See rosjava_core readme.
Note that there is no master branch. Rosjava messages creates message artifacts from the released rosdistros. Subsequently there is no meaning for a master branch.
If building rosjava sources from master branches, use the branch from rosjava_messages that corresponds to the rosdistro that you are compiling with.
Your regular rosjava message project does not need this complexity!
The gradle magic in settings.gradle is responsible for kickstarting the following very useful things for a big release of message packages:
- Generating an artifact for every listed (in the package.xml) ros message package.
- Discovering & applying the underlying package version and dependency information to the artifact.
It manages the correspondence by dynamically spawning subprojects from a template build.gradle. This saves a very large amount of tedious and repetitive gradle management.
The package version and dependency information is extracted by the catkin gradle plugin that can be found in rosjava_bootstrap.