The mechanism of annotations in Symfony - how does it all work?

Question

I have started to learn Symfony (4.1) and I have a question about annotations.

As far as I know, annotation are just comments in terms of php and they are not a part of the language itself. However they are rather powerful thing in Symfony.

I want to know, how it all works.

• Is there a code preprocessor which parses the source files dynamically and creates new php entities?
• But if it's so, how does it affect the performance of an application?
• Why should I use special namespaces for certain annotations?

Put simply, I would like to know how do annotations in Symfony work, the mechanism of this feature.

Answer 1

Yes, indeed, annotations are not part of the language itself. But they're also not the part of Symfony framework.

Annotations are usually handled by doctrine/annotations package (most common). It utilizes reflection to read and parse these comments and transform them into annotation objects (every annotation has an annotation class which it represents). Then, its up to the library to make use of generated objects representing these annotations.

So to answer first question - yes, there is a preprocessor. But it doesn't "create new php entities", because its the job for the library that uses those annotations (e.g. Symfony framework or Doctrine ORM).

How it affects the performance, depends on the library that uses them. If they would be parsed on every request, that would indeed affect performance. So e.g. Symfony and Doctrine ORM cache this data or create proxy classes etc.

So the answer to second question is - it might if used incorrectly, but it usually is not (in production environment) as they are simply not parsed every time.

The last question doesn't really relates to annotations. Since annotations are really classes, the reason for namespacing them is also the same. To avoid conflicts between libraries and for sake of readability.