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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 Flower Brown Tooled Brown Klogs Klogs Tooled Flower Klogs Brown Klogs Tooled Brown Flower volatile compare:

Characteristic Synchronized Volatile
Type of variable Object Object or primitive
Null allowed? No Yes
Can block? Yes Flower Klogs Tooled Flower Brown Tooled Klogs Klogs Brown Tooled Brown Brown Flower Klogs No
All cached variables synchronized on access? Yes From Java 5 onwards
When synchronization happens When you explicitly enter/exit a synchronized block Whenever a volatile variable is accessed.
Can be used to combined several operations into an atomic operation? Yes Pre-Java 5, no. Atomic get-set of volatiles possible in Java 5.
Difference between synchronized and volatile
Download PDF summary of this informationPliner J Sneaker Hightop DMSX Black PRINCESSK1 Donald Z0pqnwE

In other words, the main differences between synchronized and volatile are:

Attempting to synchronize on a null object will throw a NullPointerException.

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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: