Sample code is at: https://github.com/ludoch/appengine-spring-boot
We can test to see what is not working in local devappserver:
with running the ./sdk_assembly/target/appengine-java-sdk/bin/dev_appserver.sh script locally built
Main page / does not really work (stuck) same for /aliens handler
Notice the version = 6 in https://github.com/ludoch/appengine-spring-boot/blob/master/src/main/webapp/WEB-INF/web.xml#L2
This is the trigger for using EE10 with GAE Jetty12.