java - 如何在方法之间保存数据

Question

我有一个 Controller ，可以从 pdf(pdfBox) 创建许多图像。我将所有图片保存到 List 。而且我需要在按下按钮时为每个 View 发送一张图片。或输入值

public class Pdf{
     public byte[] PDFCREATE(){
      byte[] pdf = ...;
        //Code for creating pdf using Itext
       return pdf;

    }
}

    public List<byte[]> getImage(byte[] pdf) throws Exception{
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
       List<byte> listImg = new List()<>;
        try (final PDDocument document = PDDocument.load(file)){
            PDFRenderer pdfRenderer = new PDFRenderer(document);
            for (int page = 0; page < document.getNumberOfPages(); ++page)
            {
                BufferedImage bim = pdfRenderer.renderImage(page)
                ImageIO.write(bim, "png",baos);
                   listImg.add(baos)

            }
            document.close();
        } catch (IOException e){
            System.err.println("Exception while trying to create pdf document - " + e);
        }
        return baos.toByteArray();

查看我发布:

   public ResponseEntity<?> CreateFont(int pictureNumber ) {
        PDF pdf= new PDF();
PdfToImg pdfToImg = new PdfToImg();
List<byte []> fileBytes = pdfToImg.getImage(pdf.PDFCREATE());
byte[] encoded= Base64.encodeBase64(fileBytes.get(pictureNumber ));
String encodedString = new String(encoded);

            return new ResponseEntity<String>(
                    "<img src='data:image/jpeg;base64,"  + encodedString + "' alt='' width='420' height='580'>",
                    HttpStatus.OK);

但是这里我每次都必须创建pdf。是否可以将 getImage 中的值保存到列表中？当用户每次输入值(图片编号)或按下按钮时不创建新的 pdf。而不是从列表中获取值。并创建刷新页面后的新列表我需要将列表文件字节存储在某处，但是在哪里？

对不起我的英语

Answer 1

这可能对您有帮助:

private static Map<Integer, String> pictureMap = new HashMap<>();
private static PDF pdf;

public ResponseEntity<?> CreateFont(int pictureNumber) {
    if(pictureMap.containsKey(pictureNumber))
        return response(pictureMap.get(pictureNumber));
    if(pdf == null)
        pdf = new PDF();
    PdfToImg pdfToImg = new PdfToImg();
    List<byte[]> fileBytes = pdfToImg.getImage(pdf.PDFCREATE());
    byte[] encoded = Base64.encodeBase64(fileBytes.get(pictureNumber));
    String encodedString = new String(encoded);
    pictureMap.put(pictureNumber, encodedString);

    return response(encodedString);
}


private ResponseEntity response(String encodedString){
    return new ResponseEntity<String>(
        "<img src='data:image/jpeg;base64," + encodedString + "' alt='' width='420' height='580'>",
        HttpStatus.OK);
}