为什么我在STRUTS中配置validator时总是出现如下错误，我的XML文件配置没问题？？？

2006/04/11 13:58:39 org.apache.struts.validator.DynaValidatorForm validate
致命的: No such validation method: org.apache.struts.validator.FieldChecks.validateRequired(java.lang.Object, org.apache.commons.validator.ValidatorAction, org.apache.commons.validator.Field, org.apache.struts.action.ActionErrors, javax.servlet.http.HttpServletRequest)
org.apache.commons.validator.ValidatorException: No such validation method: org.apache.struts.validator.FieldChecks.validateRequired(java.lang.Object, org.apache.commons.validator.ValidatorAction, org.apache.commons.validator.Field, org.apache.struts.action.ActionErrors, javax.servlet.http.HttpServletRequest)
at org.apache.commons.validator.ValidatorAction.loadValidationMethod(ValidatorAction.java:603)
at org.apache.commons.validator.ValidatorAction.executeValidationMethod(ValidatorAction.java:533)
at org.apache.commons.validator.Field.validateForRule(Field.java:766)
at org.apache.commons.validator.Field.validate(Field.java:846)
at org.apache.commons.validator.Form.validate(Form.java:290)
at org.apache.commons.validator.Validator.validate(Validator.java:354)
at org.apache.struts.validator.DynaValidatorForm.validate(DynaValidatorForm.java:112)
at org.apache.struts.action.RequestProcessor.processValidate(RequestProcessor.java:921)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:206)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1158)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:397)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:237)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:157)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)
at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at org.apache.catalina.core.StandardContextValve.invokeInternal(StandardContextValve.java:198)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:152)
at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:137)
at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:118)
at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:102)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929)
at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
at java.lang.Thread.run(Thread.java:534)