Difference Between @Controller and @RestController Annotation in Spring

In Spring, both @Controller and @RestController annotations are used to define a controller in a Spring MVC application. However, they serve slightly different purposes, particularly concerning the type of response they produce.

Here's a breakdown of their differences:

@Controller:
- Primary Purpose: It's used to mark classes as Spring MVC Controllers.
- Response Type: By default, methods in a class annotated with @Controller are expected to return a view name (like the name of a JSP, Thymeleaf template, etc.).
- For RESTful Responses: If you wish a method in a @Controller-annotated class to return a raw response (like JSON or XML), you'd use the @ResponseBody annotation on that method. This annotation tells Spring to serialize the return value and send it in the response body.
- Usage Example:
```
@Controller
public class MyController {
    @RequestMapping("/greet")
    @ResponseBody
    public String greet() {
        return "Hello, World!";
    }
}
```
@RestController:
- Primary Purpose: It's a specialized version of @Controller that's intended for RESTful web services. Introduced in Spring 4.
- Response Type: All methods in a class annotated with @RestController will have @ResponseBody semantics by default, meaning their return values will be automatically serialized to JSON/XML and returned in the response body.
- For View Responses: Generally, if you're using @RestController, you're not returning views. If you need to return views, consider using @Controller or a mix of both based on requirements.
- Usage Example:
```
@RestController
public class MyRestController {
    @RequestMapping("/greet")
    public String greet() {
        return "Hello, World!";
    }
}
```

Key Takeaway:

@Controller is used when you're creating a traditional web application with server-side view rendering. For methods that should return a data response instead of a view, the @ResponseBody annotation is used.
@RestController is used when you're building a RESTful API or any backend that returns data directly to the client (e.g., AJAX calls from a SPA - Single Page Application). With @RestController, you don't need to use @ResponseBody on each method; it's applied to all methods by default.

It's crucial to choose the appropriate annotation based on the type of response your application or specific endpoint is designed to produce.

Examples of @Controller and @RestController in Spring applications:

Here's a simple example of @Controller:

@Controller
public class MyController {
    @GetMapping("/hello")
    public String sayHello() {
        return "hello";
    }
}

And an example of @RestController:

@RestController
public class MyRestController {
    @GetMapping("/hello")
    public String sayHello() {
        return "hello";
    }
}

In the @RestController example, the return value "hello" is directly sent as the response content, without rendering a view.