This pattern suggests that you create one and only one instance and provide a global point of access to the created object:

The DB connection in the preceding diagram is intended to be a singleton and provides a getter for its only object.

Here are its benefits:

• Controlled access to a sole instance
• Reduced namespace
• Flexibility to refinement of operations and representations
• More flexible than class operations

Impacts are as follows:

• Carry states for the whole lifetime of the application, creating additional overhead for unit tests
• Some level of violation of single responsibility principle
• By using singleton as a global instance, it hides the dependencies of the application; rather, it should get exposed through interfaces