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);   
            }