Java: Why is volatile used in this example of double checked locking

[Richards]

I'm reading Head First design patterns and in the chapter on the singleton pattern it shows you how to implement double checked locking as follows:\[code\]public class Singleton { private volatile static Singleton instance; private Singleton() {} public static Singleton getInstance() { if (instance == null) { synchronized (Singleton.class) { if (instance == null) { instance = new Singleton(); } } return instance; }}\[/code\]I don't understand why volatile is being used. Doesn't using volatile defeat the purpose of using double checked locking i.e performance?