请求下面这个关于struts验证产生的错误的帮助

异常信息: 
org.apache.commons.validator.ValidatorException: 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:627) 
at org.apache.commons.validator.ValidatorAction.executeValidationMethod(ValidatorAction.java:557) 
at org.apache.commons.validator.Field.validateForRule(Field.java:811) 
at org.apache.commons.validator.Field.validate(Field.java:890) 
at org.apache.commons.validator.Form.validate(Form.java:174) 
at org.apache.commons.validator.Validator.validate(Validator.java:367) 
at org.apache.struts.validator.ValidatorForm.validate(ValidatorForm.java:112) 
at fenye.StudentActionForm.validate(StudentActionForm.java:89) 
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:1164) 
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:415) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:709) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) 
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252) 
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) 
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213) 
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178) 
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126) 
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105) 
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107) 
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148) 
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869) 
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664) 
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527) 
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80) 
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684) 
at java.lang.Thread.run(Thread.java:595)