struts.xml中为什么加上<constant name="struts.devMode" value="true" />就出错

严重: Exception starting filter struts2  
java.lang.NullPointerException  
    at com.opensymphony.xwork2.util.FileManager$FileRevision.needsReloading(FileManager.java:209)  
    at com.opensymphony.xwork2.util.FileManager.fileNeedsReloading(FileManager.java:60)  
    at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.needsReload(XmlConfigurationProvider.java:325)  
    at org.apache.struts2.config.StrutsXmlConfigurationProvider.needsReload(StrutsXmlConfigurationProvider.java:168)  
    at com.opensymphony.xwork2.config.ConfigurationManager.conditionalReload(ConfigurationManager.java:220)  
    at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:61)  
    at org.apache.struts2.dispatcher.Dispatcher.getContainer(Dispatcher.java:774)  
    at org.apache.struts2.dispatcher.ng.InitOperations.initStaticContentLoader(InitOperations.java:77)  
    at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.init(StrutsPrepareAndExecuteFilter.java:49)  
    at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:275)  
    at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:397)  
    at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:108)  
    at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:3800)  
    at org.apache.catalina.core.StandardContext.start(StandardContext.java:4450)  
    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:526)  
    at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:987)  
    at org.apache.catalina.startup.HostConfig.deployDirectories(HostConfig.java:909)  
    at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:495)  
    at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1206)  
    at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:314)  
    at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)  
    at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1053)  
    at org.apache.catalina.core.StandardHost.start(StandardHost.java:722)  
    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:583)  
    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:597)  
    at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)  
    at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)  
2010-3-1 22:26:44 org.apache.catalina.core.StandardContext start  
严重: Error filterStart