AjaxUpload与SSH框架结合做上传图片

Question

在action中

String path=ServletActionContext.getServletContext().getRealPath(uploadDir)

path总为null (uploadDir为保存文件的目录，在struts中进行了如下配置

<action name="edituserphoto" class="userPhotoAction" method="upload">
	      <interceptor-ref name="defaultStack" /> 
	      <result type="json">
          <param name="ignoreHierarchy">false</param> 
          <param name="uploadDir">/userphotos</param>
          </result>
	 </action>

求解