The src sub-directory contains modules included in Apache SIS releases. Those modules have no prerequisites other than Java Development Kit (JDK) and Gradle for building from the sources. The build command is:
gradle assemble
The JAR files will be in the build/libs/ directory. See the parent directory for more information.
The optional directory contains modules that are also part of Apache SIS releases, but requiring prerequisites such as acceptance of license agreements more restrictive than Apache 2 license.