iText7: Error at file pointer when merging two pdfs - itext

Question

We are in the last steps of evaluating iText7. We use iText 7.1.0 and html2pdf 2.0.0.
What we do: we send a json_encoded collection with pdf-data (which includes html for header, body and footer) to our Java app. There we iterate over the collection, create a byteArrayOutputStream for each pdf-data element and merge them together. We then send the results to a script which echoes it to e.g. a browser. Although the pdf is displayed correctly, we encounter errors while creating it:
com.itextpdf.io.IOException: Error at file pointer 226,416.
...
Caused by: com.itextpdf.io.IOException: xref subsection not found.
... 73 common frames omitted
If we create only one part of the collection, no error is thrown.
Iterate over collection and merge:
#RequestMapping(value = "/pdf", method = RequestMethod.POST, produces = MediaType.APPLICATION_PDF_VALUE)
public byte[] index(#RequestBody PDFDataModelCollection elements, Model model) throws IOException {
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
PdfWriter writer = new PdfWriter(byteArrayOutputStream);
try (PdfDocument resultDoc = new PdfDocument(writer)) {
for (PDFDataModel pdfDataModel : elements.getElements()) {
PdfReader reader = new PdfReader(new ByteArrayInputStream(creationService.createDatasheet(pdfDataModel)));
try (PdfDocument sourceDoc = new PdfDocument(reader)) {
int n = sourceDoc.getNumberOfPages(); //<-- IOException on second iteration
for (int i = 1; i <= n; i++) {
PdfPage page = sourceDoc.getPage(i).copyTo(resultDoc);
resultDoc.addPage(page);
}
}
}
}
return byteArrayOutputStream.toByteArray(); //outputs the final pdf
}
Creation of part:
public byte[] createDatasheet(PDFDataModel pdfDataModel) throws IOException {
PdfWriter writer = new PdfWriter(byteArrayOutputStream);
//Initialize PDF document
PdfDocument pdfDoc = new PdfDocument(writer);
try (
Document document = new Document(pdfDoc)
) {
//header, footer, etc
//body
for (IElement element : HtmlConverter.convertToElements(pdfDataModel.getBody(), this.props)) {
document.add((IBlockElement) element);
}
footer.writeTotalNumberOnPages(pdfDoc);
}
return byteArrayOutputStream.toByteArray();
}
We are grateful for any suggestion.

Answer 1

In createDatasheet you appear to re-use some byteArrayOutputStream without clearing it first.
In the first iteration, therefore, everything works as desired, at the end of createDatasheet you have a single PDF file in it.
In the second iteration, though, you have two PDF files in that byteArrayOutputStream, one after the other. This concatenation does not form a valid single PDF.
Thus, byteArrayOutputStream.toByteArray() returns something broken.
To fix this, either make the byteArrayOutputStream local to createDatasheet and create a new instance every time or alternatively reset byteArrayOutputStream at the start of createDatasheet:
public byte[] createDatasheet(PDFDataModel pdfDataModel) throws IOException {
byteArrayOutputStream.reset();
PdfWriter writer = new PdfWriter(byteArrayOutputStream);
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