الـ Singleton عديم الحالة وأمان الخيوط (Thread safety)

في هذا الدرس

explain why singletons must not hold request state.

الموضوعات المخطط لها

• One singleton serves many requests at the same time, on many threads.
• A mutable field (a counter, "current user") is shared by all of them: races and wrong data.
• Keep singletons stateless: only final dependencies, data in method variables.
• State that must be shared belongs in a database or a thread-safe type, handled later.