java给PFD添加文字水印

可以通过使用Java的Graphics2D类来添加文字水印到PDF文件中。以下是一个示例代码：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.PDPage;
import org.apache.pdfbox.pdmodel.PDPageContentStream;
import org.apache.pdfbox.pdmodel.font.PDType1Font;

public class AddTextWatermarkToPDF {
    public static void main(String[] args) {
        String inputFile = "input.pdf";
        String outputFile = "output.pdf";
        String watermarkText = "Confidential";

        try {
            PDDocument document = PDDocument.load(new File(inputFile));
            for (PDPage page : document.getPages()) {
                PDPageContentStream contentStream = new PDPageContentStream(document, page, PDPageContentStream.AppendMode.APPEND, true, true);

                // set font and font size
                contentStream.setFont(PDType1Font.HELVETICA_BOLD, 72);

                // set text color to red
                contentStream.setNonStrokingColor(Color.RED);

                // rotate the text 45 degrees
                AffineTransform transform = new AffineTransform();
                transform.rotate(Math.toRadians(45), 0, 0);
                contentStream.setTextMatrix(transform);

                // get the width and height of the text
                float textWidth = PDType1Font.HELVETICA_BOLD.getStringWidth(watermarkText) / 1000 * 72;
                float textHeight = PDType1Font.HELVETICA_BOLD.getFontDescriptor().getFontBoundingBox().getHeight() / 1000 * 72;

                // set the position of the text
                float x = (page.getMediaBox().getWidth() - textWidth) / 2;
                float y = (page.getMediaBox().getHeight() - textHeight) / 2;

                // write the text to the content stream
                contentStream.beginText();
                contentStream.newLineAtOffset(x, y);
                contentStream.showText(watermarkText);
                contentStream.endText();

                contentStream.close();
            }

            // save the PDF document with the watermark
            document.save(new FileOutputStream(outputFile));
            document.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这个示例代码使用Apache PDFBox库来操作PDF文件。它打开一个PDF文件，遍历每个页面，然后在每个页面上添加一个旋转的、红色的、加粗的文本水印。如果需要更改水印文本、字体、颜色或位置，可以修改代码中的相关行。