关于把struts2项目修改为开发模式的问题

我在struts.xml中增加了这样一句话：
<constant name="struts.devMode" value="true" />
就是把它改成开发模式，但是我重新部署之后出现一个异常时怎么回事？异常如下：
严重: Exception starting filter struts2
java.lang.NullPointerException
at com.opensymphony.xwork2.util.FileManager$FileRevision.needsReloading(FileManager.java:209)
at com.opensymphony.xwork2.util.FileManager.fileNeedsReloading(FileManager.java:60)
at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.needsReload(XmlConfigurationProvider.java:325)
at org.apache.struts2.config.StrutsXmlConfigurationProvider.needsReload(StrutsXmlConfigurationProvider.java:168)
at com.opensymphony.xwork2.config.ConfigurationManager.conditionalReload(ConfigurationManager.java:220)
at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:61)
at org.apache.struts2.dispatcher.Dispatcher.getContainer(Dispatcher.java:774)
at org.apache.struts2.dispatcher.ng.InitOperations.initStaticContentLoader(InitOperations.java:77)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.init(StrutsPrepareAndExecuteFilter.java:49)
at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:275)
at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:397)
at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:108)
at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:3693)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4342)
at org.apache.catalina.startup.HostConfig.checkResources(HostConfig.java:1105)
at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1203)
at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:293)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:120)
at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1307)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1571)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1580)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1560)
at java.lang.Thread.run(Thread.java:619)
2010-12-19 22:57:29 org.apache.catalina.core.StandardContext start
严重: Error filterStart
2010-12-19 22:57:29 org.apache.catalina.core.StandardContext start
请问一下各位大虾是怎么回事啊？求答案。。