[jira] [Commented] (TOMEE-1934) Incorrect double-check locking

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TOMEE-1934?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15489679#comment-15489679 ] 

ASF GitHub Bot commented on TOMEE-1934:
---------------------------------------

GitHub user SvetlinZarev opened a pull request:

    https://github.com/apache/tomee/pull/41

    Fix the double-check locking

    Due to the java memory model it was
    possible to create more than one instance
    of the WebappBeanManager. More information
    at: http://www.cs.umd.edu/%7Epugh/java/memoryModel/DoubleCheckedLocking.html
    
    Bug: https://issues.apache.org/jira/browse/TOMEE-1934

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/SvetlinZarev/tomee sync_1

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/tomee/pull/41.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #41
    
----
commit 2ceb4bb3715a29c0ff3c532078eebdc940aa7382
Author: Svetlin Zarev <sv...@sap.com>
Date:   2016-09-14T07:24:53Z

    Fix the double-check locking
    
    Due to the java memory model it was
    possible to create more than one instance
    of the WebappBeanManager. More information
    at: http://www.cs.umd.edu/%7Epugh/java/memoryModel/DoubleCheckedLocking.html

----


> Incorrect double-check locking
> ------------------------------
>
>                 Key: TOMEE-1934
>                 URL: https://issues.apache.org/jira/browse/TOMEE-1934
>             Project: TomEE
>          Issue Type: Bug
>            Reporter: Svetlin Zarev
>            Priority: Trivial
>
> http://www.cs.umd.edu/%7Epugh/java/memoryModel/DoubleCheckedLocking.html
> {code}
>  public BeanManagerImpl getBeanManagerImpl() {
>         if (bm == null) { // should be done in the constructor
>             synchronized (this) {
>                 if (bm == null) {
>                     bm = new WebappBeanManager(this);
>                 }
>             }
>         }
>         return bm;
>     }
> {code}
> Unless **bm** is volatile, multiple instances of the bean manager can be created under concurrent conditions. 
> PS: I saw the suppress warning. I'm not sure if PMD just warns against DCL or that it was broken. 



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