itext对pdf签章提问

请假下，最近在写itext对pdf进行电子签章，发现盖章成功后印章图片是在文字上层的，完全挡住了盖章位置的文字。查了下基本没有这方面的资料文档，有没有遇到过类似问题的朋友支援下
代码和网上的大致一样


```java
BouncyCastleProvider provider = new BouncyCastleProvider();
            Security.addProvider(provider);
            KeyStore ks = KeyStore.getInstance("PKCS12", new BouncyCastleProvider());
            fos = new FileInputStream(keyStorePath);
            
            // 私钥密码 为Pkcs生成证书是的私钥密码 123456
            ks.load(fos, password.toCharArray()); 
            String alias = (String) ks.aliases().nextElement();
            // 获取私钥
            PrivateKey key = (PrivateKey) ks.getKey(alias, password.toCharArray());
            // 获取证书链
            Certificate[] chain = ks.getCertificateChain(alias);
            reader = new PdfReader(signPdfSrc);
            signPDFData = new ByteArrayOutputStream();
            // 临时pdf文件
            File temp = new File(signPdfSrcFile.getParent(), System.currentTimeMillis() + ".pdf");
            stp = PdfStamper.createSignature(reader, signPDFData, '\0', temp, true); 
            stp.setFullCompression();
            PdfSignatureAppearance sap = stp.getSignatureAppearance();
            sap.setReason("数字签名，不可改变");
            // 使用png格式透明图片
            Image image = Image.getInstance(signImage);
            sap.setImageScale(0);
            sap.setSignatureGraphic(image);
            sap.setRenderingMode(RenderingMode.GRAPHIC);
            // 是对应x轴和y轴坐标
            sap.setVisibleSignature( new Rectangle(x, y, x + 185, y + 68), 1,  UUID.randomUUID().toString().replaceAll("-", ""));
            stp.getWriter().setCompressionLevel(5);
            
            ExternalDigest digest = new BouncyCastleDigest();
            ExternalSignature signature = new PrivateKeySignature(key, DigestAlgorithms.SHA512, provider.getName());
            MakeSignature.signDetached(sap, digest, signature, chain, null, null, null, 0, CryptoStandard.CADES);
            stp.close();
            reader.close();

```