单例模式常见有哪几种？

package com.zhuyca.singleton;

// 饿汉式
public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return instance;
    }
}

// 懒汉式
class Singleton2 {
    private static Singleton2 instance = null;
    private Singleton2() {}
    public static Singleton2 getInstance() {
        if(instance == null) {
            instance = new Singleton2();
        }
        return instance;
    }
}

// 懒汉加锁
class Singleton3 {
    private static Singleton3 instance = null;
    private Singleton3() {}
    public static synchronized Singleton3 getInstance() {
        if(instance == null) {
            instance = new Singleton3();
        }
        return instance;
    }
}

// 懒汉双检锁
class Singleton4 {
    private static Singleton4 instance = null;
    private Singleton4() {}
    public static Singleton4 getInstance() {
        if(instance == null) {
            synchronized(Singleton4.class) {
                if(instance == null) {
                    instance = new Singleton4();
                }
            }
        }
        return instance;
    }
}

// 静态内部类实现单例
class Singleton5 {
    private Singleton5() {}
    public static final Singleton5 getInstance() {
        return SingletonHolder.INSTANCE;
    }
    private static class SingletonHolder {
        private static final Singleton5 INSTANCE = new Singleton5();
    }
}

参考链接：http://developer.51cto.com/art/201804/569738.htm