我在JSP中用了struts标签，出现如下状况：
严重: Servlet.service() for servlet jsp threw exception
javax.servlet.jsp.JspException: No selector attribute (cookie/header/name/parameter) was specified
at org.apache.struts.taglib.logic.PresentTag.condition(PresentTag.java:163)
at org.apache.struts.taglib.logic.NotPresentTag.condition(NotPresentTag.java:93)
at org.apache.struts.taglib.logic.ConditionalTagBase.doStartTag(ConditionalTagBase.java:218)
at org.apache.jsp.Welcome_jsp._jspx_meth_logic_notPresent_0(org.apache.jsp.Welcome_jsp:203)
at org.apache.jsp.Welcome_jsp._jspService(org.apache.jsp.Welcome_jsp:97)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:322)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
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:856)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:744)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
------解决方法--------------------------------------------------------
用logic标签测试一下list对象是否存在
List list = ?
