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1. Overview

iText PDF is a Java library for creating and manipulating PDF filesWatermarks help protect confidential information.

In this tutorial, we'll explore the iText PDF library by creating a new PDF file with watermarks. We'll also add watermarks to existing PDF files.

2. Maven Dependencies

In this tutorial, we'll use Maven to manage our dependencies. We'll need iText dependency to get started with the iText PDF library. Also, we'll need AssertJ dependency for testing. We'll add the two dependencies to our pom.xml:

<dependency>
    <groupId>com.itextpdf</groupId>
    <artifactId>itext7-core</artifactId>
    <version>7.2.4</version>
    <type>pom</type>
</dependency>
<dependency>
    <groupId>org.assertj</groupId>
    <artifactId>assertj-core</artifactId>
    <version>3.23.1</version>
    <scope>test</scope>
</dependency>

3. Watermarks

Watermarks help overlay or underlay a text or logo on a document or image file. It's vital for copyright protection, marketing of a digital product, preventing counterfeiting, etc.

In this tutorial, we'll add a confidential watermark to our generated PDF. The watermark will prevent unauthorized usage of our generated PDF:

watermark on PDF

4. Generating PDF with iText

In this article, let's curate a story and use the iText PDF library to convert our story to PDF format. We'll write a simple program, StoryTime. First, we'll declare two variables of String type. We'll store our stories in the declared variables:

public class StoryTime {
    String aliceStory = "I am ...";
    String paulStory = "I am Paul ..";
}

We'll shorten the String values for simplicity. Then, let's declare a variable of String type that will store the output path of our generated PDF:

public static final String OUTPUT_DIR = "output/alice.pdf";

Finally, let's create a method that will contain the program logic. We'll create an instance of PdfWriter to specify our output path and name.

Next, we'll create an instance of PdfDocument to handle our PDF file. To add our String values to the PDF document, we'll create a new instance of Document:

public void createPdf(String output) throws IOException {
    
    PdfWriter writer = new PdfWriter(output);
    PdfDocument pdf = new PdfDocument(writer);
    try (Document document = new Document(pdf, PageSize.A4, false)) {
        document.add(new Paragraph(aliceSpeech)
          .setFont(PdfFontFactory.createFont(StandardFonts.TIMES_ROMAN)));
        document.add(new Paragraph(paulSpeech)
          .setFont(PdfFontFactory.createFont(StandardFonts.TIMES_ROMAN)));
        document.close();
    }
}

Our method will generate a new PDF file and store it in OUTPUT_DIR

5. Adding Watermarks to Generated PDF

In the last section, we generated a PDF file using the iText PDF library. Generating PDF first helps know the page size, rotation, and the number of pages. This help to effectively add watermarks. let's add more logic to our simple program. Our program will add watermarks to generated PDF. 

First, let's create a method that will specify the attributes of our watermark. We'll set the FontfontSize, and Opacity of our watermark:

public Paragraph createWatermarkParagraph(String watermark) throws IOException {
    
    PdfFont font = PdfFontFactory.createFont(StandardFonts.HELVETICA);
    Text text = new Text(watermark);
    text.setFont(font);
    text.setFontSize(56);
    text.setOpacity(0.5f);
    return new Paragraph(text);
}

Next, let's create a method that contains the logic of adding watermarks to our PDF document. The method will take DocumentParagraph, and offset as an argument. We'll calculate the position and rotation to place our watermark paragraph:

public void addWatermarkToGeneratedPDF(Document document, int pageIndex, 
  Paragraph paragraph, float verticalOffset) {
    
    PdfPage pdfPage = document.getPdfDocument().getPage(pageIndex);
    PageSize pageSize = (PageSize) pdfPage.getPageSizeWithRotation();
    float x = (pageSize.getLeft() + pageSize.getRight()) / 2;
    float y = (pageSize.getTop() + pageSize.getBottom()) / 2;
    float xOffset = 100f / 2;
    float rotationInRadians = (float) (PI / 180 * 45f);
    document.showTextAligned(paragraph, x - xOffset, y + verticalOffset, 
      pageIndex, CENTER, TOP, rotationInRadians);
}

We added the watermark paragraph to our document by invoking the showTextAligned() method. Next, let's write a method that generates a new PDF and adds a watermark. We'll invoke the createWatermarkParagraph() method and addWatermarkToGeneratedPDF() method:

public void createNewPDF() throws IOException {
    
    StoryTime storyTime = new StoryTime();
    String waterMark = "CONFIDENTIAL";
    PdfWriter writer = new PdfWriter(storyTime.OUTPUT_FILE);
    PdfDocument pdf = new PdfDocument(writer);
        
    try (Document document = new Document(pdf)) {
        document.add(new Paragraph(storyTime.alice)
          .setFont(PdfFontFactory.createFont(StandardFonts.TIMES_ROMAN)));
        document.add(new Paragraph(storyTime.paul));
        Paragrapgh paragraph = storyTime.createWatermarkParagraph(waterMark);
        for (int i = 1; i <= document.getPdfDocument().getNumberOfPages(); i++) {
            storyTime.addWatermarkToGeneratedPDF(document, i, paragraph, 0f);
        }
    }
}

Finally, let's write a unit test to verify the presence of watermarks:

@Test
public void givenNewTexts_whenGeneratingNewPDFWithIText() throws IOException {
 
    StoryTime storyTime = new StoryTime();
    String waterMark = "CONFIDENTIAL";
    LocationTextExtractionStrategy extStrategy = new LocationTextExtractionStrategy();
    try (PdfDocument pdfDocument = new PdfDocument(new PdfReader(storyTime.OUTPUT_FILE))) {
        for (int i = 1; i <= pdfDocument.getNumberOfPages(); i++) {
            String textFromPage = getTextFromPage(pdfDocument.getPage(i), extStrategy);
            assertThat(textFromPage).contains(waterMark);
        }
    }
}

Our test verifies the presence of watermarks in our generated PDF.

6. Adding Watermarks to Existing PDF

iText PDF library made it easy to add watermarks to existing PDFs. We'll first load our PDF document into our program. And use the iText library to manipulate our existing PDF.

To get started, we'll need to create a method to add a watermark paragraph. Since we created one in the last section, we can also use it here.

Next, we'll create a method that contains the logic that will help us add a watermark to our existing PDF. The method will accept DocumentParagraphPdfExtGState, pageIndex, and offSet as an argument. In the method, we'll create a new instance of PdfCanvas to write data to our PDF content stream. 

Then, we'll calculate the position and rotation of our watermark on the PDF. We'll flush the document and release the state to improve performance:

public void addWatermarkToExistingPDF(Document document, int pageIndex,
  Paragraph paragraph, PdfExtGState graphicState, float verticalOffset) {
    
    PdfDocument pdfDocument = document.getPdfDocument();
    PdfPage pdfPage = pdfDocument.getPage(pageIndex);
    PageSize pageSize = (PageSize) pdfPage.getPageSizeWithRotation();
    float x = (pageSize.getLeft() + pageSize.getRight()) / 2;
    float y = (pageSize.getTop() + pageSize.getBottom()) / 2;
    
    PdfCanvas over = new PdfCanvas(pdfDocument.getPage(pageIndex));
    over.saveState();
    over.setExtGState(graphicState);
    float xOffset = 14 / 2;
    float rotationInRadians = (float) (PI / 180 * 45f);
    
    document.showTextAligned(paragraph, x - xOffset, y + verticalOffset, 
      pageIndex, CENTER, TOP, rotationInRadians);
    document.flush();
    over.restoreState();
    over.release();
}

Finally, let's write a method to add a watermark to an existing PDF. We'll invoke the createWatermarkParagraph() to add a watermark paragraph. Also, we'll invoke addWatermarkToExistingPDF() to handle the task of adding watermarks to the pages:

public void addWatermarkToExistingPdf() throws IOException {
    
    StoryTime storyTime = new StoryTime();
    String outputPdf = "output/aliceNew.pdf";
    String watermark = "CONFIDENTIAL";
    
    try (PdfDocument pdfDocument = new PdfDocument(new PdfReader("output/alice.pdf"), 
      new PdfWriter(outputPdf))) {
        Document document = new Document(pdfDocument);
        Paragraph paragraph = storyTime.createWatermarkParagraph(watermark);
        PdfExtGState transparentGraphicState = new PdfExtGState().setFillOpacity(0.5f);
        for (int i = 1; i <= document.getPdfDocument().getNumberOfPages(); i++) {
            storyTime.addWatermarkToExistingPage(document, i, paragraph, 
              transparentGraphicState, 0f);
        }
    }
}

Let's write a unit test to verify the presence of watermarks:

@Test
public void givenAnExistingPDF_whenManipulatedPDFWithITextmark() throws IOException {
    StoryTime storyTime = new StoryTime();
    String outputPdf = "output/aliceupdated.pdf";
    String watermark = "CONFIDENTIAL";
        
    LocationTextExtractionStrategy extStrategy 
      = new LocationTextExtractionStrategy();
    try (PdfDocument pdfDocument = new PdfDocument(new PdfReader(outputPdf))) {
        for (int i = 1; i <= pdfDocument.getNumberOfPages(); i++) {
            String textFromPage = getTextFromPage(pdfDocument.getPage(i), extStrategy);
            assertThat(textFromPage).contains(watermark);
        }
    }
}

Our test verifies the presence of watermarks in our existing PDF.

7. Conclusion

In this tutorial, we explored the iText PDF library by generating a new PDF. We added watermarks to our generated PDF and later add watermarks to an existing PDF. iText library looks powerful in manipulating PDFs. The complete code is available over on GitHub.

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