What are the possible disadvantages of the Singleton pattern and why is it considered bad practice?

The Singleton pattern is often criticized for several reasons:

• Global state: Singleton essentially creates a global object, which complicates tracking changes and leads to hidden dependencies.
• Testing difficulties: Due to the global state, isolating tests becomes difficult, which hampers modular testing.
• Multithreading issues: In Java, the Singleton implementation must be thread-safe; otherwise, errors may occur during concurrent access.
• Violation of SOLID principles: Singleton violates the single responsibility principle and dependency inversion.

Because of these drawbacks, Singleton is often considered a bad practice, and alternative approaches such as dependency injection are recommended.