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https://issues.apache.org/jira/browse/VELOCITY-536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12486071
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Nathan Bubna commented on VELOCITY-536:
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But the DCL'ed block only needs to happen once per instance. Isn't the whole
point to make the second thread skip the block completely? We only need one
thread per instance to go through, period. Once one thread has started going
through the block, it will finish and keep other threads out of there for that
instance, order notwithstanding.
And again, this is not a constructor nor a completely abstract use of DCL that
we're talking about. This is a very specific use.
> Velocity Engine throws NullPointer Exception when two people click on the
> same page at the same time for the first time
> -----------------------------------------------------------------------------------------------------------------------
>
> Key: VELOCITY-536
> URL: https://issues.apache.org/jira/browse/VELOCITY-536
> Project: Velocity
> Issue Type: Bug
> Components: Engine
> Affects Versions: 1.5
> Reporter: Lei Gu
> Attachments: 536-patch.txt, ASTDirective.java, ASTSetDirective.java,
> VelocimacroProxy.java
>
>
> Multi-thread concurrency issue
> During our concurrency testing, we observed NullPointer exceptions being
> thrown when two people hit the same page at the same time for the first time.
> Upon further investigation, it turns out that we need to synchronize the init
> method on ASTDirective, ASTSetDirective, and render method on
> ASTSetDirective, and VelocimacroProxy.
> Basically, the problem is introduced as the following; when two threads
> attempts to parse and render the same template at the same time. Thread1
> finishes parsing first and proceeds to the render method call, while thread 2
> is still busy parsing and will overwrite the existing parse tree that is
> being used by thread 1 for rendering purpose. Thus under certainly condition
> a NullPointer exception will be thrown.
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