What does the value in @RestController annotation do?

Creating a simple Spring Boot application using Maven. I have given a value with RestController annotation, but it doesn't work. If I don't use the RestController's value, it works. I want to know, why it's not working and What's the use of value in @RestController?

http://localhost:9090/app/hello this gives error

http://localhost:9090/hello this works fine

@RestController("/app") What's the purpose of "/app" this value inside @RestController annotation?

P.S: I know, I can use @RequestMapping("/app") on ScraperResource class.

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

@RestController("/app")
public class ScraperResource {
	@GetMapping("hello")
	public String testController() {
		return "Hello";
	}
}

application.properties

server.port=9090

That is because the "/app" inside your RestController has nothing to do with your URL mapping, but rather with a "logical component" name being used internally Spring.

You should do this instead, if you want to prefix all your controller methods with /app (or just leave it out).

@RestController
@RequestMapping("/app")
public class ScraperResource {

    @GetMapping("hello")
    public String testController() {
        return "Hello";
    }
}

Without @RestController Spring won't know that this class should handle HTTP calls, so it is a needed annotation.

As per the Java Doc associated with the @RestController annotation, this is the meaning of the value that you are passing to it:

/**
	 * The value may indicate a suggestion for a logical component name,
	 * to be turned into a Spring bean in case of an autodetected component.
	 * @return the suggested component name, if any (or empty String otherwise)
	 * @since 4.0.1
	 */
	@AliasFor(annotation = Controller.class)
	String value() default "";

As such, it does not influence or affect what URL your endpoint is accessible with. If you want to add a top-level mapping you can use the @RequestMapping("/app") on the class-level as you mentioned.

The parameter in the @Controller annotation allows you to name the Controller. In cases where there are multiple beans of the same type, the bean name can be used along with the @Qualifier annotation to let Spring know which component to inject during autowiring.

From the documentation:

> When a component is autodetected as part of the scanning process, its
> bean name is generated by the BeanNameGenerator strategy known to that
> scanner. By default, any Spring stereotype annotation (@Component,
> @Repository, @Service, and @Controller) that contains a name value
> thereby provides that name to the corresponding bean definition.
>
> If such an annotation contains no name value or for any other detected
> component (such as those discovered by custom filters), the default
> bean name generator returns the uncapitalized non-qualified class
> name.

Read more about autowire disambiguation, and how to use the component name with @Qualifier here.

> By default, Spring resolves @Autowired entries by type. If more than
> one bean of the same type is available in the container, the framework
> will throw a fatal exception.
>
> To resolve this conflict, we need to tell Spring explicitly which bean
> we want to inject.
>
> When there are multiple beans of the same type, it's a good idea
> to use @Qualifier to avoid ambiguity.
>
> Please note that the value of the @Qualifier annotation matches with
> the name declared in the @Component annotation of our FooFormatter
> implementation.