Questions about learning Symfony

1、Can a service only be retrieved from the container if its class is explicitly defined in services.yaml?

No.
A service can be retrieved from the container in two cases:
*1. Explicit definition in services.yaml. *
If the class is not in a PRS-4 autoload directory or its constructor arguments have to be declared mannually, the service class has to be defined in services.yaml.
2. Implicit(Auto-registeration)
The class is in a PSR-4 autoload directory AND Not excluded in services.yaml

2、Does the #[Route]Attribute Require the Controller to Be in src/Controller?​

No.
Symfony’s #[Route]attribute works ​​regardless of where the controller is located​​, as long as:

1. The class is ​​registered as a service​​ (auto-registered via PSR-4 autoloading or explicitly defined in services.yaml).
2. The route is properly ​​configured in Symfony’s routing system​​.

3. ​​Does Symfony’s #[Route]Attribute Have a Fallback Mechanism?​

No. The #[Route]attribute itself does ​​not​​ have a built-in fallback mechanism. However, you can implement ​​fallback behavior​​ in Symfony using:
1. ​​Route priorities​​ (e.g., /blog/{slug}vs. /blog/list).

#[Route('/blog/{slug}', name: 'blog_show')]
public function show(string $slug): Response { ... }

#[Route('/blog/list', name: 'blog_list')]
public function list(): Response { ... }

A request to /blog/list match blog_list, not blog_show. However, /blog/other_list match blog_show.

#[Route('/{any}', name: 'fallback', priority: -10)]
public function fallback(): Response
{
    // Handle 404 or redirect
    throw $this->createNotFoundException();
}

Place this ​​last​​ (low priority) to avoid overriding other routes.

2. ​​Custom route conditions​​ (e.g., requirements, methods).
3. ​​Event listeners​​ (e.g., kernel.exceptionfor 404 handling).

4. How to custom route conditions? Give me an example.

5. Service object gotten by the container is a singleton object?

YES.
By default, the service object obtained from the container is a singleton. However, the object is reset per each request. Obviously, the service is stateless.

6. In my legacy application, there are many global variables. How to handle them in Symfony?

Use a factory to create them and pass DI(Dependency Injection) as contructor arugments to use them somewhere else.