I don't find any example on how to secure spring boot app.
There is a very popular generator based on Spring-boot and angular called JHipster.
Jhipster already supports OAuth2. See the sample https://github.com/jhipster/jhipster-sample-app-oauth2. So I dont think it will be difficult to integrate Keycloak and Jhipster.
Recently they introduced module(plug-in) feature where anyone can write a plugin.
It will be great if you can add a module on showing how to integrate Keycloak with JHipster.
Keycloak is such a wonderful project but it is not known to world. Jhipster is a very popular project now. If these two are integrated then Keycloak will be automatically advertised to whole new world. Number of users and contributors will be drastically increased.