使用java如何实现给pdf文件添加水印

//读取原来的pdf
PdfReader reader = new PdfReader("/test/" + "1.pdf");
//生成以后的pdf
 PdfStamper stamp = new PdfStamper(reader, new FileOutputStream("/test/" + "createSamplePDF2.pdf"));
 int max = reader.getNumberOfPages();
 //max =2;
 // 文字水印
 for (int i = 1; i <= max; i++) {
 PdfContentByte over = stamp.getOverContent(i);
 over.beginText();
 BaseFont bf = BaseFont.createFont(BaseFont.HELVETICA, BaseFont.WINANSI, BaseFont.EMBEDDED);
 over.setFontAndSize(bf, 16);
 over.setTextMatrix(10, 10);
 over.setColorFill(BaseColor.GRAY);
 over.showTextAligned(Element.ALIGN_LEFT, "java blog java-er.com ", 150,670, 0);
// 0 0 0 表示左下脚 最后一个0是角度，0表示横着 45 表示斜着
 over.endText();
 }
 
 reader.close();
 stamp.close();
 
 }