三种高效优雅的、线程安全的Singleton实现方式

1、double check add volatile

    private static volatile SingletonObject5 instance;

    private SingletonObject5() {
        //
    }

    //double check add volatile
    public static SingletonObject5 getInstance() {

        if (null == instance) {
            synchronized (SingletonObject4.class) {
                if (null == instance)
                    instance = new SingletonObject5();
            }
        }
        return SingletonObject5.instance;
    }

2、InstanceHolder

    private SingletonObject6() {

    }

    private static class InstanceHolder {
        private final static SingletonObject6 instance = new SingletonObject6();
    }

    public static SingletonObject6 getInstance() {
        return InstanceHolder.instance;
    }

3、enum

    private SingletonObject7() {

    }

    private enum Singleton {
        INSTANCE;

        private final SingletonObject7 instance;

        Singleton() {
            instance = new SingletonObject7();
        }

        public SingletonObject7 getInstance() {
            return instance;
        }
    }

    public static SingletonObject7 getInstance() {
        return Singleton.INSTANCE.getInstance();
    }

    public static void main(String[] args) {
        IntStream.rangeClosed(1, 100)
                .forEach(i -> new Thread(String.valueOf(i)) {
                    @Override
                    public void run() {
                        System.out.println(SingletonObject7.getInstance());
                    }
                }.start());
    }