Singleton Pattern: Thread-Safe Config Manager

Problem

Design an AppConfig singleton that loads configuration from environment/file once and provides global read access. Understand WHY naive singletons break in multithreaded environments and implement a thread-safe version.

Requirements

AppConfig.getInstance() always returns the same instance
Config loaded once at first getInstance() call (lazy initialization)
Config is read-only after loading: get(key), getInt(key), getBool(key)
AppConfig.reload() — only for testing/hot reload, protected by lock
Show the race condition in a naive singleton (no locks)
Fix with double-checked locking or initialization-on-demand pattern

Constraints

–No public constructor
–getInstance() must be safe to call from multiple threads simultaneously

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