What is the volatile keyword useful for?

Question

At work today, I came across the volatile keyword in Java. Not being very familiar with it, I found this explanation.

Given the detail in which that article explains the keyword in question, do you ever use it or could you ever see a case in which you could use this keyword in the correct manner?

Answer 1

volatile has semantics for memory visibility. Basically, the value of a volatile field becomes visible to all readers (other threads in particular) after a write operation completes on it. Without volatile, readers could see some non-updated value.

To answer your question: Yes, I use a volatile variable to control whether some code continues a loop. The loop tests the volatile value and continues if it is true. The condition can be set to false by calling a "stop" method. The loop sees false and terminates when it tests the value after the stop method completes execution.

The book "Java Concurrency in Practice," which I highly recommend, gives a good explanation of volatile. This book is written by the same person who wrote the IBM article that is referenced in the question (in fact, he cites his book at the bottom of that article). My use of volatile is what his article calls the "pattern 1 status flag."

If you want to learn more about how volatile works under the hood, read up on the Java memory model. If you want to go beyond that level, check out a good computer architecture book like Hennessy & Patterson and read about cache coherence and cache consistency.

Answer 2

“… the volatile modifier guarantees that any thread that reads a field will see the most recently written value.” - Josh Bloch

If you are thinking about using volatile, read up on the package java.util.concurrent which deals with atomic behaviour.

The Wikipedia post on a Singleton Pattern shows volatile in use.