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

Lew wrote:

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 =

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....