Code · July 10, 2026

The Singleton Pattern Explained in 2026

Singleton guarantees exactly one instance of a class exists and gives it a global access point. Here is what it is good for in 2026, and why many teams now avoid it by default.

By ByteLedger Team

TL;DR

Singleton restricts a class to one instance — the class controls its own instantiation and hands out the same object to every caller instead of letting callers create new ones.
It solves shared-state and resource-coordination problems — a single logger, config store, or connection pool is a legitimate reason to want exactly one instance.
Global state is the real cost, not the pattern itself — the criticism singleton draws is mostly about hidden global state making code hard to test, not about the one-instance guarantee.
Dependency injection replaces most legitimate uses today — a container that hands out a single shared instance gives you the same guarantee without the global access point.
Skip: reaching for singleton as a default way to avoid passing parameters — that is a convenience shortcut, not a real one-instance requirement, and it is the version that causes testing pain.

Singleton is a creational design pattern that restricts a class to exactly one instance and provides a single, well-known way to access it. The class typically hides its constructor and exposes a static method — often called getInstance() — that creates the instance on first call and returns the same one on every call after that. It is one of the oldest and most recognizable patterns in the Gang of Four catalog, and also one of the most argued about.

What changed in 2026

Dependency injection containers absorbed most legitimate singleton use cases. Modern frameworks let you register a service as a singleton scope in a container, getting the one-instance guarantee without a hardcoded static accessor.
Testing tooling made the downside more visible. As test suites grew and parallel test execution became standard, the shared mutable state that singletons introduce became a more obvious source of flaky, order-dependent tests.
Language-level module systems reduced the need for it. In languages where a module is only evaluated once, a plain exported object often gives you singleton behavior without the pattern's ceremony.

How it works

class Config {
  private static instance: Config;
  private constructor() { /* load config */ }

  static getInstance(): Config {
    if (!Config.instance) {
      Config.instance = new Config();
    }
    return Config.instance;
  }
}

Every call to Config.getInstance() returns the same object. The constructor is private, so nothing outside the class can create a second instance directly. This is the mechanism — the debate is entirely about when using that mechanism is the right call.

When it is a legitimate choice

Singleton fits problems that are genuinely about coordinating a single shared resource: a connection pool that should not be duplicated, a logger that writes to one file, a cache that needs one source of truth in memory. In these cases, having two instances would be a bug, not just a style issue — the one-instance guarantee is solving a real problem, not just avoiding a function parameter.

Singleton vs the alternatives

Approach	One-instance guarantee	Testability	Global access
Classic singleton	Yes, self-enforced	Poor — hard to swap/mock	Yes, global
Dependency injection (singleton scope)	Yes, container-enforced	Good — swap for a test double	No, injected explicitly
Module-level export	Yes, if module runs once	Medium	Yes, but scoped to imports
Plain instance passed explicitly	No, up to the caller	Best	No

Why it draws criticism

The complaint about singleton is rarely about the one-instance guarantee itself — it is about the global access point. A class that can be reached from anywhere becomes a hidden dependency: any function that calls Config.getInstance() has a dependency on global state that does not show up in its signature. That makes the function harder to test in isolation, since you cannot substitute a different config without reaching into global state to do it. This is the same underlying issue that shows up in discussions of the DRY vs WET tradeoff — a pattern applied for convenience rather than necessity tends to cost more than it saves once the codebase grows.

Common pitfalls

Using singleton to dodge passing a parameter. If the only reason for the singleton is avoiding an extra constructor argument, dependency injection gives the same convenience without the hidden coupling.
Making mutable singletons in multi-threaded or concurrent code. A shared mutable instance accessed from multiple threads without synchronization is a classic source of race conditions.
Singleton-izing something that should scale per request. A per-user or per-request object forced into a singleton either leaks data across requests or requires awkward internal keying to fake per-request state.

FAQ

Is singleton considered an anti-pattern now? Opinions vary. It is not inherently wrong, but the hardcoded, self-managed version is now considered inferior to a dependency-injected singleton scope for most application code, since it gives the same guarantee with better testability.

Does every logger need to be a singleton? No. A single shared logger instance is often convenient, but passing a logger explicitly through a constructor is just as effective and easier to substitute during tests.

What is the difference between a singleton and a static class? A static class has no instance at all — every member is accessed directly on the class. A singleton is a real object instance; it just happens to be limited to exactly one. That distinction matters if you ever need the object to implement an interface or be passed around.

Can singleton cause memory issues? Rarely directly, but a singleton that accumulates state over the life of an application (a cache that never evicts, for example) can grow unbounded precisely because nothing ever creates a fresh instance to replace it.