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https://issues.apache.org/jira/browse/OPENJPA-2507?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14016419#comment-14016419
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Thomas Darimont commented on OPENJPA-2507:
------------------------------------------
Hello,
I just had a look at this and I think a solution that fixes this issue.
I noticed by debugging the provided example app that an instance of
{{org.apache.openjpa.persistence.criteria.CriteriaQueryImpl}} is shared between
multiple instances of {{org.apache.openjpa.jdbc.kernel.JDBCStoreQuery}}. In
{{CriteriaQueryImpl}} the {{_contexts}} field is read and written by multiple
threads.
I think the problem lies in
{{org.apache.openjpa.persistence.criteria.CriteriaQueryImpl.getQueryExpressions(ExpressionFactory)}}:
{code:java}
QueryExpressions getQueryExpressions(ExpressionFactory factory) {
_contexts = new Stack<Context>();
Context context = new Context(null, null, null);
_contexts.push(context);
return new CriteriaExpressionBuilder().getQueryExpressions(factory,
this);
}
{code}
Since the (shared) {{_context}} field is modified and read by multiple threads
this causes the spurious NPE / EmptyStackExceptions to appear.
One (relatively easy) way to solve this would be to just wrap the {{_context}}
in a {{ThreadLocal}} - I added a patch file that does exactly that. Note that
we remove the stored ThreadLocal value when we leave {{getQueryExpressions}}
With this fixed I could run the provided example app without any problems. This
patch would work for openjpa 2.3.0 as well as 2.4.0.
I tested this with openjpa 2.3.0 directly. I created the patch from the latest
svn head for 2.4.0.
Cheers,
Thomas
> Weird EmptyStackException in CriteriaQueryImpl
> ----------------------------------------------
>
> Key: OPENJPA-2507
> URL: https://issues.apache.org/jira/browse/OPENJPA-2507
> Project: OpenJPA
> Issue Type: Bug
> Components: criteria
> Affects Versions: 2.2.0, 2.3.0
> Environment: openjpa 2.2.0 & 2.3.0, spring-data-jpa up to 1.4.2
> Reporter: Ioannis Alexandrakis
> Priority: Minor
> Labels: criteria_api, openjpa
> Attachments: exceptions.txt, openjpa-exception-demo.zip
>
>
> Using spring-data-jpa with openjpa, I sometimes encounter a strange error
> (not all the times, but under heavy load it makes its appearance, sparsely
> though):
> ....
> Caused by: java.util.EmptyStackException
> at java.util.Stack.peek(Stack.java:<arbitrary line>)
> at
> org.apache.openjpa.persistence.criteria.CriteriaQueryImpl.ctx(CriteriaQueryImpl.java:<arbitrary
> line>
> ....
> I do not know which behaviour triggers it, however I think it would be an
> improvement to change (I did not know where exactly to file it, because it is
> both an improvement and a bug in my opinion), in
> org.apache.openjpa.persistence.criteria.CriteriaQueryImpl, method: Context
> ctx(), this:
> return _contexts == null || _contexts.isEmpty() ? null : _contexts.peek();
> to something like this:
> try {
> return _contexts == null || _contexts.isEmpty() ? null :
> _contexts.peek();
> } catch (EmptyStackException e) {
> return null;
> }
> , in order to prevent a case where multiple threads modify the "_contexts"
> between the evaluation of the inline "if".
> I am not able to reproduce it all the time, thus I can't create a useful
> test, neither have I created a patch due to the simplicity of the 'fix'.
> However I believe it is a harmless fix which could be considered a minor
> improvement.
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