how to remove seal When I sign many times - itext7

Question

How I sign pdf by p12:
signer.setFieldName(fieldName);
signer.signDetached(digest, pks, chain, null, null, null, 0, PdfSigner.CryptoStandard.CMS);
How I try to remove one sign:
PdfAcroForm.removeField(fieldName);
PdfAcroForm.flattenFields();

Answer 1

If one looks for an example for removing digital signatures on the iText site, one finds this example (as you did). This is a bit misleading as it strictly speaking is an example for flattening a signature, i.e. removing the whole signature field but making its visual appearance part of the page content. Actually the code is flattening not only the signature field but all fields.
You clarified in a comment to your question, though:
i want to remove the signature value and the visualization thereo by fieldName
So you in particular want to change only the signature field, you don't want to actually remove it but merely clear it, and you don't want to keep its appearance. Thus, the example you found does not match your requirements in multiple ways.
First of all, you can only clear the most recently signed signature field(s); otherwise you invalidate any more recent signatures. So we can assume that the signature field you want to clear is the outermost one.
To clear the signature field as you want to, you need to remove its value (the actual signature and some metadata) and the appearances of the field widgets (strictly speaking ISO 32000-2 allows only a single widget but there are a number of PDFs in the wild with signature fields with multiple visualizations), e.g. using this method:
String clearLastSignature(PdfDocument pdfDocument) {
SignatureUtil signatureUtil = new SignatureUtil(pdfDocument);
PdfAcroForm acroForm = PdfAcroForm.getAcroForm(pdfDocument, false);
List<String> signatureNames = signatureUtil.getSignatureNames();
if (signatureNames != null && signatureNames.size() > 0) {
String lastSignatureName = signatureNames.get(signatureNames.size() - 1);
PdfFormField lastSignatureField = acroForm.getField(lastSignatureName);
if (null != lastSignatureField.getPdfObject().remove(PdfName.V))
lastSignatureField.getPdfObject().setModified();
for (PdfWidgetAnnotation pdfWidgetAnnotation : lastSignatureField.getWidgets()) {
if (pdfWidgetAnnotation.getPdfObject().remove(PdfName.AP) != null)
pdfWidgetAnnotation.getPdfObject().setModified();
}
return lastSignatureName;
}
return null;
}
(from ClearSignature.java)
This method additionally returns the name of the cleared field which can be helpful if you want to resign it soon after.
You can use this method like this:
try ( PdfReader pdfReader = new PdfReader(source);
PdfWriter pdfWriter = new PdfWriter(clearedFile);
PdfDocument pdfDocument = new PdfDocument(pdfReader, pdfWriter, new StampingProperties().useAppendMode())) {
String clearedSignatureName = clearLastSignature(pdfDocument);
}
(ClearSignature test testClearLastSignatureIncrementally)
Applied to the output "step6_signed_by_alice_bob_carol_and_dave.pdf" of the iText signature whitepaper example C2_11_SignatureWorkflow
you get
and after resigning the signature field you get this
Beware: Depending on PDF permissions and restrictions this may fail for some PDF documents.