@RestController vs @Controller.

@RestController is literally @Controller + @ResponseBody — that one fact answers the question, and the rest is the consequence. Both make a class a Spring MVC handler discovered by component scanning; the difference is what happens to a method's return value.

@Controller

The classic web-MVC stereotype. A handler method's return value is interpreted as a logical view name, and a ViewResolver renders a template (Thymeleaf, JSP) to produce HTML. You populate a Model for the template to consume.

@Controller
class OrderPageController {
    @GetMapping("/orders/{id}")
    String show(@PathVariable Long id, Model model) {
        model.addAttribute("order", service.find(id));
        return "order-detail"; // -> resolves to a template, renders HTML
    }
}

To make a single method in a @Controller return data instead of a view, you annotate it with @ResponseBody — then its return value goes through an HttpMessageConverter and is serialized straight to the body.

@RestController

A meta-annotation that bundles @Controller and @ResponseBody, so every method behaves as if @ResponseBody-annotated. Return values bypass view resolution entirely and are written to the response body via a message converter — JSON by default with Jackson on the classpath.

@RestController
@RequestMapping("/api/orders")
class OrderApiController {
    @GetMapping("/{id}")
    OrderDto get(@PathVariable Long id) {
        return service.find(id); // serialized to JSON, no view resolution
    }
}

When each fits

Use @Controller for server-rendered pages where the output is HTML from a template. Use @RestController for REST/JSON (or XML) APIs where you return data objects. A common hybrid is a @Controller with a few @ResponseBody methods, but if the whole class is an API, @RestController is the honest choice.