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

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

[Description: cid:[email protected]]

5750 Centre Avenue, Suite 500

Pittsburgh, PA 15206

Tel  412-204-0116

[email protected]<mailto:[email protected]>

www.d3onc.com<../../../../../mullerjr/AppData/Local/Microsoft/Windows/Temporary%20Internet%20Files/Content.Outlook/CEWCLQXA/www.d3onc.com>

Fax 412-365-0749


This e-mail may contain confidential information of the sending organization. Any unauthorized or improper disclosure, copying, distribution, or use of the contents of this e-mail and attached document(s) is prohibited. The information contained in this e-mail and attached document(s) is intended only for the personal and confidential use of the recipient(s) named above. If you have received this communication in error, please notify the sender immediately by e-mail and delete the original e-mail and attached document(s).

------------------------------------------------------------------------------
Simplify data backup and recovery for your virtual environment with vRanger.
Installation's a snap, and flexible recovery options mean your data is safe,
secure and there when you need it. Discover what all the cheering's about.
Get your free trial download today. 
http://p.sf.net/sfu/quest-dev2dev2

_______________________________________________
FreeMarker-user mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/freemarker-user