public static Singleton getInstance()
{
if (instance == null)
{
synchronized(Singleton.class) { //1
if (instance == null) //2
instance = new Singleton(); //3
}
}
return instance;
}

可能会导致如下的问题：

mem = allocate(); //Allocate memory for Singleton object.
instance = mem; //Note that instance is now non-null, but
//has not been initialized.
ctorSingleton(instance); //Invoke constructor for Singleton passing
//instance.

那么如果这样，会不会有所不同呢：

public static Singleton getInstance()
{
if (instance == null)
{
synchronized(Singleton.class) { //1
if (instance == null)
{ //2
Singleton localVar = new Singleton();
localVar.DoSomething();
instance = localVar; //3
}
}
}
return instance;
}

会不会由于乱序执行导致仍然有同样的问题呢？