iText7多重签名

我想用iText7在pdf文档中进行多个签名,但问题是当我签名一次时,它就可以工作了。如果我签名两次,第一个签名无效。看起来是这样的:

unsigned PDF

signed PDF

这是我的代码:

@Test
public void testMutiSign() {

    iTextSignerUtil1.SignMultPDF(getBytes(unsignedPath), destPath1);

    iTextSignerUtil2.SignMultPDF(getBytes(destPath1), destPath2);

}


 IExternalSignatureContainer externalP7DetachSignatureContainer = new IExternalSignatureContainer() {
    @Override
    public byte[] sign(InputStream data) throws GeneralSecurityException {


        //byte[] hashData = HashUtil.hash(data , "SHA256");

        byte signData = null;
        signData = signUtil.signP7DetachData(data);

        return signData;

    }


    @Override
    public void modifySigningDictionary(PdfDictionary signDic) {
        signDic.put(PdfName.Filter, PdfName.Adobe_PPKLite);
        signDic.put(PdfName.SubFilter, PdfName.Adbe_pkcs7_detached);
    }
};


public void SignMultPDF(byte[] pdfFile , String destPath , String name , String fname , String value){

    boolean success = false;

    int estimatedSize = 300000;

    while (!success) {
        try {

            PdfReader pdfReader = new PdfReader(new ByteArrayInputStream(pdfFile));
            PdfSigner pdfSigner = new PdfSigner(pdfReader, new FileOutputStream(destPath), true);

          pdfSigner.signExternalContainer(externalP7DetachSignatureContainer, estimatedSize);

            success = true;

        } catch (IOException e) {
            e.printStackTrace();
            estimatedSize += 1000;
        } catch (GeneralSecurityException e) {
            e.printStackTrace();
        }
    }
}

以上是我的全部示例代码。

还有一个问题,我是否错过了创建签名字段?