Question

So I am reading this book titled Java Concurrency in Practice and I am stuck on this one explanation which I cannot seem to comprehend without an example. This is the quote:

When thread A writes to a volatile
variable and subsequently thread B
reads that same variable, the values
of all variables that were visible to
A prior to writing to the volatile
variable become visible to B after
reading the volatile variable.

Can someone give me a counterexample of why “the values of ALL variables that were visible to A prior to writing to the volatile variable become visible to B AFTER reading the volatile variable”?

I am confused why all other non-volatile variables do not become visible to B before reading the volatile variable?

Answer 1

Thread B may have a CPU-local cache of those variables. A read of a volatile variable ensures that any intermediate cache flush from a previous write to the volatile is observed.

For an example, read the following link, which concludes with “Fixing Double-Checked Locking using Volatile”:

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html