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);
      }
    }