[jira] Issue Comment Edited: (WW-3576) SessionMap is not thread-safe

["Dave Newton (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/WW-3576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12995774#comment-12995774 ] 

Dave Newton edited comment on WW-3576 at 2/17/11 11:59 AM:
-----------------------------------------------------------

DCL doesn't work and is JVM-implementation-dependent.

http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html

      was (Author: newton_dave):
    DCL doesn't work.

  
> 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: S. 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 support 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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