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https://issues.apache.org/jira/browse/WW-3576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12996430#comment-12996430
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Sylvain Veyrié commented on WW-3576:
------------------------------------
After some thinking about a quick patch, there are some other issues and
questions.
Can SessionMap be extended? The class is non-final and its properties
(including "session") are "protected". The only one example I found is a mock
object in SessionMapTestCase.
If we want SessionMap to be extendable, we should care about how to share the
lock, and that's not easy - see Brian Goets, /Java Concurrency in practice/
§3.5.
Another thing: currently, there are locks on "session" and "this" when
"session" is to be initialized, but both are guarding "session".
I think the best should be to have an unique and specific private lock object,
and set all properties to private.
> SessionMap is not thread-safe
> -----------------------------
>
> Key: WW-3576
> URL: https://issues.apache.org/jira/browse/WW-3576
> Project: Struts 2
> Issue Type: Bug
> Components: Other
> Affects Versions: 2.2.1
> Reporter: Sylvain Veyrié
>
> Searching for a bug after some stress test (Exception on "getAttribute":
> already invalidated session), I reviewed SessionMap.java.
> Following WW-239, SessionMap has been made thread-safe.
> All methods are like this :
> public void doSomething() {
> if (session == null) {
> return;
> }
>
> synchronized (session) {
> session.doSometing();
> }
> }
> For example:
> public void invalidate() {
> if (session == null) {
> return;
> }
>
> synchronized (session) {
> session.invalidate();
> session = null;
> entries = null;
> }
> }
> IMHO this is not thread-safe. With the example of invalidate(), if there is a
> context switch just before the synchronized, the nullity is no more checked.
> If another invalidate() is called at least a NPE can be thrown. There are
> probably other side-effects like my exception problem.
> As now Struts 2 only supports Java 5+, there is two ways to fix it :
> * use a double-check-locking (DCL) and set session "volatile" (easy way)
> * use java.util.concurrent instead of synchronized keyword
> If you agree and choose one of those fixes, I can provide a patch.
> For the moment, I don't know if my bug is resolved if we directly use javax
> session, without this wrapper.
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