iText is a great Java PDF library. They also have an API for creating barcodes. You don't need to be creating a PDF to use it.
This page has the details on creating barcodes. Here is an example from that site:
BarcodeEAN codeEAN = new BarcodeEAN();
codeEAN.setCodeType(codeEAN.EAN13);
codeEAN.setCode("9780201615883");
Image imageEAN = codeEAN.createImageWithBarcode(cb, null, null);
The biggest thing you will need to determine is what type of barcode you need. There are many different barcode formats and iText does support a lot of them. You will need to know what format you need before you can determine if this API will work for you.
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