Why is volatile used in double checked locking

Question

From Head First design patterns book, the singleton pattern with double checked locking has been implemented as below:

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;
    }
}

I don't understand why volatile is being used. Doesn't volatile usage defeat the purpose of using double checked locking i.e performance?

Answer 1

A good resource for understanding why volatile is needed comes from the JCIP book. Wikipedia has a decent explanation of that material as well.

The real problem is that Thread A may assign a memory space for instance before it is finished constructing instance. Thread B will see that assignment and try to use it. This results in Thread B failing because it is using a partially constructed version of instance.