Re: AtomicReferenceArray writes and visibility

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 31 Mar 2011 22:24:33 -0700

Message-ID:

<5rSdnZ3R1d8O_AjQnZ2dnUVZ_qWdnZ2d@earthlink.com>

On 3/31/2011 9:58 PM, markspace wrote:

On 3/31/2011 9:54 PM, Peter Duniho wrote:

On 3/31/11 9:22 PM, markspace wrote:

[...]
Read the source code for the AtomicReferenceArray. set() doesn't use any
synchronization. It calls sun.misc.Unsafe#putObjectVolatile(). I don't
see how that's going to create a happens-before.

"volatile" creates the happens-before. All writes before a volatile
write in the same thread must be visible in a given thread after a
volatile read in the same thread from the same location.

I didn't notice the keyword volatile there. Did I miss it?

The relevant use of "volatile" is in the java.util.concurrent.atomic
package documentation,
http://download.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/package-summary.html

"The memory effects for accesses and updates of atomics generally follow
the rules for volatiles, as stated in The Java Language Specification,
Third Edition (17.4 Memory Model):"

It goes on to map atomic operations to volatile operations. For example
"get has the memory effects of reading a volatile variable."

Regardless, the lack of any guarantees in the docs of
AtomicReferenceArray would kill it for me. You can't be sure how the
implementation will change over time.

Does that include the guarantees that it provides indirectly, by
reference to the package documentation?

Patricia