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

Declaring the variable as volatile guarantees that all accesses to it actually read its current value from memory.

Without volatile, the compiler may optimize away the memory accesses to the variable (such as keeping its value in a register), so only the first use of the variable reads the actual memory location holding the variable. This is a problem if the variable is modified by another thread between the first and second access; the first thread has only a copy of the first (pre-modified) value, so the second if statement tests a stale copy of the variable's value.