How to Convert XML to PDF

1. Overview

Java provides several libraries and APIs for working with XML and PDF documents. Converting XML to PDF in Java involves parsing the XML data, applying styles and formatting, and generating the PDF output.

This article explores different methods and libraries to convert XML to PDF in Java.

2. Understanding the Conversion Process

Before discussing implementation details, let’s highlight the essential steps to convert XML to PDF. This process typically entails two primary steps:

1. The first step is XML parsing, where the XML content is analyzed, and its structure and textual data are extracted. In Java, developers have access to various XML parsing libraries such as DOM (Document Object Model), SAX (Simple API for XML), and StAX (Streaming API for XML).
2. The second step involves PDF generation. This step includes creating PDF components such as paragraphs, tables, images, and other elements. These components are then organized and formatted according to the structure defined within the XML document.

3. Using Apache FOP (Formatting Objects Processor)

Apache FOP is a robust open-source library for converting XML data into various output formats, including PDF. Furthermore, FOP transforms XML content according to XSL-FO stylesheets, ultimately generating high-quality PDF documents.

3.1. How Apache FOP Works

Apache FOP works through the following key stages:

• XML Parsing: Apache FOP begins by parsing the input XML data. This process involves extracting the structure and content of the XML document, which typically represents the data to be presented in the final PDF output.
• XSL-FO Transformation: FOP applies an XSL-FO stylesheet to format XML elements into corresponding PDF elements like paragraphs, tables, and images, ensuring adherence to specified styles and layout rules.
• PDF Rendering: After transforming the content into XSL-FO format, Apache FOP renders it into a visually appealing PDF document that accurately reflects the original XML content.
• Output Generation: Finally, FOP generates a standalone PDF file encapsulating the formatted content, ready for saving, display, or distribution, suitable for various printing and viewing purposes.

3.2. Example: Converting XML to PDF using Apache FOP

To use the Apache FOP library and its features for converting XML to PDF, it is necessary to integrate the Apache FOP dependency into our project’s build configuration.

If we’re using Maven, we can achieve this by including the FOP dependency in our pom.xml file:

<dependency>
    <groupId>org.apache.xmlgraphics</groupId>
    <artifactId>fop</artifactId>
    <version>2.9</version>
</dependency>

Now, let’s create a method to convert XML to PDF using Apache FOP in Java:

void convertXMLtoPDFUsingFop(String xmlFilePath, String xsltFilePath, String pdfFilePath) throws Exception {
    FopFactory fopFactory = FopFactory.newInstance(new File(".").toURI());
    FOUserAgent foUserAgent = fopFactory.newFOUserAgent();

    try (OutputStream out = new BufferedOutputStream(Files.newOutputStream(new File(pdfFilePath).toPath()))) {
        Fop fop = fopFactory.newFop(MimeConstants.MIME_PDF, foUserAgent, out);
        TransformerFactory factory = TransformerFactory.newInstance();
        Transformer transformer = factory.newTransformer(new StreamSource(new File(xsltFilePath)));
        Source src = new StreamSource(new File(xmlFilePath));
        Result res = new SAXResult(fop.getDefaultHandler());
        transformer.transform(src, res);
    }
}

The above example highlights the key steps involved in the conversion process, which include:

• Initialization: we first initialize Apache FOP by creating instances of FopFactory and FOUserAgent.
• Output Stream: we specify the output stream for the resulting PDF file.
• FOP Instance Creation: a new Fop instance is created using the FopFactory, specifying the PDF output format.
• XSLT Transformation: we create a Transformer instance from the XSLT stylesheet specified in the xsltFilePath parameter.
• Transformation Application: the XML data defined in the xmlFilePath parameter is transformed using the XSLT stylesheet, and the resulting FO (Formatting Object) is sent to the FOP instance for rendering.
• Output Generation: finally, the method generates the PDF output and saves it to the specified file path provided in the pdfFilePath parameter.

4. Using IText Library

The iText library is a robust and flexible solution for generating and managing PDF files. Its comprehensive capabilities enable seamless conversion of XML content into PDF documents, offering tailored customization and adaptability.

4.1. How IText Works

IText works through the following key stages:

• HTML to PDF Conversion: iText converts XML data to PDF using HTML as an intermediate format. XML is transformed into HTML, leveraging iText’s HTML parsing capabilities for seamless integration into PDF documents.
• XML Parsing and Rendering: iText parses XML content and renders it directly into PDF. It supports various XML formats like XHTML, SVG, and MathML and can apply CSS styles for precise control over layout and appearance.
• PDF Generation: After parsing, iText generates PDF elements such as text, images, and tables. Developers can customize the output with headers, footers, and other elements, ensuring compliance with PDF standards for printing and viewing.

4.2. Converting XML to PDF using iText in Java

To use the iText library for PDF generation in Java, We must incorporate the iTextPDF dependency in our project configuration. For Maven, we can add the iText dependency to our pom.xml file:

<dependency>
    <groupId>com.itextpdf</groupId>
    <artifactId>itextpdf</artifactId>
    <version>5.5.13.3</version>
</dependency>

Here’s a simple example demonstrating how to convert XML to PDF using iText in Java:

public static void convertXMLtoPDFUsingIText(String xmlFilePath, String pdfFilePath) throws Exception {
    try (FileOutputStream outputStream = new FileOutputStream(pdfFilePath)) {
        Document document = new Document();
        PdfWriter.getInstance(document, outputStream);
        document.open();

        String xmlContent = new String(Files.readAllBytes(Paths.get(xmlFilePath)));
        document.add(new Paragraph(xmlContent));
        document.close();
    }
}

The above example illustrates a straightforward method for converting XML to PDF using iText in Java. First, we create a new PDF document object. Next, we open the document to write content. Following this, we read the XML content from the specified file path and embed it into the PDF document.

Finally, we close the document and the output stream, ensuring the saved PDF file contains the XML content in a structured format.

5. Conclusion

Exploring XML to PDF conversion with FOP and iText in this article has provided us with valuable knowledge and practical skills. Mastery of these techniques enables us to efficiently convert XML data into refined PDF documents, enhancing the functionality of our Java applications.

As always, the source code is available over on GitHub.