The volatile keyword in Java

Topics covered include:  Synchronized vs volatile  When to use volatile?  Atomic updates of volatile fields

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. Essentially, volatile is used to indicate that a variable's value will be modified by different threads.

Declaring a volatile Java variable means:

• The value of this variable will never be cached thread-locally: all reads and writes will go straight to "main memory";
• Access to the variable acts as though it is enclosed in a synchronized block, synchronized on itself.

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:

Difference between synchronized and volatile

Characteristic	Synchronized	Volatile
Type of variable	Object	Object or primitive
Null allowed?	No	Yes
Can block?	Yes	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.

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

• a primitive variable may be declared volatile (whereas you can't synchronize on a primitive with synchronized);
• an access to a volatile variable never has the potential to block: we're only ever doing a simple read or write, so unlike a synchronized block we will never hold on to any lock;
• because accessing a volatile variable never holds a lock, it is not suitable for cases where we want to read-update-write as an atomic operation (unless we're prepared to "miss an update");
• a volatile variable that is an object reference may be null (because you're effectively synchronizing on the reference, not the actual object).

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