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From "Jeffrey Clary" <jcl...@actuate.com>
Subject Thread safeness using EmbeddedDriver
Date Thu, 21 Jun 2007 19:07:17 GMT
Folks,

 

I've run onto what looks like a Derby multithreading bug, but before
submitting it to JIRA I thought I'd give this list a chance to comment.
I did a keyword search, but it is possible that this has been reported
or is commonly known.  Is the EmbeddedDriver even expected to be thread
safe?

 

I've attached a short test program that displays the error for me.  It
creates a test database with a table, and a view onto that table.  Then
it creates N threads, each of which repeatedly (M times) create and drop
a view onto the previously created view.  Each thread has its own view
name to create and drop, so they are not contending for that view.  But
all the created/dropped views depend on the same underlying view.

 

When the error happens, it is a NullPointerException in
org.apache.derby.iapi.sql.dictionary.TableDescriptor.getObjectName.  An
instance member named referencedColumnMap is checked for null at the top
of the method, but later when it is dereferenced it is null, because it
was set to null by another thread.  I am not sure what getObjectName is
used for other than error reporting.  I have considered a fix of just
saving the non-null reference as a method variable, to avoid the later
NullPointerException.   But I don't know what unintended consequences
this may have.  

 

We tried synchronizing the getObjectName and the setReferencedColumnMap
methods, but this tanked performance in our application.

 

I have successfully seen the error with N=100 and M=100, on a dual-core
3GHz Pentium machine running Windows Vista Business, using JDK 1.4.2_13
and Derby trunk revision 548822.  Sometimes I have to run the program
several times before I see the error.  

 

I'd appreciate any insight before I officially report this as a bug.

 

Thanks,

Jeff Clary

 


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