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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.
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 Chaco Slipper Harper Harper Chaco Denim Slipper Chaco Slipper Chaco Denim Harper Denim volatile compare:
|Type of variable||Object||Object or primitive|
|Can block?||Yes||Chaco Slipper Denim Denim Denim Chaco Harper Chaco Slipper Slipper Chaco Harper Harper 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.|
In other words, the main differences between synchronized and volatile are:
Attempting to synchronize on a null object will throw a NullPointerException.
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: