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实现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。然后查看StreamResult的源代码才发现是因为InputStream为null的缘故。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,问题得以解决,问题的原因是这个流的realPath路径错误。