Re: what is the point of volatile?

Knute Johnson <>
Thu, 22 Nov 2007 12:43:21 -0800
Daniel Pitts wrote:

Knute Johnson wrote:

Daniel Pitts wrote:

Lew wrote: wrote:

Can anyone tell me what the value is of 'volatile' please? It seems to
me to be a lazy way of protecting a private data member without having
to write a synchronized getter and setter. But it only works for types
that can be updated atomically. This sounds a bit tricky to me....

The point of volatile is to provide a kind of lightweight

I don't know why you call it "lazy". Nor is it limited to private
members; volatility is orthogonal to access.

Reads from a volatile member are guaranteed to see previous writes
to that variable. In fact, reads from a volatile variable guarantee
that all writes prior to the latest write to that variable are
visible. This is not so for non-volatile variables. That is the
new (as of 5) memory model for Java.

It's worth pointing out that volatile doesn't mean atomic:
volatile int a;

++a; // Still two separate accesses to a

I didn't fully understood volatile until I read about it in the book
Java Concurrency In Practice

The only real use I've found for it is for a simple
you-should-shut-down-now flag to a tight-looped thread. That thread
only needs to read, and other threads only need to write.

volatile is useful for any variable that will be read in one or many
threads and assigned in another.

To further abstract...
volatile is useful if any *one* thread doesn't both read and write that
variable. Any thread can do one and not the other, and the outcome is

A little complicated for us simpletons but yes :-).


Knute Johnson
email s/nospam/knute/

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