What is?
This is a class-level annotation that tells Spring that your class is a controller. A controller is an entry point for a web application. This allows you to define a path to communicate with your backend using REST methods or by serving and responding html forms.
This annotation is more general and allows your controller to serve REST endpoints and serve webpage content, which is very common in the MVC pattern.
Why is this important
This annotation is what allows your front end to communicate with the backend, allowing you to define routes, pages, dynamic content, etc. If you are working with web applications, this annotation is one of the most important ones.
How to use @Controller
Defining a controller class
import org.springframework.stereotype.Controller;
@Controller
public class MyController {
// controller methods go here
}
Defining an endpoint and its respective HTTP method
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
@Controller
public class MyController {
@GetMapping("/greeting")
public String greeting() {
return "greeting"; // resolves to a view named "greeting"
}
}
Add a page in the location src/main/resources/templates/greeting.html with the same name as the string returned by the controller to return an HTML page from the server
<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
<meta charset="UTF-8" />
<title>Greeting</title>
</head>
<body>
<h1>Hello!</h1>
</body>
</html>
Testing
Testing @Controller without REST
Using curl to call our controller
curl -i http://localhost:8080/greeting
The result should look like this*:*
HTTP/1.1 200
Content-Type: text/html;charset=UTF-8
Content-Language: pt-BR
Transfer-Encoding: chunked
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<title>Greeting</title>
</head>
<body>
<h1>Hello!</h1>
</body>
</html>
Creating a REST endpoint with @ResponseBody
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ResponseBody;
@Controller
public class RestControllerExample {
@GetMapping("/api/message")
@ResponseBody
public ResponseEntity<String> getMessage() {
return ResponseEntity.ok("Hello, World!");
}
}
Testing @Controller with @ResponseBody
When calling with curl :
curl -i http://localhost:8080/api/message
It should return like this:
HTTP/1.1 200
Content-Type: text/plain;charset=UTF-8
Content-Length: 13
Hello, World!
Conclusion
This is one of the most important and common annotations when it comes to creating web applications with Spring. The usage is straightforward, making it easier to add new controllers and endpoints to your application.
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