String path = fileName + ".pdf";
title = URLEncoder.encode(title, "UTF-8");
FtpUtils.getInstance("def").getFile(path, bos);
os = response.getOutputStream();
         InputStream is = new ByteArrayInputStream(bos.toByteArray());
response.reset(); // 清除下載檔案的空白行
response.resetBuffer();
response.setContentType(contentType);
response.setHeader("Content-Disposition", showMode + "; filename=" + title);
convertPdf2Image(is, os);//將圖片顯示在頁面

//傳入一個輸入流，把pdf轉化為圖片之後流轉到輸出流中
public void convertPdf2Image(InputStream is, OutputStream os) throws IOException {
    int pagen = 1;
    PDFFile pdffile = null;
    byte[] byt = this.toByteArray(is);
    try {
        ByteBuffer buf = ByteBuffer.allocate(byt.length);//若長度不夠會報outinex錯誤
        buf.put(byt);
        pdffile = new PDFFile(buf);
    } catch (Exception e) {
        e.printStackTrace();
    }
    if (pagen < pdffile.getNumPages())
        return;

    //設定將第pagen也生成png圖片
    PDFPage page = pdffile.getPage(pagen);
    int width = (int) page.getBBox().getWidth();
    int height = (int) page.getBBox().getHeight();
    BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
    Graphics2D g2 = img.createGraphics();
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    PDFRenderer renderer = new PDFRenderer(page, g2,
            new Rectangle(0, 0, width, height), null, Color.white);//這個color為渲染出來的圖片的背景顏色
    try {
        page.waitForFinish();
    } catch (Exception e) {
        e.printStackTrace();
    }
    renderer.run();
    g2.dispose();

    try {
        ImageIO.write(img, "png", os);
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

//將一個輸入流轉化為位元組陣列

public static byte[] toByteArray(InputStream in) throws IOException {
    ByteArrayOutputStream out = new ByteArrayOutputStream();
    byte[] buffer = new byte[1024 * 20];
    int n = 0;
    while ((n = in.read(buffer)) != -1) {
        out.write(buffer, 0, n);
    }
    return out.toByteArray();
}