java导出excel(用的是poi jar包),出现以下错误，下载对话框没有弹出来，请问，这是什么原因？怎样改？
2010-10-7 19:12:38 org.apache.catalina.core.ApplicationDispatcher invoke
严重: Servlet.service() for servlet jsp threw exception
java.lang.IllegalStateException: getOutputStream() has already been called for this response
at org.apache.catalina.connector.Response.getWriter(Response.java:610)
at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
at javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:178)
at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
at org.apache.jsp.user_005flist_jsp._jspService(user_005flist_jsp.java:712)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
......
BaseAction代码如下（Struts2+Hibernate+Spring）:
protected void downloadExcel(HSSFWorkbook workbook, HttpServletResponse response, String filename)
throws IOException {
OutputStream out = response.getOutputStream();
response.setHeader("Content-disposition", "attachment;filename=" + URLEncoder.encode(filename, "UTF-8"));
response.setContentType("application/msexcel;charset=UTF-8");
workbook.write(out);
//out.flush();
out.close();
}
Action代码如下:
public String exportExcel() throws Exception {
UserInfoVO user = new UserInfoVO();
List list = userInfoService.exportUser(user);
// 填充数据
HSSFWorkbook workbook = this.fillDataToExcel(list);
Date date = new Date();
String currentDate = DateFormatUtil.getStringToDate(date, "yyyyMMdd");
if(null != workbook){
this.downloadExcel(workbook, this.getResponse(), "用户信息("+currentDate+").xls");
}
return "list";
}
------解决方案--------------------
返回null