Java 单例模式的3种写法

public class Singleton {    /*1.    private Singleton() {}    public static class Holder {        private final static Singleton instance = new Singleton();    }    public static Singleton getInstance() {        return Holder.instance;    }    */    /*2.    private static Singleton singleton = new Singleton();    public static Singleton getInstance() {        return singleton;    }    */    /*3.    private static volatile Singleton singleton;    public static Singleton getInstance() {        if (singleton != null) {            synchronized (Singleton.class) {                if (singleton != null) {                    singleton = new Singleton();                }            }        }        return singleton;    }    */}