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@Controller and @RestController

@Controller

1. Returning Views with Controller

Traditional Spring MVC controller
Mainly used to return Views
- Returns a ViewName!
A ViewResolver is used to render a View from the ViewName returned by the Controller
- Finds and renders a View according to the ViewResolver configuration

2. Returning Data with Controller

When data needs to be returned instead of a View, the @ResponseBody annotation must be used for data return
- The @ResponseBody annotation allows a Controller to return data in JSON format!
- The returned object is serialized to JSON and returned to the user
When returning objects through a Controller, they are generally wrapped in ResponseEntity
- To return objects, HttpMessageConverter operates instead of ViewResolver
- HttpMessageConverter has several Converters registered, and the Converter used varies depending on the data to be returned
  - For strings: StringHttpMessageConverter
  - For objects: MappingJackson2HttpMessageConverter
Spring combines the client's Http Accept header and the server's Controller return type information to select the appropriate HttpMessageConverter to process it

@RestController

Restful web service controller
@Controller with @ResponseBody added
- An HTTP Response Body is generated
- The behavior is exactly the same as attaching @ResponseBody to @Controller!
- Allows using @ResponseBody, which was previously declared on each method, all at once
The main purpose is to return object data in JSON format!
Mainly used when developing REST APIs, and objects are wrapped in ResponseEntity for return