Programatically Filling Out PDFs in Java

I just got done dealing with an interesting problem. I had one of those PDFs that’s a form you can fill out and was tasked with programatically filling it out. So, I busted out my google-fu and came across PDFBox. It’s a handy and fairly no-nonsense little utility not just for filling out forms, but for manipulating PDFs in general. I had no idea something like this existed (mainly because I’d never really thought about it).

I downloaded the jar for PDFBox and wrote a simple class to test out my theory. In setting up the class and poking around randomly in the documentation, I saw that the main object of interest was a PDDocument. So, I set about instantiating one and discovered that you needed to use something called a COSDocument, which took something called a RandomAccess (not the standard version of the file, but a special version from PDFBox), and then my eyes started to cross and I pulled back and discovered that this is really what I want:

private static PDDocument _pdfDocument;

private static void populateAndCopy(String originalPdf, String targetPdf) throws IOException, COSVisitorException {
	_pdfDocument = PDDocument.load(originalPdf);
	
	_pdfDocument.save(targetPdf);
	_pdfDocument.close();
}

Much easier. Now, as I got down to business of trying this out, I discovered via runtime exception that I needed two external dependencies. One was apache commons logging and the other was something called fontbox that was right there along with the PDFBox download, but I ignored in the beginning. Probably with this code alone you wouldn’t necessarily hit both of those problems, but you will eventually, so better to add those jars right up front.

So far I was able successfully to open a PDF and save it as another file, which isn’t exactly a new capability for any programming language with file I/O, so I added something a little more concrete to the mix:

private static void populateAndCopy(String originalPdf, String targetPdf) throws IOException, COSVisitorException {
	_pdfDocument = PDDocument.load(originalPdf);
	
	_pdfDocument.getNumberOfPages();
	
	setField("SomeFieldName", "SomeFieldValue");
	_pdfDocument.save(targetPdf);
	_pdfDocument.close();
}

Lo and behold, liftoff. I actually got the right number of pages in the document. Now I was getting somewhere. Time to get down to the real business. I did this by going to the “Cookbook” section of the project and seeing what was under form generation. Seeing that this just took me to the javadoc for examples, I went and downloaded the example code and pasted it into my project (modifying it to conform to the Egyptian-style braces. In this fashion, I had a method that would print out all of the fields in the PDF as well as a method that would let me set fields by name. When I ran the one that printed out all of the fields, I got a runtime exception about some deprecated method and I discovered that in the source code for that method, it just threw an exception. Presumably, the written examples predated some change that had deprecated that method — deprecated it with extreme prejudice!

Well, I’d like to say that I fought the good fight, but I didn’t. I just deleted the offending call since it was just writing to console. So here is the end result of that effort:

public class Populater {

	private static PDDocument _pdfDocument;
	
	public static void main(String[] args) {
		
		String originalPdf = "C:\\blah\\blah\\input.PDF";
		String targetPdf = "C:\\blah\\blah\\input.PDF";
		
		try {
			populateAndCopy(originalPdf, targetPdf);
		} catch (IOException | COSVisitorException e) {
			e.printStackTrace();
		}
		
		System.out.println("Complete");
	}

	private static void populateAndCopy(String originalPdf, String targetPdf) throws IOException, COSVisitorException {
		_pdfDocument = PDDocument.load(originalPdf);
		
		_pdfDocument.getNumberOfPages();
		//printFields();  //Uncomment to see the fields in this document in console
		
		setField("SomeFieldName", "SomeFieldValue");
		_pdfDocument.save(targetPdf);
		_pdfDocument.close();
	}
	
    public static void setField(String name, String value ) throws IOException {
        PDDocumentCatalog docCatalog = _pdfDocument.getDocumentCatalog();
        PDAcroForm acroForm = docCatalog.getAcroForm();
        PDField field = acroForm.getField( name );
        if( field != null ) {
            field.setValue(value);
        }
        else {
            System.err.println( "No field found with name:" + name );
        }
    }

    @SuppressWarnings("rawtypes")
	public static void printFields() throws IOException {
        PDDocumentCatalog docCatalog = _pdfDocument.getDocumentCatalog();
        PDAcroForm acroForm = docCatalog.getAcroForm();
        List fields = acroForm.getFields();
        Iterator fieldsIter = fields.iterator();

        System.out.println(new Integer(fields.size()).toString() + " top-level fields were found on the form");

        while( fieldsIter.hasNext()) {
            PDField field = (PDField)fieldsIter.next();
               processField(field, "|--", field.getPartialName());
        }
    }
    
    @SuppressWarnings("rawtypes")
	private static void processField(PDField field, String sLevel, String sParent) throws IOException
    {
        List kids = field.getKids();
        if(kids != null) {
            Iterator kidsIter = kids.iterator();
            if(!sParent.equals(field.getPartialName())) {
               sParent = sParent + "." + field.getPartialName();
            }
            
            System.out.println(sLevel + sParent);
            
            while(kidsIter.hasNext()) {
               Object pdfObj = kidsIter.next();
               if(pdfObj instanceof PDField) {
                   PDField kid = (PDField)pdfObj;
                   processField(kid, "|  " + sLevel, sParent);
               }
            }
         }
         else {
             String outputString = sLevel + sParent + "." + field.getPartialName() + ",  type=" + field.getClass().getName();
             System.out.println(outputString);
         }
    }
}

Going forward, I’ll certainly factor this into a new class and probably extract some methods and improve warning avoidance, but that’s the gist of it. This didn’t exactly take a long time, but it probably could have gone quicker if I’d known a little more up-front and had all example code in one place. Hopefully it helps you in that capacity.