content: package com.iluwatar.singleton;
  
  /**
   *
   * Singleton pattern ensures that the class (IvoryTower) can have only one
   * existing instance per java classloader instance and provides global access to it. 
   * 
   * http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
   *
   * The risk of this pattern is that bugs resulting from setting a singleton up in a distributed environment can 
   * be tricky to debug, since it will work fine if you debug with a single classloader. Additionally, these
   * problems can crop up a while after the implementation of a singleton, since they may start out synchronous and
   * only become async with time, so you it may not be clear why you are seeing certain changes in behaviour.
   * 
   * http://stackoverflow.com/questions/17721263/singleton-across-jvm-or-application-instance-or-tomcat-instance
   */
  public class App {
  
  	public static void main(String[] args) {
  
  		// eagerly initialized singleton
  		IvoryTower ivoryTower1 = IvoryTower.getInstance();
  		IvoryTower ivoryTower2 = IvoryTower.getInstance();
  		System.out.println("ivoryTower1=" + ivoryTower1);
  		System.out.println("ivoryTower2=" + ivoryTower2);
  
  		// lazily initialized singleton
  		ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower1 = ThreadSafeLazyLoadedIvoryTower
  				.getInstance();
  		ThreadSafeLazyLoadedIvoryTower threadSafeIvoryTower2 = ThreadSafeLazyLoadedIvoryTower
  				.getInstance();
  		System.out.println("threadSafeIvoryTower1=" + threadSafeIvoryTower1);
  		System.out.println("threadSafeIvoryTower2=" + threadSafeIvoryTower2);
  
  		// enum singleton
  		EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
  		EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
  		System.out.println("enumIvoryTower1=" + enumIvoryTower1);
  		System.out.println("enumIvoryTower2=" + enumIvoryTower2);
  		
  		InitializingOnDemandHolderIdiom demandHolderIdiom = InitializingOnDemandHolderIdiom.getInstance();
  		System.out.println(demandHolderIdiom);
  		InitializingOnDemandHolderIdiom demandHolderIdiom2 = InitializingOnDemandHolderIdiom.getInstance();
  		System.out.println(demandHolderIdiom2);
  		
  		ThreadSafeDoubleCheckLocking dcl1 = ThreadSafeDoubleCheckLocking.getInstance();
  		System.out.println(dcl1);
  		ThreadSafeDoubleCheckLocking dcl2 = ThreadSafeDoubleCheckLocking.getInstance();
  		System.out.println(dcl2);
  	}
  }
  