Struts2 下载成功，但是后台打印有异常

   现象:
   Can not find a java.io.InputStream with the name [inputStream] in the invocation stack. Check the <param name="inputName"> tag specified for this action.

   原因:
        你要下载的文件不存在，导致inputStream为null,所以报了这个错误；查看官方源代码，
如下：

           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);
            }

  解决方法:
         a) 配置文件:
            <result type="stream" name="SUCCESS">
                <param name="inputName">inputStream</param>
            </result>
         b）java代码:
              定义变量:
                     private InputStream inputStream;
                  并生成getter/setter方法
              方法里:
                    filePath = ServletActionContext.getServletContext().getRealPath("/Webcontent下的文件夹名/") + filePath;
            File file = new File(filePath);
            inputStream = new FileInputStream(file);