Synchronization and concurrency
Deadlock (and avoiding it)
Java 5: ConcurrentHashMap
The volatile keyword in Java
It's probably fair to say that on the whole, the volatile keyword in Java is poorly documented, poorly understood, and rarely used. To make matters worse, its formal definition actually changed as of Java 5. On this and the following pages, we will cut through this mess and look at what the Java volatile keyword does and when it is used. We will also compare it to other mechanisms available in Java which perform similar functions but under subtly different other circumstances.
What is the Java volatile keyword?
Essentially, volatile is used to indicate that a variable's value will be modified by different threads.
Declaring a volatile Java variable means:
We say "acts as though" in the second point, because to the programmer at least (and probably in most JVM implementations) there is no actual lock object involved. Here is how synchronized and volatile compare:
In other words, the main differences between synchronized and volatile are:
Attempting to synchronize on a null object will throw a NullPointerException.
Volatile variables in Java 5
We mentioned that in Java 5, the meaning of volatile has been tightened up. We'll come back to this issue in a moment. First, we'll look at a typical example of using volatile. Later, we'll look at topics such as:
NOTE: The original authentic version of this article is published on the Javamex web site. It has been ripped off on various blogs and sites such as scribd. If you are reading this article on any site other than javamex.com then you are reading a copyright infringing version which may potentially be out of date.
Copyright © Neil Coffey 2013. All rights reserved.