Use annotations to configure your web application.

@SpringBootApplication - uses @Configuration,

@EnableAutoConfiguration and @ComponentScan.

@EnableAutoConfiguration - make Spring guess the

configuration based on the classpath.

@Controller - marks the class as web controller,

capable of handling the requests.

@RestController - a convenience annotation of a

@Controller and @ResponseBody.

@ResponseBody - makes Spring bind method's

return value to the web response body.

@RequestMapping - specify on the method in the

controller, to map a HTTP request to the URL to this method.

@RequestParam - bind HTTP parameters into

method arguments.

@PathVariable - binds placeholder from the URI to

the method parameter. 

https://www.baeldung.com/spring-mvc-annotations