tomcat5.0之间的关于 TransformerFactoryImpl 类的冲突造成的。
tomcat-5.0.28\common\endorsed下有两个jar包：xercesImpl.jar和xml-apis.jar，其中的类 javax.xml.transform.TransformerFactory 与jdk1.5中的类org.apache.xalan.processor.TransformerFactoryImpl其实是同一个类。
in tomcat java is called with the following argument:
-Djava.endorsed.dirs="X:\my_app\Portal\tomcat\common\endorsed"
In this directory you find two jar files: xercesImpl.jar and xml-apis.jar needed by tomcat and that must be loaded before all xmsl stuff present in the jdk (1.4 naming problem). And in the file xml-apis.jar the TransformerFactoryImpl is set to "org.apache.xalan.processor.TransformerFactoryImpl".
解决办法：
1. 将xml-apis.jar移出endorsed文件夹。
2. 用xalan系列jar包替换原来的xercesImpl.jar和xml-apis.jar。
xalan系列jar包：serializer.jar、xalan.jar、xercesImpl.jar和xml-apis.jar。
3. For other application, just check this file or dom3-xml-apis.jar in your class path.