iText-检查附件是否存在/链接附件多次

Question

protected HashMap<String, PdfFileSpecification> cache = new HashMap<>();
private final byte[] BUFFER = new byte[1024];

public PdfFileSpecification getPdfFileSpecification(final PdfWriter pdfWriter, final String name, final byte[] data) throws IOException {

    String hash = createMD5Hash(data);
    PdfFileSpecification pdfFileSpecification = cache.get(hash);

    if (pdfFileSpecification == null) {
        pdfFileSpecification = PdfFileSpecification.fileEmbedded(pdfWriter, null, name, data);
        cache.put(hash, pdfFileSpecification);
        return pdfFileSpecification;
    }
    System.out.println(String.format("Name: %s Hash: %s", name, hash));
    return pdfFileSpecification;
}

private String createMD5Hash(final byte[] data) {

MessageDigest messageDigest;

    try {
        messageDigest = MessageDigest.getInstance("MD5");
    } catch (NoSuchAlgorithmException e) {
        return null;
    }

    ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(data);

    try {
        int i;
        while ((i = byteArrayInputStream.read(BUFFER)) != -1) {
            messageDigest.update(BUFFER, 0, i);
        }
        byteArrayInputStream.close();
    } catch (IOException e) {
        return null;
    }
    byte[] mdbytes = messageDigest.digest();

    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < mdbytes.length; i++) {
        sb.append(Integer.toString((mdbytes[i] & 0xff) + 0x100, 16).substring(1));
    }

    return sb.toString();
}

所以每次我要处理一个新的附件时，我都是这样做的：

PdfFileSpecification fs = getPdfFileSpecification(pdfWriter, name, data)
PdfAnnotation an = PdfAnnotation.createFileAttachment(pdfWriter, rectangle, name, fs);

Answer 1

请允许我拿着您的代码，介绍一些伪代码，向您展示我会如何做到这一点：

protected Map<String, PdfFileSpecification> cache =
    new HashMap<String, PdfFileSpecification>();

public void cellLayout(final PdfPCell pdfPCell, final Rectangle rectangle, final PdfContentByte[] pdfContentBytes) {
    String hasheddata = createHash(attachment);
    PdfFileSpecification fs = cache.get(hasheddata);
    if (fs == null) {
        fs = PdfFileSpecification.fileEmbedded(writer, null, displayname, attachment);
        cache.put(hasheddata, fs);
    }
    PdfAnnotation an = PdfAnnotation.createFileAttachment(writer, rectangle, displayname, fs);
    writer.addAnnotation(an);
}

这段代码无法编译，因为我遗漏了一些与问题无关的部分。我只保留了解释为文件规范创建缓存的概念的内容。

我创建附件字节的哈希值以节省内存。您必须使用您选择的哈希算法实现createhash()方法。在创建将向PDFWriter写入字节的新FileSpecification之前，我要检查是否不能重用已有的文件规范。如果存在，我将在注释中重用它。如果它不存在，我创建一个新的文件规范。