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Explain @Controller annotation example

Explain @Controller annotation example

The `@Controller` annotation in Spring is used to define a controller class that handles web requests. It’s a specialization of the `@Component` annotation, allowing Spring to detect and register it as a Spring bean. Unlike `@RestController`, which returns data directly to the client, `@Controller` typically returns a view name, which is resolved to an actual view by a `ViewResolver`.

@Controller annotation

Example of Controller annotation example

Step 1: Create a Spring Boot Application

Ensure you have the necessary dependencies in your `pom.xml` (for Maven) or `build.gradle` (for Gradle).

Example
Maven Dependency:
```xml
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
```

Gradle Dependency:
```groovy
implementation 'org.springframework.boot:spring-boot-starter-web'
```

Step 2: Create the Main Application Class

Create the main class for the Spring Boot application.

Example
```java
package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}
```

Explanation:

  • @SpringBootApplication: Indicates a configuration class that declares one or more @Bean methods and also triggers auto-configuration and component scanning.

Step 3: Create a Controller

Create a simple controller using the @Controller annotation.

Example
```java
package com.example.demo.controller;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
@RequestMapping("/web")
public class HelloController {

    @GetMapping("/hello")
    public String sayHello(Model model) {
        model.addAttribute("message", "Hello, World!");
        return "hello";
    }
}
```

- Explanation:
  - `@Controller`: Indicates that the class is a Spring MVC controller.
  - `@RequestMapping("/web")`: Base URL for all endpoints in this controller.
  - `@GetMapping("/hello")`: Maps HTTP GET requests to the `sayHello` method.
  - `Model model`: Used to pass data to the view.
  - `model.addAttribute("message", "Hello, World!")`: Adds an attribute to the model that can be accessed in the view.
  - `return "hello"`: Returns the view name to be resolved.

Step 4: Create a View

Create an HTML file named hello.html in the src/main/resources/templates directory.

Example
```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Hello</title>
</head>
<body>
    <h1>${message}</h1>
</body>
</html>
```

Explanation:

  • ${message}: Placeholder that will be replaced by the actual message passed from the controller.

Step 5: Running the Application

Run the application from the main class (DemoApplication). When you navigate to http://localhost:8080/web/hello in your web browser, you should see the message: “Hello, World!” displayed on the page.

Conclusion

  • @Controller: Used to define a Spring MVC controller that returns view names to be resolved by a ViewResolver.
  • Example: Demonstrated how to use @Controller to create a simple web page that displays a message.

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