复习了一遍 Singleton 的 5 种写法

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这是一个创建于 3420 天前的主题，其中的信息可能已经有所发展或是发生改变。

https://gist.github.com/opoo/e48dbc0b3c5d0afddfc0

好吧，github gist 貌似暂时不能访问了，贴在下面。

public class SingletonDemo {
    public static class Singleton1{
        private static final Singleton1 instance = new Singleton1();
        private Singleton1(){}
        public static Singleton1 getInstance(){
            return instance;
        }
    }

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

    public static class Singleton3{
        private static volatile Singleton3 instance;
        private Singleton3(){}
        public static Singleton3 getInstance(){
            if(instance == null){
                synchronized (Singleton3.class){
                    if(instance == null){
                        instance = new Singleton3();
                    }
                }
            }
            return instance;
        }
    }

    public static class Singleton4{
        private static final Singleton4 instance;
        static{
            instance = new Singleton4();
        }
        private Singleton4(){}
        public static Singleton4 getInstance(){
            return instance;
        }
    }

    public static class Singleton5{
        private static class SingletonHolder{
            private static final Singleton5 instance = new Singleton5();
        }
        private Singleton5(){}
        public static Singleton5 getInstance(){
            return SingletonHolder.instance;
        }
    }
}