strust2 form标签加上validate="true"后出错，求问问题原因

struts2.3.16.3如下

运行出错，提示信息

Method public java.util.List org.apache.struts2.components.Form.getValidators(java.lang.String) threw an exception when invoked on org.apache.struts2.components.Form@1f269fc
The problematic instruction:
----------
==> list tag.getValidators("${tagName}") as aValidator [on line 66, column 9 in template/~~~xhtml/form-close-validate.ftl]
 in include "/${parameters.templateDir}/${parameters.expandTheme}/form-close-validate.ftl" [on line 25, column 1 in template/xhtml/form-close.ftl]
----------

Java backtrace for programmers:
----------
freemarker.template.TemplateModelException: Method public java.util.List org.apache.struts2.components.Form.getValidators(java.lang.String) threw an exception when invoked on org.apache.struts2.components.Form@1f269fc
at freemarker.ext.beans.SimpleMethodModel.exec(SimpleMethodModel.java:130)
at freemarker.core.MethodCall._getAsTemplateModel(MethodCall.java:93)
at freemarker.core.Expression.getAsTemplateModel(Expression.java:89)
at freemarker.core.IteratorBlock.accept(IteratorBlock.java:94)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.IteratorBlock$Context.runLoop(IteratorBlock.java:167)
at freemarker.core.Environment.visit(Environment.java:428)
at freemarker.core.IteratorBlock.accept(IteratorBlock.java:102)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.MixedContent.accept(MixedContent.java:92)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.ConditionalBlock.accept(ConditionalBlock.java:79)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.MixedContent.accept(MixedContent.java:92)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.Environment.include(Environment.java:1508)
at freemarker.core.Include.accept(Include.java:169)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.MixedContent.accept(MixedContent.java:92)
at freemarker.core.Environment.visit(Environment.java:221)
at freemarker.core.Environment.process(Environment.java:199)
at freemarker.template.Template.process(Template.java:259)
at org.apache.struts2.components.template.FreemarkerTemplateEngine.renderTemplate(FreemarkerTemplateEngine.java:158)
at org.apache.struts2.components.UIBean.mergeTemplate(UIBean.java:593)
at org.apache.struts2.components.UIBean.end(UIBean.java:547)
at org.apache.struts2.views.jsp.ComponentTagSupport.doEndTag(ComponentTagSupport.java:42)
at org.apache.jsp.WEB_002dINF.WebContent.login_jsp._jspx_meth_s_005fform_005f0(login_jsp.java:186)
at org.apache.jsp.WEB_002dINF.WebContent.login_jsp._jspService(login_jsp.java:104)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:315)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.NullPointerException
at com.opensymphony.xwork2.validator.AnnotationActionValidatorManager.buildValidatorKey(AnnotationActionValidatorManager.java:237)
at com.opensymphony.xwork2.validator.AnnotationActionValidatorManager.getValidators(AnnotationActionValidatorManager.java:94)
at com.opensymphony.xwork2.validator.AnnotationActionValidatorManager.getValidators(AnnotationActionValidatorManager.java:90)
at org.apache.struts2.components.Form.getValidators(Form.java:280)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at freemarker.ext.beans.BeansWrapper.invokeMethod(BeansWrapper.java:866)
at freemarker.ext.beans.SimpleMethodModel.exec(SimpleMethodModel.java:106)
... 55 more

网上搜到的两个解决方法

 方法1、form标签去掉validate="true"(改用服务端验证)

 方法2、action="login"改为action="login.action"，即login加上.action后缀

求问why???

特别是方法2，struts框架并不要求aciton属性必需以.action后缀结尾，而在这里加上.action后缀后就不出错，是struts2的bug吗？？