Singleton

The Java programming language solutions provided here are all thread-safe but differ in supported language versions and lazy-loading.

Traditional simple way
This solution is thread-safe without requiring special language constructs, but it may lack the laziness of the one below. The INSTANCE is created as soon as the Singleton class is initialized. That might even be long before getInstance() is called. It might be (for example) when some static method of the class is used. If laziness is not needed or the instance needs to be created early in the application's execution, or your class has no other static members or methods that could prompt early initialization (and thus creation of the instance), this (slightly) simpler solution can be used:

 public class Singleton {
   private static final Singleton INSTANCE = new Singleton();
 
   // Private constructor prevents instantiation from other classes
   private Singleton() {}
 
   public static Singleton getInstance() {
      return INSTANCE;
   }
   public Object clone() throws CloneNotSupportedException {
      throw new CloneNotSupportedException();
   }
 }

The solution of Bill Pugh
University of Maryland Computer Science researcher Bill Pugh has written about the code issues underlying the Singleton pattern when implemented in Java.[8] Pugh's efforts on the "Double-checked locking" idiom led to changes in the Java memory model in Java 5 and to what is generally regarded as the standard method to implement Singletons in Java. The technique known as the initialization on demand holder idiom, is as lazy as possible, and works in all known versions of Java. It takes advantage of language guarantees about class initialization, and will therefore work correctly in all Java-compliant compilers and virtual machines.

The nested class is referenced no earlier (and therefore loaded no earlier by the class loader) than the moment that getInstance() is called. Thus, this solution is thread-safe without requiring special language constructs (i.e. volatile or synchronized).

 public class Singleton {
   // Private constructor prevents instantiation from other classes
   private Singleton() {}
 
   /**
    * SingletonHolder is loaded on the first execution of Singleton.getInstance() 
    * or the first access to SingletonHolder.INSTANCE, not before.
    */
   private static class SingletonHolder { 
     private static final Singleton INSTANCE = new Singleton();
   }
 
   public static Singleton getInstance() {
     return SingletonHolder.INSTANCE;
   }
 }

Traditional simple way using synchronization
This solution is thread-safe in Java 5.0 and later:

public class Singleton {
   // volatile is needed so that multiple thread can reconcile the instance
   // semantics for volatile changed in Java 5.
   private volatile static Singleton singleton;
 
   private Singleton(){}
 
   // synchronized keyword has been removed from here
   public static Singleton getSingleton(){
     // needed because once there is singleton available no need to acquire
     // monitor again & again as it is costly
     if(singleton==null) {
       synchronized(Singleton.class){
          // this is needed if two threads are waiting at the monitor at the
          // time when singleton was getting instantiated
          if(singleton==null)
          singleton= new Singleton();
       }
    }
   return singleton;
  }
 
}

http://en.wikipedia.org/wiki/Singleton_pattern