这个Singleton是线程安全的吗?我看不出怎么回事?

考虑Singleton:

public final class MySingleton {
    private static class Nested 
    {
        private static final MySingleton INSTANCE = new MySingleton();
    }

    private MySingleton() 
    {
        if (Nested.INSTANCE != null) 
        {
            throw new IllegalStateException("Already instantiated");
        }
    }

    public static MySingleton getInstance() 
    {
        return Nested.INSTANCE;
    }
}

我没有设置任何锁,但为什么这是Singleton问题的线程安全解决方案?