itext签名后不执行签名域属性怎么解决？

通过手动签名以后可以正常执行签名域的属性但是通过代码完成签名以后，签名属性不能执行。标记的只读的域还是能操作。
代码如下

    KeyStore ks = KeyStore.getInstance("pkcs12");
        ks.load(new FileInputStream(certSrc), certPassword.toCharArray());
        String alias = (String) ks.aliases().nextElement();
        PrivateKey key = (PrivateKey) ks.getKey(alias, certPassword.toCharArray());
        Certificate[] chain = ks.getCertificateChain(alias);
        PdfReader reader = new PdfReader(pdfSrc,pdfPassword.getBytes()); // 
        FileOutputStream fout = new FileOutputStream(new File(donePdfSrc));
        PdfStamper stp = PdfStamper.createSignature(reader, fout, '\0',null,false);
        PdfSignatureAppearance sap = stp.getSignatureAppearance();
        sap.setReason(reason);
        sap.setLocation(location);
        Image image = Image.getInstance(imageSrc);
        sap.setSignatureGraphic(image);
        String url = SignPDF.class.getResource("simsun.ttc").toString()+",0";
        BaseFont bfChinese = BaseFont.createFont(url.toString(),BaseFont.IDENTITY_H,BaseFont.NOT_EMBEDDED);//
        Font fontChinese = new Font(bfChinese,12,Font.NORMAL);
        sap.setLayer2Font(fontChinese);//
        sap.setLayer2Text(reason);//
        sap.setLayer4Text("setLayer4Text");//
        sap.setRenderingMode(RenderingMode.GRAPHIC_AND_DESCRIPTION);
        sap.setVisibleSignature(fieldName);
        stp.getWriter().setCompressionLevel(5);
        Security.addProvider(new BouncyCastleProvider());
        ExternalSignature es = new PrivateKeySignature(key,"SHA-256","BC");
        ExternalDigest digest = new BouncyCastleDigest();
        MakeSignature.signDetached(sap, digest, es, chain, null, null, null, 0, CryptoStandard.CMS);

有没有哪个大神帮忙解答下，

http://bbs.csdn.net/topics/390166269

请教下，用itext 向PDF最加一个签名域生成新的PDF文件，用adober reader 去打开签名的时候发现，签名域不可用，签名 不允许，这个怎么通过itext改变成可用呢？？