It wasn't obvious when I searched for that interface in the doc. See http://stackoverflow.com/questions/35217354/how-to-add-custom-applicationcontextinitializer-to-a-spring-boot-application/35217768#35217768
Comment From: snicoll
This is already documented here
Comment From: wimdeblauwe
Correct link is now https://docs.spring.io/spring-boot/how-to/application.html#howto.application.customize-the-environment-or-application-context I believe.