Some consider the Singleton pattern an anti-pattern because it can lead to tightly coupled code that is difficult to maintain. The Singleton pattern is a design pattern that restricts the instantiation of a class to one object, ensuring that all code that needs to access that object uses the same instance.
The following are some reasons why the Singleton pattern can be considered an anti-pattern,
1. Global state
The Singleton pattern can create a global state that is accessible from any part of the code, making it difficult to reason about the behavior of the system.
2. Hidden dependencies
Because the Singleton pattern can be used to access an object from any part of the code, it can create hidden dependencies between different parts of the system, making it difficult to modify and test the code.
3. Testing difficulties
The Singleton pattern can make testing difficult because isolating the Singleton object is often impossible when writing unit tests.
When unit testing two classes, Singletons break the Single Responsibility Principle because one class implements Singleton and the other does not. We need to pass in the Singleton as a parameter to the constructor, allowing the tester to easily mock out the Singleton class. The Singleton then does not have to enforce its own singularity. This can be done by factory classes eliminating the global state.
5. Thread-safety issues
The Singleton pattern can create thread-safety issues if multiple threads access the Singleton object simultaneously.
6. lazily initialization
If we have several lazily initialized Singletons, the compiler cannot fold multiple SingletonDemo.getInstance() calls into one, and Singletons decrease performance. This is because each call to getInstance() will cause a branch instruction and possibly a cache mismatch. Singletons promote tight coupling between classes. Singletons tightly couple the code to the exact object type and remove the scope of polymorphism.
7. Extensions
A programmer/developer must use different decorator patterns to change the behavior. Extending Singletons is not easy.
8. Garbage Collection
In a garbage-collected system, Singletons can become very tricky regarding memory management.
There are situations where the Singleton pattern may be appropriate, but it should be used calculated and with careful consideration of its implications. In general, it's better to favor other design patterns, such as dependency injection, that promote loose coupling and testability.