<%
String pieGraphURL = request.getContextPath() + "/DisplayChart?filename=" + piefilename;
%>

再看一下DisplayChart调用的核心源码：

public static void sendTempFile(File file, HttpServletResponse response,
                                    String mimeType) throws IOException {

        if (file.exists()) {
            BufferedInputStream bis = new BufferedInputStream(
                new FileInputStream(file)
            );

            //  Set HTTP headers
            if (mimeType != null) {
                response.setHeader("Content-Type", mimeType);
            }
            response.setHeader("Content-Length", String.valueOf(file.length()));
            SimpleDateFormat sdf = new SimpleDateFormat(
                "EEE, dd MMM yyyy HH:mm:ss z"
            );
            sdf.setTimeZone(TimeZone.getTimeZone("GMT"));
            response.setHeader(
                "Last-Modified", sdf.format(new Date(file.lastModified()))
            );

            BufferedOutputStream bos = new BufferedOutputStream(
                response.getOutputStream()
            );
            byte[] input = new byte[1024];
            boolean eof = false;
            while (!eof) {
                int length = bis.read(input);
                if (length == -1) {
                    eof = true;
                }
                else {
                    bos.write(input, 0, length);
                }
            }
            bos.flush();
            bis.close();
            bos.close();
        }
        else {
            throw new FileNotFoundException(file.getAbsolutePath());
        }
        return;
    }