Singleton Pattern

The singleton pattern ensures a class has only one object instantiated during the program’s lifecycle. It is used to guarantee the control of a resource through its implementation.

UML Design

Java Example

Generally, singletons are lazily loaded and ensure thread safety. Below is a Java example outlining the most basic form of a singleton. The .getInstance() method creates a new instance (if one does not exist), and the synchronized keyword ensures two threads can not use this method at the same time.

public class Singleton {
  // The singleton instance to be returned by getInstance()
  private static Singleton instance = null;

  // Constructor is made private to stop creation through 'new' keyword outside of getInstance()
  private Singleton() {}

  // Returns instance when called
  public static synchronized Singleton getInstance() {
    // Creates new instance if none exists
    if (instance == null) {
      instance = new Singleton();
    }
    return instance;
  }
}