JSP 下载文件~~~防止多次调用抛异常

<%
String filenamedownload =(String) request.getAttribute("fileName");
String filenamedisplay =(String) request.getAttribute("fileDisplayName");

response.reset();
response.setContentType("application/bin");  //告诉浏览器，我服务器发送的data属于何种MIME类型

filenamedisplay = URLEncoder.encode(filenamedisplay, "UTF-8");
response.setHeader("Content-Disposition", "attachment;filename="
+ filenamedisplay);

OutputStream output = null;
FileInputStream fis = null;
try {
output = response.getOutputStream();
fis = new FileInputStream(filenamedownload);

byte[] b = new byte[1024];
int i = 0;

while ((i = fis.read(b)) > 0) {
output.write(b, 0, i);
}
output.flush();
} catch (IOException ex) {
ex.printStackTrace();
} finally {
if (fis != null) {
fis.close();
fis = null;
}
if (output != null) {
output.close();
output = null;
}
out.clear();
out = pageContext.pushBody();
}

%>