It’s really easier to just integrate via Intent. More reliable, and you get updates automatically. While I don’t advise it…
The complete source code is available from the ZXing project. You want to build core.jar from core/ and put it in your Android lib/ folder to include the core decoder in your app. You do not want to include javase. You can look to the code in android/ as it is the source to Barcode Scanner, but as the authors we’d suggest you not just copy and paste it.
It is Apache licensed which means you can use it freely, as long as you essentially give the user access to the license terms.