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

今天在用struts2就行文件下载时出现如下错误：

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)

说实话这个提示真有误导人的嫌疑，刚开始还以为是名称不对，估计一般人看到这个提示都这样想。然后查看StreamResult的源代码才发现是因为InputStream为null的缘故，汗一个。看下源码：

 

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);
            }

大家如果也碰到此类问题,直接打印
InputStream in=ServletActionContext.getServletContext().getResourceAsStream(realPath);
System.out.println(in);

如果打印为NULL的话,恭喜您,问题得以解决,问题的原因是这个流的realPath路径错误,

 还没明白的往下看

怪呀,我的配置应该没错呀
页面上:
<a href=" fileDownload.action?fileName=<s:property value ="imageName" />">下载此图片</a>
struts.xml中
----------------------------------------------------------
<!-- 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>
----------------------------------------------------------
action中
----------------------------------------------------------
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;
}