Re: Exceptions due to freemarker's use of non-thread-safe DOM library

FreeMarker data-models are in general not shareable between threads.
After all, they may contain POJO-s which aren't thread-safe (like
Xerces DOM trees, it seems), so it's hopeless to guarantee
thread-safety in FreeMarker. Since sharing data-models among threads
wasn't a design goal, TemplateModel-s (the things that wrap the
POJO-s) aren't thread-safe either (unless by pure accident). Is there
some part in the documentation that made you believe otherwise?

Still, a data-model can be thread-safe, but then it has to be built
carefully. Like, it seems, you would need to write a
SyncronizedNodeModel class or like...

-- 
Best regards,
 Daniel Dekany


Friday, June 3, 2011, 6:04:46 PM, Newman, John W wrote:

> Hello,
>  
> In production under load, occasionally we’ll get this stack trace
> from a template (using freemarker servlet).
>  
> java.lang.NullPointerException: 
> (no message) 
> at org.apache.xerces.dom.ParentNode.nodeListItem(Unknown Source) 
> at org.apache.xerces.dom.ParentNode.item(Unknown Source) 
> at freemarker.ext.dom.NodeListModel.<init>(NodeListModel.java:89) 
> at freemarker.ext.dom.NodeModel.getChildNodes(NodeModel.java:302) 
> at freemarker.ext.dom.ElementModel.get(ElementModel.java:124) 
> at freemarker.core.Dot._getAsTemplateModel(Dot.java:76) 
> at freemarker.core.Expression.getAsTemplateModel(Expression.java:89) 
> at freemarker.core.IteratorBlock.accept(IteratorBlock.java:94) 
> at freemarker.core.Environment.visit(Environment.java:209) 
> [ a few dozen stack frames visit/accept/macro/ifblock from template itself ]
> at freemarker.core.Environment.visit(Environment.java:209) 
> at freemarker.core.Environment.process(Environment.java:189) 
> at freemarker.template.Template.process(Template.java:237) 
> at
> freemarker.ext.servlet.FreemarkerServlet.process(FreemarkerServlet.java:458)
> at
> freemarker.ext.servlet.FreemarkerServlet.doGet(FreemarkerServlet.java:397)
> at javax.servlet.http.HttpServlet.service(HttpServlet.java:690) 
> at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)  
>  
>  
> Our application uses XML elements that are sometimes cloned from
> another element.  The freemarker templates do a lot of various xml
> operations, ${element.@Attribute} , ${element[xpath]}, etc.  things
> like that that normally works perfectly fine.  In investigating the
> stack trace, it really looks like 
> https://issues.apache.org/jira/browse/XERCESJ-727
>  
> Please read through that, essentially the org.apache.xerces.dom
> library is deliberately not thread safe by design.  [This is news to
> me, a bit unexpected…]  Apparently these element implementations
> maintain a local cache of what has already been parsed through, and
> if a second thread jumps in it will end up stepping on this shared
> cache causing undefined behavior.  So the burden falls on the caller
> to synchronize on the elements before doing certain operations. (I’m not sure exactly which).
>  
> I’ve looked through the freemarker xml package a bit, and I didn’t
> find any sync blocks.  I tried isolating the problem in there and
> getting a good repeatable test case, but unfortunately I don’t
> understand your library well enough to do so.  The best test case I
> can provide you is attached – it’s basically an adaptation of what
> was posted in that issue.  I think the freemarker library needs to
> add a few sync blocks here and there around these xml operations,
> but unfortunately I can’t say exactly where, possibly quite a few
> places.  And hopefully it’s not one big wide block on the outer
> method that would kill performance.  Please look at this and let me
> know what you think.  I was hoping to just get a patch to send, but
> after several hours I’m not having much luck.  A simple pojo test case is attached.
>  
> Thanks,
> John
>  
>  
>  
> John W. Newman
> Programmer
>  
> 5750 Centre Avenue, Suite 500
> Pittsburgh, PA 15206
> Tel  412-204-0116
> [email protected]
> www.d3onc.com
> Fax 412-365-0749
>  
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