单例的一种形式

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;  
  }  
}