The source code is available on GitHub.
The build/dependency manangement system is Maven.
The development environment uses the Netbeans IDE, but this is not mandatory, any development environment which can work with a Maven project can be used.
Sources for this release can be found here.
Alternatively the project can be forked from here to obtain the whole source tree. Tag v4.0.0 refers to the sources relevant to this release.
Developers modifying the source code are encouraged to submit their changes back to the project using a Pull-Request (for consideration for inclusion in a future release).