Re: what is the point of volatile?

From:
Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>
Newsgroups:
comp.lang.java.help
Date:
Thu, 22 Nov 2007 09:55:20 -0800
Message-ID:
<hPWdneX8A4oUX9janZ2dnUVZ_h2pnZ2d@wavecable.com>
Lew wrote:

apm35@student.open.ac.uk 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 synchronization.

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
<http://virtualinfinity.net/wordpress/technical-book-recommendations/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.
--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

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