如何使用iText在每个页面上添加总页数？

import com.itextpdf.kernel.pdf.*;
import com.itextpdf.layout.Document;
import com.itextpdf.layout.element.Paragraph;
import com.itextpdf.layout.property.TextAlignment;
import com.itextpdf.layout.property.VerticalAlignment;
import com.itextpdf.layout.renderer.DocumentRenderer;
import com.itextpdf.layout.renderer.IRenderer;

import java.io.FileOutputStream;
import java.io.IOException;

public class AddTotalPages {

    public static void main(String[] args) throws IOException {
        PdfWriter writer = new PdfWriter("output.pdf");
        PdfDocument pdfDocument = new PdfDocument(writer);
        Document document = new Document(pdfDocument);

        // 创建一个自定义页面事件处理程序
        CustomPageEventHandler pageEventHandler = new CustomPageEventHandler();
        pdfDocument.addEventHandler(PdfDocumentEvent.END_PAGE, pageEventHandler);

        // 添加内容到文档
        for (int i = 0; i < 10; i++) {
            document.add(new Paragraph("This is page " + (i + 1)));
            document.add(new Paragraph("Total pages: "));
            document.add(new AreaBreak());
        }

        // 关闭文档
        document.close();
    }

    static class CustomPageEventHandler extends PdfPageEventHelper {

        @Override
        public void onEndPage(PdfWriter writer, Document document) {
            // 获取当前页面
            PdfPage page = writer.getPage(writer.getCurrentPageNumber());

            // 创建一个占位符，用于在页面上显示总页数
            PdfFormXObject placeholder = new PdfFormXObject(new Rectangle(50, 50, 100, 50));
            PdfCanvas canvas = new PdfCanvas(page);
            canvas.addXObject(placeholder, 300, 50);

            // 将占位符添加到文档的渲染器中，以便在文档关闭时更新总页数
            DocumentRenderer documentRenderer = (DocumentRenderer) document.getRenderer();
            IRenderer placeholderRenderer = placeholder.createRendererSubTree().setParent(documentRenderer);
            documentRenderer.addChild(placeholderRenderer);

            // 更新占位符的内容
            placeholderRenderer.setProperty(Property.TEXT_ALIGNMENT, TextAlignment.CENTER);
            placeholderRenderer.setProperty(Property.VERTICAL_ALIGNMENT, VerticalAlignment.MIDDLE);
            placeholderRenderer.setProperty(Property.FONT_SIZE, 12f);
            placeholderRenderer.setProperty(Property.FONT, PdfFontFactory.createFont());
            placeholderRenderer.setProperty(Property.TEXT_RENDERING_MODE, PdfCanvasConstants.TextRenderingMode.FILL);
            placeholderRenderer.setProperty(Property.FILL_COLOR, ColorConstants.BLACK);
        }
    }
}