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Also known as: Single Instance, Object Creation
A creational design pattern ensuring a class has only one instance and providing a global access point to it.
DESCRIPTION
The singleton pattern restricts instantiation of a class to a single object and exposes a shared instance. It’s commonly used for configuration, logging, or caches requiring a unique global resource. Implementations must consider thread safety, lazy initialization, and serialization. Overuse can hinder testability and create hidden dependencies, making it controversial. Dependency Injection is often preferred to manage lifecycles and decouple code. Some languages offer simpler alternatives (e.g., modules or static classes). Use singletons judiciously with clear boundaries and lifecycle management.
ALTERNATIVE DEFINITIONS
Wikipedia
The singleton pattern is a software design pattern that restricts the instantiation of a class to one object.
Captures the core restriction central to the pattern.
Refactoring Guru
Singleton is a creational design pattern that lets you ensure that a class has only one instance, while providing a global access point to this instance.
Emphasizes both uniqueness and global access point.
Microsoft Learn
The singleton design pattern ensures that a class has only a single instance and provides a global point of access to it.
Highlights standard implementation guidance in .NET.
RELATED TERMS
Factory Pattern Global State Inversion of Control Lazy Initialization Thread Safety Service Locator Pattern Static Class Design Patterns Multiton
COMPETING TERMS
Multiton Pattern Dependency Injection
KNOWLEDGE AREAS
Software Engineering Software Modeling
TARGET AUDIENCE
Software Developer Enterprise Architect Data Architect
EXTERNAL SOURCES
- Wikipedia - Singleton pattern
- Refactoring Guru - Detailed explanation and examples
- Microsoft - Official C# singleton guidance
- SourceMaking - Pattern summary and anti-pattern discussion
TOP QUESTIONS
When should the Singleton Pattern be used?
Use it for unique resources like configuration managers or caches. Avoid when better lifecycle management via dependency injection is available.
Category: Requirements
What is the biggest criticism of the Singleton Pattern?
It can act like global state, hindering testability and increasing hidden coupling between components.
Category: Opinion
How do you implement a thread-safe Singleton?
Use double-checked locking, static initialization, or language-level guarantees. Ensure memory visibility and guard against reflection/serialization pitfalls.
Category: Example
Is a static class the same as a Singleton?
No. Static classes can’t implement interfaces or manage stateful lifecycle the same way; singletons are instances with lifecycle control.
Category: Misunderstanding
What are alternatives to the Singleton Pattern?
Dependency Injection with scoped or singleton lifetimes provides testable, explicit lifecycle management without hard-coded globals.
Category: Alternatives
Can Singletons cause memory leaks?
Yes, long-lived singletons retaining references can prevent garbage collection. Keep references minimal and expose disposal hooks if needed.
Category: Risks
How do Singletons affect testing?
They make tests harder due to shared state. Use interfaces and dependency injection to substitute mocks or resettable instances.
Category: Mistakes
Are Singletons bad by default?
No. They’re useful for genuinely unique resources. Problems arise from overuse and poor boundaries.
Category: Opinion
What’s a Multiton, and how does it compare?
A Multiton restricts instances to a key-based set, not a single instance. It generalizes Singleton for multiple controlled instances.
Category: Example