How does a controller determine which method to handle based on the HTTP request?

In Java web frameworks (such as Spring MVC), controllers use annotations to map HTTP requests to methods.

Main mechanism:

• Annotations, such as @GetMapping, @PostMapping, or @RequestMapping, specify the path and HTTP method.
• When a request arrives, the framework analyzes the URL and HTTP method.
• It finds the controller method with the appropriate annotation matching the request path and method.
• It invokes this method to handle the request.

Example:

@RestController
@RequestMapping("/users")
public class UserController {

    @GetMapping("/{id}")
    public User getUser(@PathVariable String id) {
        // handle GET /users/{id}
        return userService.findById(id);
    }

    @PostMapping
    public User createUser(@RequestBody User user) {
        // handle POST /users
        return userService.save(user);
    }
}

Here, a GET request to /users/123 will invoke the getUser method, and a POST to /users will invoke createUser. The framework automatically parses the HTTP request and determines the appropriate method.