PDF文档是否包含‘不可访问’内容？Java PDF库技术调研

Absolutely—PDFs absolutely can hold content that standard text extraction code (like the PDFBox/iText snippets you shared) fails to capture. Let’s walk through why this happens and how to handle it with Java libraries:

Common Types of "Unreachable" Content #

• Scanned/image-based content: If the PDF is just a collection of scanned pages (pixel images, not actual text), tools like PDFBox’s PDFTextStripper will return nothing—since there’s no text data to extract.
• Hidden text: Text might be set to the same color as the background, or marked with a "hidden" flag in the PDF’s internal structure. Default extraction often skips this.
• Content in annotations/form fields: Standard text extraction usually ignores form input values, sticky notes, or other annotation content.
• Nested/non-standard content: Some PDFs store text inside embedded XObjects, layers, or custom page structures that default extractors don’t traverse.

Fixes for Java PDF Libraries #

1. Handling Scanned PDFs (OCR Integration) #

For image-only PDFs, you’ll need to combine your PDF library with an OCR tool like Tesseract. Here’s how to do it with PDFBox:

import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.rendering.PDFRenderer;
import net.sourceforge.tess4j.Tesseract;
import java.awt.image.BufferedImage;
import java.io.File;

public class OcrPdfExtraction {
    public static void main(String[] args) throws Exception {
        PDDocument document = PDDocument.load(new File("scanned-file.pdf"));
        PDFRenderer renderer = new PDFRenderer(document);
        Tesseract tesseract = new Tesseract();
        
        StringBuilder extractedText = new StringBuilder();
        for (int page = 0; page < document.getNumberOfPages(); page++) {
            BufferedImage image = renderer.renderImageWithDPI(page, 300); // High DPI for better OCR accuracy
            extractedText.append(tesseract.doOCR(image));
        }
        
        System.out.println(extractedText);
        document.close();
    }
}

2. Extracting Hidden Text #

With PDFBox

Override PDFTextStripper to capture all text, regardless of visibility:

import org.apache.pdfbox.text.PDFTextStripper;
import org.apache.pdfbox.text.TextPosition;
import java.io.IOException;
import java.util.List;

public class HiddenTextStripper extends PDFTextStripper {
    public HiddenTextStripper() throws Exception {
        super();
    }

    @Override
    protected void writeString(String text, List<TextPosition> textPositions) throws IOException {
        // You can add logic here to check if text is truly hidden (e.g., color matching background)
        // For this example, we'll capture all text including hidden ones
        super.writeString(text, textPositions);
    }
}

// Usage in your existing code:
// HiddenTextStripper stripper = new HiddenTextStripper();
// String text = stripper.getText(document);

With iText

Use a custom text extraction strategy to capture all text fragments:

import com.itextpdf.kernel.pdf.PdfDocument;
import com.itextpdf.kernel.pdf.PdfReader;
import com.itextpdf.kernel.pdf.canvas.parser.PdfCanvasProcessor;
import com.itextpdf.kernel.pdf.canvas.parser.listener.LocationTextExtractionStrategy;

public class ITextHiddenExtractor {
    public static void main(String[] args) throws Exception {
        PdfReader reader = new PdfReader("file.pdf");
        PdfDocument pdfDoc = new PdfDocument(reader);
        
        LocationTextExtractionStrategy strategy = new LocationTextExtractionStrategy();
        for (int page = 1; page <= pdfDoc.getNumberOfPages(); page++) {
            PdfCanvasProcessor processor = new PdfCanvasProcessor(strategy);
            processor.processPageContent(pdfDoc.getPage(page));
        }
        
        System.out.println(strategy.getResultantText());
        pdfDoc.close();
        reader.close();
    }
}

(To filter hidden text specifically, extend the strategy to check text rendering modes or color properties against the page background.)

3. Extracting Annotations/Form Fields #

With PDFBox

// Extract form fields
PDAcroForm acroForm = document.getDocumentCatalog().getAcroForm();
if (acroForm != null) {
    for (PDField field : acroForm.getFields()) {
        System.out.println("Field Name: " + field.getFullyQualifiedName() + ", Value: " + field.getValueAsString());
    }
}

// Extract annotations
for (PDPage page : document.getPages()) {
    for (PDAnnotation annotation : page.getAnnotations()) {
        if (annotation instanceof PDAnnotationTextMarkup) {
            System.out.println("Annotation Text: " + ((PDAnnotationTextMarkup) annotation).getContents());
        }
    }
}

With iText

// Extract form fields
PdfAcroForm form = PdfAcroForm.getAcroForm(pdfDoc, false);
if (form != null) {
    form.getFormFields().forEach((name, field) -> 
        System.out.println("Field Name: " + name + ", Value: " + field.getValueAsString())
    );
}

// Extract annotations
for (int page = 1; page <= pdfDoc.getNumberOfPages(); page++) {
    PdfPage pdfPage = pdfDoc.getPage(page);
    if (pdfPage.getAnnotations() != null) {
        pdfPage.getAnnotations().forEach(annotDict -> 
            System.out.println("Annotation Content: " + annotDict.getAsString(PdfName.Contents))
        );
    }
}

4. Handling Nested/Complex Content #

For text stored in XObjects or layers, you’ll need to traverse the PDF’s internal structure more deeply:

• PDFBox: Extend PDFTextStripper to explicitly process embedded XObjects, or use PDPageContentStream to parse each page’s low-level operations.
• iText: Use PdfCanvasProcessor with a custom EventListener that handles XObject invocations, ensuring you process content inside nested objects.

内容的提问来源于stack exchange，提问作者Hector