单例的最佳实现方式

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 {             public static final Singleton INSTANCE = new Singleton();    }    public static Singleton getInstance() {            return SingletonHolder.INSTANCE;    }}