struts2文件下载出现Can not find a java.io.InputStream with the name的错误

    Servlet.service() for servlet default threw exception   
    java.lang.IllegalArgumentException: Can not find a java.io.InputStream with the name [imageStream] in the invocation stack.
 Check the <param name="inputName"> tag specified for this action.   
        at org.apache.struts2.dispatcher.StreamResult.doExecute(StreamResult.java:189)   
        at org.apache.struts2.dispatcher.StrutsResultSupport.execute(StrutsResultSupport.java:178)   
        at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:348)   
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:253)   
        at com.best.top.validate.TopInterceptor.intercept(TopInterceptor.java:47)   
        at com.opensymphony.xwork2.DefaultActionInvocation$2.doProfiling(DefaultActionInvocation.java:224)   
        at com.opensymphony.xwork2.DefaultActionInvocation$2.doProfiling(DefaultActionInvocation.java:223)   
        at com.opensymphony.xwork2.util.profiling.UtilTimerStack.profile(UtilTimerStack.java:455)   
        at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:221)   
        at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:50)   
        at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:504)   
        at org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:419)  

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);   
                }  

InputStream in=ServletActionContext.getServletContext().getResourceAsStream(realPath);
System.out.println(in);

<a href="fileDownload.action?fileName=<s:property value ="imageName" />">下载此图片</a>

<!-- 文件下载，支持中文附件名 -->
   <action name="fileDownload"
    class="com.test.action.filedown.FileDownloadAction">
    <result name="success" type="stream">
     <!-- 动态文件下载的，事先并不知道未来的文件类型，那么我们可以把它的值设置成为：application/octet-stream;charset=ISO8859-1 ，注意一定要加入charset，否则某些时候会导致下载的文件出错； -->
     <param name="contentType">
     application/octet-stream;charset=ISO8859-1
     </param>
     <param name="contentDisposition">
      attachment;filename="${downloadFileName}"
     </param>
     <!-- 使用经过转码的文件名作为下载文件名，downloadFileName属性
      对应action类中的方法 getDownloadFileName() 其中特殊的代码就是${downloadFileName}，它的效果相当于运行的时候将action对象的属性的取值动态的填充在${}中间的部分，我们可以认为它等价于＋action. getDownloadFileName()。 -->
     <param name="inputName">inputStream</param>
     <param name="bufferSize">4096</param>
    </result>
   </action>

private String fileName;// 初始的通过param指定的文件名属性 set get

/** 文件名 转换编码 防止中文乱码*/
public String getDownloadFileName() {
   String fileName=ServletActionContext.getRequest().getParameter("fileName");
   String downFileName = fileName;
   try {
    downFileName = new String(downFileName.getBytes(), "ISO8859-1");
   } catch (Exception e) {
    e.printStackTrace();
   }
   return downFileName;
}
//下载的流
public InputStream getInputStream() {
   String name=this.getDownloadFileName();
//  String realPath=ServletActionContext.getServletContext().getRealPath("/uploadImages")+ "/"+name; 路径错误
   String realPath="/uploadImages/"+name;
   InputStream in=ServletActionContext.getServletContext().getResourceAsStream(realPath);
   if(null==in){
    System.out.println("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.检查action中文件下载路径是否正确.");  
   }
   return ServletActionContext.getServletContext().getResourceAsStream(realPath);
}

@Override
public String execute() throws Exception {
   return SUCCESS;
}