iText 9.0.0给PDF多页添加XObject时触发空指针异常求助

使用iText 9.0.0编写Java程序生成PDF，将外部PDF作为XObject添加到目标PDF每一页。当目标PDF页数为1或2时正常，页数≥3（比如把int n = 30;改成int n = 31;）时程序崩溃，抛出空指针异常。

代码示例 #

package net.opho;
import com.itextpdf.kernel.geom.Rectangle;
import com.itextpdf.kernel.pdf.PdfDocument;
import com.itextpdf.kernel.pdf.PdfReader;
import com.itextpdf.kernel.pdf.PdfWriter;
import com.itextpdf.kernel.pdf.canvas.PdfCanvas;
import com.itextpdf.kernel.pdf.xobject.PdfXObject;
import com.itextpdf.layout.Document;
import com.itextpdf.layout.element.Paragraph;
public class Main {
    public static void main(String[] args) throws Exception {
        PdfDocument output = new PdfDocument(new PdfWriter("C:\\develop\\demo.pdf"));
        Document document = new Document(output);
        document.setTopMargin(120);
        Paragraph paragraph = (new Paragraph("Some texts...")).setFontSize(24);
        int n = 30;
        for (int i = 0; i < n; i++) {
            document.add(paragraph);
        }
        PdfDocument logoPdf = new PdfDocument(new PdfReader("C:\\develop\\OPhOLogo.pdf"));
        PdfXObject logoObject = logoPdf.getFirstPage().copyAsFormXObject(output);
        logoPdf.close();
        Rectangle logoRectangle = PdfXObject.calculateProportionallyFitRectangleWithWidth(logoObject, 20, 750, 200);
        for (int i = 1; i <= output.getNumberOfPages(); i++) {
            PdfCanvas canvas = new PdfCanvas(output, i);
            canvas.addXObjectFittedIntoRectangle(logoObject, logoRectangle);
        }
        output.close();
    }
}

抛出的异常信息 #

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Map.get(Object)" because "this.map" is null
    at com.itextpdf.kernel.pdf.PdfDictionary.get(PdfDictionary.java:471)
    at com.itextpdf.kernel.pdf.PdfDictionary.getAsNumber(PdfDictionary.java:184)
    at com.itextpdf.kernel.pdf.PdfPage.getRotation(PdfPage.java:151)
    at com.itextpdf.kernel.pdf.canvas.PdfCanvas.<init>(PdfCanvas.java:229)
    at com.itextpdf.kernel.pdf.canvas.PdfCanvas.<init>(PdfCanvas.java:271)
    at net.opho.Main.main(Main.java:25)

问题原因及解决方法 #

原因 #

通过Document添加内容生成多页PDF时，iText会延迟生成部分页面的底层PDF对象（页数较多时更明显），此时直接通过页码获取页面创建PdfCanvas，会遇到未完全初始化的页面，触发空指针异常。

解决方法 #

在操作页面之前，先调用document.close()强制完成所有页面的初始化，确保所有页面的PDF对象都已创建。修改后的代码如下：

package net.opho;
import com.itextpdf.kernel.geom.Rectangle;
import com.itextpdf.kernel.pdf.PdfDocument;
import com.itextpdf.kernel.pdf.PdfReader;
import com.itextpdf.kernel.pdf.PdfWriter;
import com.itextpdf.kernel.pdf.canvas.PdfCanvas;
import com.itextpdf.kernel.pdf.xobject.PdfXObject;
import com.itextpdf.layout.Document;
import com.itextpdf.layout.element.Paragraph;
public class Main {
    public static void main(String[] args) throws Exception {
        PdfDocument output = new PdfDocument(new PdfWriter("C:\\develop\\demo.pdf"));
        Document document = new Document(output);
        document.setTopMargin(120);
        Paragraph paragraph = (new Paragraph("Some texts...")).setFontSize(24);
        int n = 31;
        for (int i = 0; i < n; i++) {
            document.add(paragraph);
        }
        // 关闭Document，强制完成所有页面初始化
        document.close();
        
        PdfDocument logoPdf = new PdfDocument(new PdfReader("C:\\develop\\OPhOLogo.pdf"));
        PdfXObject logoObject = logoPdf.getFirstPage().copyAsFormXObject(output);
        logoPdf.close();
        
        Rectangle logoRectangle = PdfXObject.calculateProportionallyFitRectangleWithWidth(logoObject, 20, 750, 200);
        for (int i = 1; i <= output.getNumberOfPages(); i++) {
            PdfCanvas canvas = new PdfCanvas(output.getPage(i));
            canvas.addXObjectFittedIntoRectangle(logoObject, logoRectangle);
        }
        output.close();
    }
}

关键修改点 #

1. 添加完所有内容后调用document.close()，确保所有页面完全生成并初始化。
2. 创建PdfCanvas时，改用output.getPage(i)获取已初始化的页面对象，避免直接通过页码构造时的潜在问题。

内容的提问来源于stack exchange，提问作者user27326776