Singleton pattern
Gang-of-Four pattern — ensure a class has exactly one instance and give global access to it. Famously an anti-pattern as often as a pattern.
Classic Java: private constructor + static getInstance(). Thread safety requires double-checked locking or eager init. Modern variant: a dependency-injection container gives you "singleton scope" without global state.
Anti-pattern critique: makes testing hard (global state), breaks dependency injection, hides dependencies. Acceptable for truly process-global things (logger, cache). Prefer DI.