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https://issues.apache.org/jira/browse/FOP-3285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18040513#comment-18040513
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Simon Steiner commented on FOP-3285:
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It is not designed to be thread safe
> NullPointerException in Xerces DOM (fNodeListCache) during concurrent access
> to newFOP
> ---------------------------------------------------------------------------------------
>
> Key: FOP-3285
> URL: https://issues.apache.org/jira/browse/FOP-3285
> Project: FOP
> Issue Type: Bug
> Components: renderer/pdf
> Reporter: Manikanta
> Priority: Major
>
> I am encountering a concurrency issue when using FOP 2.10 in a multi-threaded
> Spring Boot environment.
> We initialize a singleton FopFactory using FopFactoryBuilder and a
> DefaultConfigurationBuilder. When multiple threads attempt to create a new
> FOP object using factory.newFop(...) simultaneously, the application crashes
> with a NullPointerException inside Xerces.
> Stack Trace:
> Caused by: java.lang.NullPointerException: Cannot read field "fLength"
> because "this.fNodeListCache" is null
> at org.apache.xerces.dom.ParentNode.nodeListGetLength(Unknown Source)
> at
> org.apache.fop.configuration.DefaultConfiguration.getChildren(DefaultConfiguration.java:138)
> at
> org.apache.fop.render.PrintRendererConfigurator.getCustomFontCollection(PrintRendererConfigurator.java:147)
> ...
> Steps to Reproduce:
> 1. Create a large fop_config.xml with many <font> entries inside a <renderer>.
> 2. Initialize FopFactory once using DefaultConfigurationBuilder PDF/A3.
> 3. Spawn 20+ threads(more than one is enough IG) that immediately call
> fopFactory.newFop() and render a PDF.
> Suggested Fix/Workaround:
> Make FopFactory.newFop() threadsafe
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