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https://issues.apache.org/jira/browse/TRINIDAD-2278?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13400400#comment-13400400
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Thomas Thrien commented on TRINIDAD-2278:
-----------------------------------------
A lock would not work as desired because using a STATIC object as a guard where
a NON-STATIC object was used before will lead into desaster.
The session object was used as the guard for the creation of the token cache to
prevent that two instances of that cache are created for the respective
session. And the session object was used for that purpose because there is not
other object available that is representing the user session (and is unique
...).
Your suggestion to use a static _LOCK_OBJECT would result in a
sequentialisation of ALL threads on that application server that are using JSF
(not only those for a particular session as for the current setup). This
sequentialisation would have a heavy impact on performance when load increases
- meaning on your home server it would not make a difference, but on a box with
hundreds or thousands of parallel sessions, it would be remarkable.
> HttpSession inside a synchronized block in the class TokenCache of
> trinidad-impl-1.0.10.jar could cause to deadlock while used with WAS 7
> -----------------------------------------------------------------------------------------------------------------------------------------
>
> Key: TRINIDAD-2278
> URL: https://issues.apache.org/jira/browse/TRINIDAD-2278
> Project: MyFaces Trinidad
> Issue Type: Bug
> Affects Versions: 1.0.10-core
> Reporter: Reshmi Choudhury
>
> Hi,
> We are using trinidad-impl-1.0.10.jar in our product. I see that HttpSession
> is used in a synchronized block in the class TokenCache of
> trinidad-impl-1.0.10.jar. Could you please clarify if this could cause to
> deadlock while used with WAS 7.
> Please find the below code snippet for your quick reference.
> static public TokenCache getTokenCacheFromSession( FacesContext context,
> String cacheName, boolean createIfNeeded, int defaultSize) {
> ExternalContext external = context.getExternalContext();
> Object session = external.getSession(true);
> assert(session != null);
> TokenCache cache;
> // Synchronize on the session object to ensure that
> // we don't ever create two different caches
> synchronized (session)
> {
> cache = (TokenCache) external.getSessionMap().get(cacheName);
> if ((cache == null) && createIfNeeded)
> {
> // Seed the token cache with the session ID (if available)
> int seed = 0;
> if (_USE_SESSION_TO_SEED_ID)
> {
> if (session instanceof HttpSession)
> {
> String id = ((HttpSession) session).getId();
> if (id != null)
> seed = id.hashCode();
> }
> }
> cache = new TokenCache(defaultSize, seed);
> external.getSessionMap().put(cacheName, cache);
> }
> }
> return cache;
> }
> This information is required as IBM informs the developer not to synchronize
> the session or it will cause deadlocks. WAS 7 implements the Servlets 2.5
> specification and manages the thread safety for any modification to the
> session map. Please refer to the URL
> http://pic.dhe.ibm.com/infocenter/wasinfo/v7r0/index.jsp?topic=/com.ibm.websphere.nd.multiplatform.doc/info/ae/ae/uprs_rsession_manager.html
> for more details. This URL contains this text:
> “Applications cannot synchronize session objects inside of their servlets or
> Java Server Pages because a deadlock with the session manager may occur. The
> deadlock occurs because the session manager does not expect the use of more
> than one locking mechanism”
> Here is an excellent article on this topic, which explains when explicit
> synchronization is needed in the application code and when the servlet
> container's locking mechanism is sufficient:
> http://www.ibm.com/developerworks/library/j-jtp09238/index.html
>
> It would be really helpful if we receive the information at the earliest as
> this is priority at our end.
> Thanks in advance.
> Regards,
> Reshmi
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