How do I start two instances of Spring Boot on different ports?

I tried to use this answer to start another instance of Spring Boot

Spring Boot - how to configure port

mvn spring-boot:run -Dserver.port=8081

It gave an error

***************************
APPLICATION FAILED TO START
***************************

Description:

The Tomcat connector configured to listen on port 8080 failed to start.

It also restarted the other instance (live reload?).


It worked with

env SERVER_PORT=8081 mvn spring-boot:run

It also works with (Spring Boot 1.5.x)

mvn spring-boot:run -Drun.arguments="--server.port=8081"