This project is a demonstration project. It's meant to show my abilities to design and maintain a system, write qualitative code (including tests, of course), learn and use some of the technologies which I use on a daily basis at work and be one more reason to explore the latest changes of used frameworks and libraries.
Sub-projects use different build tools (maven and gradle).
Note that to be on the safe side, you should run git submodule update with the --init flag in case commits you just pulled added new submodules.
If you want to automate this process, you can add the --recurse-submodules flag to the git pull command (since Git 2.14). This will make Git run git submodule update right after the pull, putting the submodules in the correct state.