FirstSpringInitializrApplication.java is the launcher for Spring Boot:

    package com.mastering.spring;

    import org.springframework.boot.SpringApplication;
    import org.springframework.boot.autoconfigure.SpringBootApplication;

    @SpringBootApplication
    public class FirstSpringInitializrApplication {

      public static void main(String[] args) {
        SpringApplication.run(FirstSpringInitializrApplication.class, args);
      }

    }