Simply Singleton

An alternative thread-safe singleton implementation

Example 7 lists a simple, fast, and thread-safe singleton implementation:

Example 7. A simple singleton

public class Singleton {
   public final static Singleton INSTANCE = new Singleton();
   private Singleton() {
         // Exists only to defeat instantiation.
      }
}

The preceding singleton implementation is thread-safe because static member variables created when declared are guaranteed to be created the first time they are accessed. You get a thread-safe implementation that automatically employs lazy instantiation; here's how you use it:

Singleton singleton = Singleton.INSTANCE;
      singleton.dothis();
      singleton.dothat();
      ...

Of course, like nearly everything else, the preceding singleton is a compromise; if you use that implementation, you can't change your mind and allow multiple singleton instances later on. With a more conservative singleton implementation, instances are obtained through a getInstance() method, and you can change those methods to return a unique instance or one of hundreds. You can't do the same with a public static member variable.