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Posted to dev@velocity.apache.org by "Lei Gu (JIRA)" <de...@velocity.apache.org> on 2007/04/02 16:50:32 UTC

[jira] Updated: (VELOCITY-536) Velocity Engine throws NullPointer Exception when two people click on the same page at the same time for the first time

     [ https://issues.apache.org/jira/browse/VELOCITY-536?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Lei Gu updated VELOCITY-536:
----------------------------

    Attachment: VelocimacroProxy.java
                ASTDirective.java
                ASTSetDirective.java

Fixes for the multi-threaded concurrency are to synchronized properly on init method on ASTSetDirective, ASTDirective, render method on ASTSetDirective and VelocimacroProxy.

> 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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