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 {
            


        

Answer 1

A volatile read is not really expensive in itself.

You can design a test to call getInstance() in a tight loop, to observe the impact of a volatile read; however that test is not realistic; in such situation, programmer usually would call getInstance() once and cache the instance for the duration of use.

Another impl is by using a final field (see wikipedia). This requires an additional read, which may become more expensive than the volatile version. The final version may be faster in a tight loop, however that test is moot as previously argued.