I can successfully start spring-boot with mvn spring-boot, the documentation mentions to gracefully exit the application hit ctrl-c.
mvn spring-boot
ctrl-c
Here is what I do on Mac:
kill `lsof -i -n -P | grep TCP | grep 8080 | tr -s " " "\n" | sed -n 2p`
It finds the PID using 8080 and kills it.