How to fill a PDF/A-1 in Java

I've got a pdf/a-1 form which I have to fill in Java.

The problem is, it works perfectly fine with normal pdf but not with pdf/a-1.

Using a normal pdf form, I display the form fields and implement an HashMap from which I generate myself an fdf file.

Then I import my fdf file into the normal pdf form using pdftk.exe and it works fine.

Using the pdf/a-1 form, I can't display fields and If I try to import an fdf file, it doesn't fill the form.

I don't know if there is a simple method which can fill a pdf/a-1 form or if I should use some mechanisms to make my pdf/a-1 fillable?

I've tried pdfbox and itext but didn't really find anything.

Hope you could help.

Best regards.