求救：struts2实现文件下载抛出异常
利用struts2实现文件下载抛出异常：

2012-08-14 14:48:17.708:WARN::Committed before 500 STREAM
2012-08-14 14:48:17.709:WARN::/software/download.do
java.lang.IllegalStateException: Committed
at org.mortbay.jetty.Response.resetBuffer(Response.java:1024)
at org.mortbay.jetty.Response.sendError(Response.java:240)
at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:852)
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:534)
at org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:432)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at webui.filter.FilterIsLogin.doFilter(FilterIsLogin.java:34)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:440)
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:926)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:549)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
struts action的xml文件配置为：
<!DOCTYPE struts PUBLIC
"-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"
"http://struts.apache.org/dtds/struts-2.3.dtd">
<struts>
<package name="software" extends="struts-default" namespace="/software">
<action name="download" class="webui.action.SoftwareAction" method="softwareDownload">
<result name="success">
/Jsp/software/software_list.jsp
</result>
<result name="error">/Jsp/error/error.jsp</result>
</action>
</package>

service层的代码为：
HttpServletResponse response = ServletActionContext.getResponse();
File file = new File(downloadPath);
FileInputStream in = new FileInputStream(file);
response.setHeader("content-disposition", "attachment;filename=" + URLEncoder.encode(fileName, "UTF-8"));
OutputStream out = response.getOutputStream();
int len = 0;
byte buffer[] = new byte[1024];
while ((len = in.read(buffer)) > 0) {
out.write(buffer, 0, len);
}
out.close();

这个函数返回的是个void类型
action调用这个函数
可以实现文件下载的功能，但在console里面会抛出以上异常，请哪位大侠能够帮帮忙看看，先在这里谢谢了！！
我试过了很多方法也不行，救命啊

------解决方案--------------------

探讨

我的理解是，你的response已经返回，你又获取到，导致再相应客户端后操作response出现的不合法的状态异常!
由于你download.do的代码没有提供所以不好精确判断