CAS 单例模式

public final class SingleInstance {

    private static final AtomicReference<SingleInstance> instanceRef = new AtomicReference<>();

    private SingleInstance() {

    }

    public static SingleInstance getInstance() {

        for (; ; ) {

            SingleInstance instance = instanceRef.get();

            if (instance != null) {

                return instance;

            }

            instanceRef.compareAndSet(null, new SingleInstance());

        }

    }

}