Understanding the @Controller Annotation in Spring MVC

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質問：

What does @Controller annotation?

回答：

The @Controller annotation is part of the Spring Framework, specifically within the Spring MVC (Model-View-Controller) module. It is used to indicate that a particular class serves as a controller in the MVC architecture. Here’s a breakdown of its purpose and functionality:

Purpose of @Controller

  1. Component Scanning: When you annotate a class with @Controller, it becomes a candidate for component scanning. This means that Spring will automatically detect this class during the component scanning process and register it as a Spring bean in the application context.

  2. Handling Requests: Classes annotated with @Controller are typically used to handle web requests. They contain methods that are mapped to specific URL patterns, allowing them to respond to HTTP requests (GET, POST, etc.).

  3. Returning Views: In a typical Spring MVC application, methods in a controller class can return a view name (e.g., a JSP page or a Thymeleaf template) that should be rendered in response to a request. The view resolver will then resolve this view name to the actual view.

  4. Model Binding: Controllers can also handle data binding, where they can accept input from web forms and bind it to Java objects (models). This is often done using @ModelAttribute or method parameters.

Example Usage

Here’s a simple example of a Spring MVC controller using the @Controller annotation:

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

@Controller
@RequestMapping("/greeting")
public class GreetingController {

    @GetMapping
    public String greeting(Model model) {
        model.addAttribute("message", "Hello, World!");
        return "greeting"; // This will resolve to a view named "greeting"
    }
}

Key Points

  • Annotation: @Controller is a specialization of the @Component annotation, which means it inherits all the characteristics of a Spring component.
  • Request Mapping: You can use additional annotations like @RequestMapping, @GetMapping, @PostMapping, etc., to specify the URL patterns that the controller methods should handle.
  • View Resolution: The return value of the controller method is typically a logical view name, which is resolved to an actual view by a view resolver.

In summary, the @Controller annotation is a fundamental part of building web applications with Spring MVC, allowing developers to create structured and maintainable web applications by separating concerns between the controller, model, and view.

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