Mustangproject is a tool to write, validate and parse Factur-X/ZUGFeRD and (CII)XRechnung e-invoices.It is a (permissive) open source cross platform command line tool and a Java library. The command line tool e.g. allows to extract and combine PDF and XML from/to Factur-X as well as validate e-invoices, convert XML to HTML or migrate from ZUGFeRD v1 to v2. The library additionally allows to generate the XML to be embedded either by implementing interfaces or actively creating objects.
minor feature: XRechnung 2.1.1 validation is now possible and a PDF issue (#249) has been fixed