@Controller vs @RestController in Spring Boot

1. Introduction

In Spring Framework, @Controller and @RestController are annotations used to create web controllers. @Controller is used to mark a class as a web request handler, a part of Spring MVC that allows you to create views. @RestController combines @Controller and @ResponseBody, indicating that the return value of the methods should be bound to the web response body and is typically used for APIs.

2. Key Points

1. @Controller is a stereotype annotation indicating the class is a web controller.

2. @RestController is a specialized version of the controller that is a convenience annotation for creating RESTful controllers.

3. @Controller is typically used in combination with @ResponseBody or a view resolver for rendering views.

4. @RestController assumes every method returns a domain object instead of a view, which is directly written to the HTTP response as JSON or XML.

3. Differences

@Controller	@RestController
Used for creating MVC controllers.	Used for creating RESTful controllers.
Methods can return a view name.	Methods return a domain object instead of a view.
Requires @ResponseBody to indicate method response should be bound to the web response body.	Automatically binds method response to the web response body.

4. Example

// Example using @Controller
@Controller
public class MyViewController {

    @RequestMapping("/greeting")
    public String getGreeting(Model model) {
        model.addAttribute("message", "Hello World");
        return "greeting"; // Name of the view
    }
}

// Example using @RestController
@RestController
public class MyRestController {

    @RequestMapping("/data")
    public MyData getData() {
        return new MyData("Hello World"); // Data is directly written as JSON
    }
}

public class MyData {
    private String message;
    // Constructor and getter
}

Output:

// Output from @Controller would be the rendered view greeting.
// Output from @RestController would be the JSON string: {"message":"Hello World"}

Explanation:

1. MyViewController with @Controller is a traditional Spring MVC controller that returns a view name to be resolved and rendered.

2. MyRestController with @RestController directly returns a MyData object that will be automatically converted to JSON format and sent in the response body.

5. When to use?

- Use @Controller when building a web application that renders HTML views.

- Use @RestController when developing a RESTful web service that returns data in a format such as JSON or XML for consumption by various clients.