请哪位高手来解答一下！！ 谢谢了！！

/ 新建一个SmartUpload对象  
SmartUpload su = new SmartUpload();  
// 初始化  
su.initialize(pageContext);  
// 设定contentDisposition为null以禁止浏览器自动打开文件，  
//保证点击链接后是下载文件。若不设定，则下载的文件扩展名为  
su.setContentDisposition(null);  // 下载文件  
su.downloadFile("D://workspace/Car/WebRoot/upload/cars.xml"); 弄完之后出现以下问题是怎么回事！！！ 
java.lang.IllegalStateException: getOutputStream() has already been called for this response 
at org.apache.catalina.connector.Response.getWriter(Response.java:599) 
at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:195) 
at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:124) 
at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:117) 
at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:191) 
at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:115) 
at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:75) 
at org.apache.jsp.MyJsp_jsp._jspService(MyJsp_jsp.java:90) 
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) 
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332) 
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314) 
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) 
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252) 
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) 
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213) 
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178) 
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126) 
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)