The purpose of using multi module application is to create a re-usable library and then used that library to build an application.
To Build the Spring Boot Application : Right Click on the Main Project (spring-boot-multi-module-app) -> Run As -> Maven Build...(configure with "clean install" in goals), then Apply & Run.
To Run the Spring Boot Application : Right Click on the Application Module (application) -> Run As -> Spring Boot App
Access your application endpoint by using the URL: http://localhost:8080/customer/100