Singleton Pattern

One instance, one access point

Singleton ensures a class has exactly one instance and provides a global point to reach it — useful for a shared config, connection pool, or cache. Overusing it turns the app into a web of hidden global state that's hard to test.

The safe way: an enum

A single-element enum is the simplest thread-safe singleton — the JVM guarantees exactly one instance is created, and it's serialization-safe for free.

enum AppConfig {
    INSTANCE;
    private final Properties props = load();
    public String get(String key) { return props.getProperty(key); }
}

String url = AppConfig.INSTANCE.get("db.url");

Prefer DI over a hand-rolled singleton

In a Spring app a @Bean or @Component is already a singleton by default (one instance per container) — no manual pattern needed, and it's swappable in tests. Reach for a hand-rolled singleton only outside a DI framework.