I’d recommend checking out the IIQ javadoc at <iiqLocation>/doc/javadoc
and looking at the sailpoint.plugin
package.
Here is a presentation from developer days this year and there is a download for the plugin example as well: Introduction to Plugin Development - #2 by henrik_kaaber
There are a bunch of plugins available here for reference: IdentityIQ Plugins-Compass
And a developer guide here: https://community.sailpoint.com/t5/Plugin-Framework/Plugin-Development/ta-p/145288