ONOS applications can hook into the ONOS CLI and GUI. When generating your application, you can use overlays to generate the classes needed to give your application access to these services.
Command Line Overlay
To allow your application access to add commands to the ONOS CLI, overlay the CLI interface like this:
onos> sample Hello World
User Interface Overlays
To allow your application to add to the ONOS web UI, overlay the UI interface like this:
onos-create-app ui org.foo.app foo-app 1.0.0
Finally your application is loaded and running withing ONOS. Also, the generation process has generated an entire project which can be loaded into your favourite JAVA editor.