Singleton implementation - Double check

[sandeep lokhande]

In implementing Singleton Class,

public Singleton getInstance(){
     if(_INSTANCE == null){
         synchronized(Singleton.class){
         //double checked locking - because second check of Singleton instance with lock
                if(_INSTANCE == null){
                    _INSTANCE = new Singleton();
                }
            }
         }
     return _INSTANCE;
}

Why we check twice(LN 2) when we are checking later(LN 5) ???

[sandeep lokhande]

sandeep lokhande wrote:In implementing Singleton Class,

public Singleton getInstance(){
     if(_INSTANCE == null){
         synchronized(Singleton.class){
         //double checked locking - because second check of Singleton instance with lock
                if(_INSTANCE == null){
                    _INSTANCE = new Singleton();
                }
            }
         }
     return _INSTANCE;
}

Why we check twice(LN 2) when we are checking later(LN 5) ???

While waiting for answer i was googling and i found satisfatory answer,
Its a huge overhead because everytime the method is invokes synchronized has to go through the process of waiting for the lock and then obtaining the lock and
its unnecessary because its only once that the instance is created and all subsequent invocations would just return that instance. :-) so00 simple ;-)

[Campbell Ritchie]

Why are you implementing a singleton in the first place?
You do realise that double‑checked locking doesn’t work in Java? Well, I think it does work in Java5+.
I think you need to create a static instance. No need to mess around with synchronisation or null checks.

public final class Foo
/*
 * Can’t subclass anything with a private constructor
 */
{
   private static final Foo instance = new Foo();

   private Foo()
   {
      // private constructor to restrict instantiation
   }

   public static Foo getInstance()
   {
      return instance;
   }

/* other methods, fields, etc */
}

I think that is correct; it looks the same as in my Effective Java™ (Joshua Bloch) page 18. But Bloch points out there is now a far better way to create a Singleton: use an enum.

[sandeep lokhande]

public enum SantaClaus {
  INSTANCE;
  
  public void distributePresents(){
    // Add some behavior 
  }
  
  /** Demonstrate use of SantaClaus. */
  public static void main(String... aArgs){
    SantaClaus fatGuy = SantaClaus.INSTANCE;
    fatGuy.distributePresents();
     }
} 

Is this a correct/good Practice? or private static final is better??