Re: Do I need to sync Get methods that return thread-safe collections

From:
Royan <romayankin@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 3 Aug 2008 03:26:37 -0700 (PDT)
Message-ID:
<25e02a5c-b392-47f7-8d09-135b6208aaf2@x35g2000hsb.googlegroups.com>
On 3 =C1=D7=C7, 11:01, Mark Space <marksp...@sbc.global.net> wrote:

Lew wrote:

Royan wrote:

I'm trying to find potential pitfall in unsynchronized methods that
return thread-safe collections. Assume i'm [sic] designing a thread-sa=

fe

class.


Read the articles on concurrency by Brian Goetz in IBM DeveloperWorks,
and his book /Java Concurrency in Practice/.


This is the best advice. =9AThread safety is complicated enough that a
couple of quick posts on Usenet won't explain everything. =9AYou need
something more thorough to give you the full picture. =9AJava Concurrency
in Practice will give an excellent understand of many thread safety and
concurrency issue.

Case in point:

=9A>> public class ThreadSafe {
=9A>>
=9A>> =9A =9A private Vector<String> vector;

=9A>> =9A =9A /** But is OK to have such method? */
=9A>> =9A =9A public Vector<String> getVector() {
=9A>> =9A =9A =9A =9A return vector;
=9A>> =9A =9A }
=9A>> }

Nope, not ok. =9AYou created an object on one thread (not shown) and then
tried to fetch it on another. =9AGuaranteed problems. =9AExample:

Let's say ThreadSafe has a constructor which Thread A calls:

=9A =9Apublic ThreadSafe() {
=9A =9A =9Avector = new Vector<String>();
=9A =9A}

Now Thread B calls getVector. =9AOops!! =9AIt may not even see the value =

of

the reference (field "vector" might be null still) or thread B might see
the Vector in a partially constructed state (there's still bits of it in
Thread A's cache which haven't been written out yet). =9AEither way, big
trouble.

=9A>> public class ThreadSafe {
=9A>>
=9A =9A =9A =9A private volatile Vector<String> vector;

=9A>> =9A =9A /** But is OK to have such method? */
=9A>> =9A =9A public Vector<String> getVector() {
=9A>> =9A =9A =9A =9A return vector;
=9A>> =9A =9A }
=9A>> }

Now you're safe. =9AMaking getVector() "synchronized" would do the same
thing. (The JVM knows to flush objects around the synchronized "memory
barrier.") =9ASee Java Concurrency in Practice for more....


Thanks a lot everyone! I think I've got the idea!

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