Singleton

Singleton is a most widely used design pattern. If a class has and only has one instance at every moment, we call this design as singleton.

Eager Approach

Rely on JVM to create the unique instance of Singleton when the class is loaded. The instance is created before any threads are able to access it.

Code

class Singleton {
  private static final Singleton INSTANCE = new Singleton();

  private Singleton() {}

  public static Singleton getInstance() {
    return INSTANCE;
  }
}

Lazy Approach

The instance is only initilizaed when it is first used. This can help to save resource usage if initializing the Singleton class is very expensive.

Use volatile keyword to ensure that all threads can see the most recently written value. Refer to Java Volatile Keyword for more information.

Code

class Singleton {
  private volatile static Singleton instance;

  private Singleton() {}

  public static Singleton getInstance() {
    if (instance == null) {
      synchronized (Singleton.class) {
        if (instance == null) {
          instance = new Singleton();
        }
      }
    }
    return instance;
  }
}