Applicability of the Singleton Design Pattern

I am a software architect working with Design Patterns. The Singleton design pattern is one of the most recognized patterns in software architecture, belonging to the Gang of Four's design patterns repertoire. It ensures that a class has only one instance and provides a global point of access to that instance. While its implementation is straightforward, the decision to employ the Singleton pattern should be made judiciously. Here are the conditions under which the Singleton pattern is most appropriate:

1. Singular Resource Access:
   Controlled Access: If you need to control access to shared resources, such as configuration management or connection pools, the Singleton pattern can be beneficial. It ensures that these resources are accessed in a controlled manner, preventing potential conflicts or resource contention.
2. State Management:
   Consistent State: For components that maintain a system-wide state, ensuring that there's a single point of truth is crucial. The Singleton pattern ensures that state is consistent and centralized, eliminating the risks associated with multiple instances.
3. Costly Instantiation:
   Resource Intensiveness: If creating an instance of a class is resource-intensive or time-consuming, and the instance is used throughout the application's lifecycle, employing a Singleton can be efficient. It ensures that the instantiation occurs only once, conserving resources.
4. Service Proxies:
   Unique Interface: In scenarios where you need a unique interface to external systems or services, the Singleton pattern can be beneficial. It ensures that all operations utilize this single interface, promoting consistency in interactions.
5. Logging Mechanisms:
   Unified Logging: For logging mechanisms where a unified point of logging is required to maintain consistency in log entries and avoid potential file access conflicts, a Singleton logger can be advantageous.
6. Configuration Management:
   Centralized Configuration: Applications often require a centralized point for managing configurations. A Singleton pattern ensures that configuration data is loaded once and accessed consistently throughout the application.
7. Avoiding Overhead:
   Reduced Overhead: If there's a need to reduce the overhead of constantly checking the existence of an instance or preventing accidental creation of additional instances, the Singleton pattern is apt.

While the Singleton pattern has its merits, it's essential to be aware of its potential pitfalls. Singleton can introduce global state, making systems harder to test due to hidden dependencies. It can also lead to overuse, where the convenience of a global instance is preferred over more suitable design choices.
The Singleton design pattern, when employed judiciously, can offer significant advantages in ensuring consistency, reducing resource consumption, and managing state. However, as with all design patterns, it's imperative to assess the specific needs of the application and the implications of the pattern before integrating it into the system architecture.

The Singleton pattern has two primary criteria for applicability:

1. The class should have exactly one instance which should be accessible from a method in the class.
2. The class can be subclassed, and clients^[1] can use the subclass as an instance of the superclass without having to change anycode.

The first criterion more or less defines the nature of the Singleton pattern.
The second criterion considers issues of reuse. In many cases there is more than one way to solve the immediate problem.

You choose your pattern more for how easily it can be modified within your system rather than for what it does.
The Singleton pattern is grouped with the other Creational patterns, although it is to some extent a non-creational pattern. There are any number of cases in programming where you need to make sure that there can be one and only one instance of a class. For example, your system can have only one window manager or print spooler, or a single point of access to a database engine. The easiest way to make a class that can have only one instance is to embed a static variable inside the class that we set on the first instance and check for each time we enter the constructor. A static variable is one for which there is only one instance, no matter how many instances there are of the class.

 
static boolean instance_flag = false;

The problem is how to find out whether creating an instance was successful or not, since constructors do not return values. One way would be to call a method that checks for the success of creation, and which simply returns some value derived from the static variable. This is inelegant and prone to error. However, because there is nothing to keep you from creating many instances of such non-functional classes and forgetting to check for this error condition.

class SingletonException extends RuntimeException{
  //new exception type for singleton classes
  public SingletonException(){
    super();
  }
  //-----------------------------------------------
  public SingletonException(String s){
    super(s);
  }
}

[1] clients: Any object or class outside the pattern; generally one that only knows about the public interface the pattern and its classes present, rather than about its private implementation.