Your Spring Boot application has an embedded copy of Tomcat inside it and that copy of Tomcat by default would listen on port 8080.
But one and only one application can use a given TCP/IP port in a single OS instance. That's a fundamental limitation of TCP/IP networking.
It's likely that you've already got a standalone Tomcat running or that you launched a copy of Tomcat from Eclipse and that app grabbed port 8080, denying it to the Tomcat instance inside your Spring Boot app.
Port 8080 is also used by other webapp servers, and they, too can hog that port if you're running one of them.
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