struts2下载出问题

if (inputStream == null) {                   // Find the inputstream from the invocation variable stack                   inputStream = (InputStream) invocation.getStack().findValue(conditionalParse(inputName, invocation));               }                 if (inputStream == null) {                   String msg = ("Can not find a java.io.InputStream with the name [" + inputName + "] in the invocation stack. " +                       "Check the <param name=\"inputName\"> tag specified for this action.");                   LOG.error(msg);                   throw new IllegalArgumentException(msg);               }