Singleton的安全实现！

public class Singleton
{
    private static Singleton instance = null;

    private Singleton()
    {
        // TODO
    }

    public static Singleton getInstance()
    {
        if (instance == null)
        {
            synchronized (Singleton.class)
            {
                Singleton inst = instance;
                if (inst == null)
                {
                    synchronized (Singleton.class)
                    {
                        inst = new Singleton();
                    }
                    instance = inst;
                }
            }
        }
        return instance;
    }
}