## Singleton Design Pattern vs. Spring Singleton Beans: When to Use Each?

Singleton Design Pattern vs Singleton Beans in Spring Container

Question:

In a Spring application, given that beans are singletons by default, does it make sense to implement the Singleton design pattern separately to manage global data, or is using Spring beans sufficient for this purpose?

Answer:

A Spring singleton bean and the Singleton design pattern are distinct concepts. The Singleton design pattern ensures that a class has only one instance across all classloaders, while Spring singletons have a narrower scope.

Spring Singleton Beans

Spring singleton beans have a "per container per bean" scope, meaning a single instance of a bean is created for each Spring IoC container. While this behavior mimics the Singleton design pattern, it is not equivalent.

Singleton Design Pattern

The Singleton design pattern enforces a single global instance for a class. This is typically implemented using static variables or a factory method that guarantees the creation of a single instance.

Comparison

• Scope: Spring singletons have a container-level scope, while the Singleton design pattern has a global scope.
• Creation: Spring singletons are automatically created by the container, whereas Singleton pattern instances are explicitly created using the factory method or static variable approach.
• Testability: Spring singletons can be injected into test classes using dependency injection, while Singleton pattern instances are harder to mock or test due to their global nature.

Use Cases

• For managing application-wide data that needs to be globally accessible, it may be more convenient to use the Singleton design pattern.
• For beans that need to be scoped to a particular request or component, Spring singletons are the preferred approach.

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