If you don’t want the embedded server to start, just set the following property in you application.properties (or .yml):
spring.main.web-application-type=none
If your classpath contains the necessary bits to start a web server, Spring Boot will automatically start it. To disable this behaviour configure the WebApplicationType in your application.properties
Source: https://docs.spring.io/spring-boot/docs/current/reference/html/howto-embedded-web-servers.html
If you application really is a Web application, then you can easily change the port using the server.port property (in your application’s .properties/.yaml file, as a command line argument at startup, etc).